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close this book Soils, Crops and Fertilizer Use
close this folder Chapter 9: Using chemical fertilizers
View the document What are chemical fertilizers?
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View the document An introduction to chemical fertilizers
View the document Common chemical fertilizers and their characteristics
View the document The effect of fertilizers on soil pH
View the document Fertilizer salt index and "burn" potential
View the document Basic application principles for N, P, and K
View the document Fertilizer application methods explained and compared
View the document Troubleshooting faulty fertilizer practices
View the document Getting the most out of fertilizer use: crop management as an integrated system
View the document Understanding fertilizer math

Chapter 9: Using chemical fertilizers

This chapter will give you a strong grounding in the use of chemical fertilizers. It covers these areas:

• Types and characteristics of chemical fertilizers.

• Understanding fertilizer labels.

• Fertilizers and their effect on soil pH.

• Timing and placement guidelines.

• Avoiding fertilizer burn.

• Fertilizer rate guidelines.

• Troubleshooting faulty fertilizer practices.

• Fertilizer as one part of integrated crop management.

• Extension guidelines for using chemical fertilizers.

• Fertilizer math skills.

What are chemical fertilizers?

As opposed to organic fertilizers which originate from plants and animals (compost, manure, etc.) or are unprocessed minerals like raw rock phosphate, chemical fertilizers are derived from a chemical manufacturing or synthesizing process. Some examples are urea fertilizer (45-46 percent N) made from carbon dioxide and ammonia, or single superphosphate 18-21 percent P2O5) made from combining rock phosphate and sulfuric acid.

Are chemical fertilizers appropriate for limited-resource farmers?

We dealt with this issue in detail at the start of Chapter 8 where the overall advantages of organic fertilizers were stressed. It was recommended that farmers be urged to maximize the practical usage of organics wherever feasible. On the other hand, it was also pointed out that many farmers may not have enough available to cover all their crop land. In such cases, chemical fertilizers are often very cost-effective if capital or credit is available. It's not unusual to receive a return of $3-$10 for every $1 spent on chemical fertilizers, especially when they're used along with other complementary management practices. However, chemical fertilizers require more skill to use than organics in terms of rate determinations, dosage calculation, timing, and placement.

An introduction to chemical fertilizers

NUTRIENT CONTENT

Chemical fertilizers contain one or more of the "Big 3" (N, P, and K) along with varying amounts of calcium and sulfur. Ordinarily, chemical fertilizers contain no magnesium or micronutrients unless these have been specially added. (Micronutrients are usually applied as separate fertilizers when needed).

The myth of "complete" fertilizer: Those fertilizers like ammonium sulfate (21% N) that contain only one of the Big 3 are called straight fertilizers. Others, like di-ammonium phosphate (18-46-0), contain two of the Big 3. Those such as 12-24-12 which contain N, P, and K are often called complete fertilizers, but this is misleading, because few of them contain all 12 plant mineral nutrients. However, some types may contain significant amounts of some secondary and micronutrients; check the label.

Some NP and NPK fertilizers are simple mechanical mixes of two or more fertilizers. Others are actual chemical combinations with every individual granule having the same nutrient content.

Color as a likely nutrient indicator: The color of a fertilizer's granules is often a useful indicator of its general composition. Grey granules usually indicate an NP, NPK, or straight P fertilizer. White granules usually indicate a straight N fertilizer like urea, ammonium nitrate, or ammonium sulfate. However, potassium sulfate (0-0-50) and most forms of potassium chloride (0-0-60) are also white; some forms of potassium chloride are reddish due to impurities.

PHYSICAL FORMS

• Most come as granules meant for soil application. Some granular fertilizers like ammonium nitrate and urea will also readily dissolve in water and can be sprayed on plant foliage in very dilute form or watered into the soil.

• Liquid formulations are available in some areas. Some can be used for soil application like granules. Others often contain NPK plus micronutrients and are meant for spray applications to the leaves (foliar applications); they are usually rather costly in relation to their nutrient content.

• Soluble powders containing NPK and/or micronutrients may also be available in your area and are meant for for foliar application.

HOW TO READ A FERTILIZER LABEL

All reputable commercial chemical fertilizers carry a label giving their nutrient content, specifying not only the NPK content, but also the amounts of secondary nutrients and micronutrients.

The 3-Number Labelling System

With a few exceptions (notably those fertilizers that originate in South Africa), most countries use a universal 3-number labelling system that indicates the N, P, and K content in that order, usually in terms of N, P2O5, and K2O. The numbers refer to percent. For example, a 12-24-12 fertilizer contains 12% N, 24% P2O5, and 12% K2O; 200 kg of 12-24-12 contains 24 kg of N, 48 kg of P2O5, and 24 kg of K2O. A 0-21-0 fertilizer contains 21% P2O5 but no N or K.

N-P2O5-K2O vs. N-P-K

The N-P2O5-K2O labelling system is traditional and dates back to the 19th Century when chemical fertilizers were first developed. The P and K contents were analyzed by burning (oxidizing) the fertilizer and then measuring the resulting P2O5 (called phosphoric acid or phosphorus pentoxide) and K2O (potash, potassium oxide) that formed. The N-P2O5K2O system is known as the oxide form of labelling.

In recent years, a few countries have switched over to to the elemental form (straight N, P, and K) for labelling and giving nutrient rates; in some cases, the label will give the fertilizer formula in both the oxide and the elemental forms.

Note that N content is given in terms of actual N in both systems.

Don't be confused by this. It really makes little difference whether a fertilizer's NPK content is expressed in the oxide or elemental form as long as the fertilizer labels and the nutrient rate recommendations given by the extension service both use the same form. A fertilizer's true nutrient content is the same whether measured in the oxide or the elemental form, just as the distance between your village and the country's capital is the same whether measured in kilometers or miles. Likewise, the sodium content of a pickle is the same whether measured as pure sodium or sodium chloride.

NOTE: Throughout this manual we'll use the N-P2O5-K2O system since it's still the most common. The terms "P" and "K" will often be used as a short form for phosphorus and potassium with no regard to either labelling system.

When the difference does matter: In some countries like the U.S., both systems are being used. In this case, you'll want to double check and be sure whether the amount of phosphorus or potassium listed on a label or given as a fertilizer recommendation is in the oxide or the elemental form. This affects the actual amount of fertilizer needed, especially in the case of phosphorus. Here's how to convert between the two systems:

P x 2.3 = P2O5

P2O5 X 0.44 = P

K x 1.2 = K2O

K2O x 0.83 = K

Here are 2 practice problems to clear up any confusion:

PROBLEM 1: Suppose soil test results recommend that Suheyla apply phosphorus at the rate of 30 kg of actual P (elemental P) per hectare. If the phosphorus content of the fertilizer is expressed in the oxide form (P2O5), how much P2O5 will be needed to supply 30 kg elemental P?

SOLUTION:

Since P2O5 = P x 2.3, you'd multiply the 30 kg actual P by 2.3 to convert it to P2O5. The answer is 69 kg P2O5.

PROBLEM 2: Suppose your country uses the elemental system in labelling fertilizers. You see a fertilizer with the formula 15-6.6-12.5 (N-P-K basis). What would the formula be in terms of N-P2O5-K2O?

SOLUTION: 6.6% P x 2.3 = 15% P2O5 12.5% K x 1.2 = 15% K2O

Therefore: 15-6.6-12.5 N-P-K formula equals 15-15-15 on an N-P2O5-K2O basis.

Why Don't the 3 Numbers Add Up to 100?

If you'll look at the fertilizer composition table in Appendix D, you'll notice that the percentages of N, P2O5, and K2O don't even come close to totalling 100. The main reason is that N, P, and K have to be combined with carriers like sulfur, calcium, oxygen, and hydrogen to become stable and usable.

EXAMPLES: Ammonium nitrate fertilizer (33-0-0) has the chemical formula NH4NO3. It contains 33% N with the rest being hydrogen and oxygen.

Single superphosphate (0-21-0) has the formula Ca(H2PO4)2CaSO4 . In addition to containing 21% phosphorus P2O5 basis), it has calcium, hydrogen, sulfur, and oxygen.

Another reason why the 3 numbers don't total 100 is that some fertilizers have fillers like sand added so that they end up with a whole-number formula. In addition, conditioning agents are sometimes added to improve handling qualities.

Another Useful Term: Fertilizer Ratio

The fertilizer ratio is the ratio between the 3 numbers in a fertilizer's formula and tells the relative proportions of N, P2O5 and K2O (or N, P, K if the elemental system is used) in the fertilizer. Some examples:

Fertilizer Formula

Fertilizer Ratio

10-20-10

1:2:1

12-24-12

1:2:1

6-12-6

1:2:1

15-15-15

1:1:1

10-10-10

1:1:1

10-30-10

1:3:1

Understanding fertilizer ratios is very useful when trying to match the kind of fertilizer to a recommendation. For instance, if soil test results recommend applying 30 kg N, 60 kg P2O5, and 30 kg KsO per hectare at planting time, this is a ratio of 1:2:1. It follows that any fertilizer with a 1:2:1 ratio could be used to supply the 3 nutrients in the right proportion and amount (i.e. 300 kg/ha of 10-20-10 or 250 kg/ha of 12-24-12).

Common chemical fertilizers and their characteristics

NOTE: Appendix D lists the nutrient content of common chemical fertilizers.

NITROGEN FERTILIZERS

Nearly all chemical N fertilizers contain either ammonium (NH4+) or nitrate (NO3-) nitrogen. The nitrate form is quicker acting because it's more immediately mobile (leachable) and reaches the roots sooner if applied to a growing crop. But, remember that ammonium is rather quickly converted to mobile nitrate in warm soils (all of it within 7-10 days).

N fertilizers and soil pH: Most N fertilizers containing ammonium N have a gradual acidifying effect on the soil; this will be covered in detail farther along.

Loss of N by volatilization: All ammonium N fertilizers will release ammonia gas when applied to soils with pH's above 7.0. If applied to the soil surface, significant amounts may be lost to the atmosphere. Urea fertilizer releases ammonia at any pH. Losses can be avoided by placing such fertilizers a few centimeters deep.

Common Nitrogen Fertilizers

Ammonium Nitrate (33-34% N)

• Contains half nitrate N and half ammonium N, so is quicker acting than straight ammonium fertilizers.

• Absorbs moisture and becomes slushy in high humidity; keep bags well sealed.

• Can become explosive if mixed with oil. Releases oxygen when exposed to fire which encourages combustion.

Ammonium Nitrate with Lime (26% N

• Same as above but is coated with dolomitic limestone to neutralize the acid-forming properties of regular ammonium nitrate and to reduce moisture absorption.

Ammonium Sulfate (20-21% N)

• In addition to N, it contains 23% sulfur (or 69% sulfate).

• Good handling and storage properties

Urea (45-46% N)

• The highest-strength solid form of N.

• Its N is initially in the amide form (NH2) but is converted to ammonium in moist warm soils within 1-2 days (a week or two in cooler soils) and then to nitrate by soil bacteria.

• Unlike ammonium N fertilizers, urea is mobile and leachable until its amide N has been converted to ammonium.

• Regardless of soil pH, some N will be lost to the atmosphere as ammonia gas if urea is left on the soil surface. Losses are highest above a soil pH of 7.0 and can reach 35% when urea is broadcast (spread) over grass pastures. Losses are minimal, however, if rainfall or irrigation occur within a few hours after such surface applications.

• Can "burn" (injure) seeds and seedlings if placed too close due to release of free ammonia.

• May sometimes contain excessive amounts of biuret ( toxic to plants) due to faulty manufacturing. Biuret is most toxic when urea is mixed with water and applied foliarly ( sprayed on the leaves).

• Tends to absorb moisture, but not as much as ammonium nitrate.

• Can be fed to ruminants like cattle as a protein source; the rumen bacteria convert the N to protein; BUT urea can be toxic at anything but very low levels and must be fed in combination with certain other feeds. Vinegar is the antidote.

Sodium Nitrate (16% N) (Chilean nitrate)

• Its nitrate N is readily leachable.

• Unlike most ammonium N fertilizers, it has a gradual basic effect on the soil.

• Can easily burn seeds and seedlings because of its very high salt content. (Fertilizer burn is covered farther along)

• Absorbs moisture and can become slushy in high humidity; keep bags well sealed.

• Expensive because of its low nutrient content relative to shipping costs.

Anhydrous Ammonia (82% N)

• Exists as a liquid under pressure and a gas when released into the soil.

• The highest-strength N fertilizer available.

• Must be injected into moist soil about 15 cm deep to avoid ammonia loss.

• Very dangerous; inhalation and facial exposure can cause blindness and fatal lung damage.

• Requires special storage and application equipment.

Aqua Ammonia (21% N)

• Made by dissolving ammonia gas in water. Has strong odor of ammonia. Unlike anhydrous ammonia, it doesn't have to be applied or stored under pressure.

• Should be applied at least 4-5 cm below the soil surface to avoid loss of ammonia.

• Requires special storage and application equipment.

• Releases irritating fumes.

Potassium Nitrate (13-0-44: See under K fertilizers.

Ammonium Phosphate Fertilizers: See under P fertilizers.

Time-Release or Slow-Release N Fertilizers: They're coated with special substances that reduce their solubility and slow down the rate at which soil bacteria convert ammonium to nitrate. Leaching losses are much lower, but they're usually too expensive to be cost effective for farmers.

PHOSPHORUS FERTILIZERS

The phosphorus in most chemical fertilizers comes from reacting rock phosphate with sulfuric, phosphoric, or nitric acids or with anhydrous ammonia.

Water-soluble vs. Citrate-soluble vs. Insoluble P

A chemical fertilizer's P can exist in several forms which should be listed on the label:

Water-soluble P: This type of P is soluble in water and moves quickly out of the granules into the soil. But, that doesn't mean it will be 100 percent available to plants, because it's still subject to the soil's ability to tie up (fix) P. When P fertilizer is placed in a band, hole, or half-circle near the row, it's recommended that at least half the fertilizer's P be water-soluble. When P fertilizer is broadcast on soils below pH 7.0, water solubility isn't important, because soil acidity helps dissolve the P.

Citrate-soluble P: This type of P isn't soluble in water but will dissolve in a weak acid solution. Heat-treated rock phosphate contains largely citrate-soluble P which is usable only in acidic soils.

Insoluble P: This type of P isn't soluble in water or a weak acid solution, so it has very limited availability to plants. Most of the P in raw rock phosphate is insoluble and only very slowly available, even in acid soils.

Common Phosphorus Fertilizers

Single Superphosphate (16-22% P2O5, 8-12% S): A common P fertilizer and also a good sulfur source. About 78% of its P is water soluble (see above). Made from rock phosphate and sulfuric acid.

Triple or Concentrated Superphosphate (42-48% P2O5): Has much more P than single super but only 1-3% sulfur. About 84% of its P is water soluble. Made from rock phosphate and phosphoric acid.

Ammonium Phosphate Fertilizers

There are 3 classes, all with 100% water-soluble P:

• Mono-ammonium phosphate (11-48-0, 12-61-0): Tends to work better than all-ammonium phosphate on alkaline soils. Low in sulfur. Less likely to cause burning than DAP.

• Di-ammonium phosphate (16-48-0, 18-46-0, 21-53-0): A good P source but can injure seeds or seedlings due to ammonia release if placed too close.

• Ammonium Phosphate sulfate (16-20-0, 13-39-0): Both are also good sources of sulfur (915% S in 16-20-0, 7% S in 13-39-0).

Miscellaneous NP and NPK Fertilizers: 20-20-0, 14-14-14, 12-24-12, etc.

Heat-treated Rock Phosphates: These vary a lot in P content and are made by heat treating rock phosphate which greatly increases its low availability. Its P isn't watersoluble but is citrate-soluble (see above) and will slowly become available in acid soils when broadcast. It may be a cheap P source in areas with phosphate deposits but is only recommended for acid soils or where organic matter is very high. It should be in a finelyground form and be applied by broadcasting to promote the release of its P through soil reaction. It doesn't become available quickly enough to be used as the sole source of added P for short-term annual crops like maize. Much higher rates are needed than for more available forms. Where mycorrhizae soil fungi are abundant (see Chapter 1), they increase the availability of rock phosphate to plant roots.

Raw rock phosphate: See Chapter 8.

Basic Slag (8-25% P2O5) A by-product of steel making. About 60-90% of its P is citrate soluble, so it's best used on acid soils, much like heat-treated rock phosphate. It has a gradual basic effect on soils.

POTASSIUM FERTILIZERS

The most common K fertilizers are:

• Potassium chloride (muriate of Potash): Contains about 60%-62% K2O

• Potassium sulfate: Contains about 48-50% K2O and 18% S.

• Potassium nitrate (13-0-44).

• NPK fertilizers like 10-20-10, etc.

NOTE: Tobacco, potatoes, and sweet potatoes are sensitive to high amounts of chlorides which affect crop quality. In this case, potassium chloride should be avoided or minimized.

SECONDARY NUTRIENT FERTILIZERS (Calcium, Magnesium, Sulfur)

Calcium and Magnesium

Even acid soils have enough calcium for most crops. Where liming is needed and magnesium is also deficient, dolomitic limestone (a mixture of calcium and magnesium carbonates) should be used. Liming with calcium only can also provoke a Mg deficiency. Gypsum has no effect on soil pH and is often used to supply calcium to crops with high needs, such as peanuts, without raising the pH.

Magnesium sulfate (epsom salts; 9-11% Mg) and potassium magnesium sulfate (11% Mg) are other sources and have no effect on soil pH. The Mg content of fertilizers is often expressed in terms of magnesium oxide (MgO); the conversion is: Mg x 1.66 = MgO MgO x 0.6 - Mg

Sulfur

Some common fertilizers are good S sources like single superphosphate (8-12% S), ammonium sulfate (23-24% S), 16-20-0 (9-15% S), and potassium sulfate (17% S). Usually, the higher the NPK content of the fertilizer, the lower the S content (i.e. triple superphosphate contains only 1-3% S).

Sulfur deficiencies are on the increase in non-industrial areas, due to the growing use of high-analysis fertilizers with lower S contents. It's usually a good idea to include a sulfurbearing fertilizer in a fertilizer program, especially on acid, sandy soils. Organic fertilizers are a good source of S. Appendix D lists the S content of chemical fertilizers.

The S content of fertilizers is often expressed in terms of SO4 (sulfate). The conversion is: S x 3 = SO4

MICRONUTRIENT FERTILIZERS

Some NP and NPK fertilizers may have added amounts of micronutrients (check the label) but usually too little to correct deficiencies. If a meaningful amount of a micronutrient is present, it may be indicated by a fourth number in the fertilizer formula, referring to it.

Separate micronutrient fertilizers like copper sulfate, ferrous sulfate (iron), zinc sulfate, manganese sulfate, and borax can be used for soil or foliage (leaf) application. Remember that soil tie-up of added manganese and iron is often a problem on deficient soils (see Chapter 6).

Micronutrient chelates: Specially synthesized forms of micronutrients called chelates are available and used where soil tie-up problems are serious. A chelate has a special molecular structure that protects the micronutrient from being tied up.

Some fungicides like Maneb (containing manganese) and Zineb (containing zinc) can supply these micronutrients in conjunction with a disease control program.

The effect of fertilizers on soil pH

Fertilizers can be acid, basic, or neutral in their effect on soil pH:

• All ammonium N fertilizers (except ammonium nitrate with lime) have a gradual acidforming effect. That's because the conversion of ammonium (NH.) to nitrate (NO3) releases acid-forming hydrogen ions (H+). The same applies to urea and most NP and NPX fertilizers. (See Table 9-1.)

• Large applications of manure or compost also have a gradual acid-forming effect.

• Nitrate N fertilizers that have their nitrate combined with a strong base have a slightly basic effect (i.e. calcium nitrate, potassium nitrate, sodium nitrate).

• The straight P or K fertilizers have no effect on soil pH. Examples: potassium chloride, potassium sulfate, and the superphosphates.

The Practical Implications of Acid-Forming Fertilizers

Continued use of acid-forming fertilizers over the years will eventually lower soil pH enough to require liming, unless the soil is very alkaline. The rate that soil pH will fall depends on the kind and amount of fertilizer applied and the buffering capacity (negative charge, C.E.C.) of the soil (see Chapter 6). Since clayey soils or those high in organic matter tend to have more buffering capacity, they're usually more resistant to pH change than sandy soils.

So why use acid-forming fertilizers?: They're usually the most available and economical; on alkaline soils, they can actually be beneficial.


TABLE 9-1 Relative Acidity of Acid-Forming Fertilizers

Why not add lime to acid-forming fertilizers?: Some fertilizer labels state the amount of lime required to neutralize the acidity produced per 100 kg of the fertilizer, but this is just a legal requirement. Mixing in lime with such a fertilizer will convert much of its ammonium into ammonia gas which is then lost to the air. Don't add lime to the soil after each fertilizer application, either; it's unnecessary and time consuming. At any rate, most limited-resource farmers won't be applying high enough rates to markedly lower the pH in a year or two.

Fertilizer salt index and "burn" potential

As with manure, some chemical fertilizers can injure or even kill seeds or plants when placed too close, and this is called fertilizer "burn". The likelihood of burn depends on the fertilizer used, its rate and placement, and the type of crop.

What Causes Fertilizer Burn?

Fertilizers are composed of various types of salts such as chlorides, sulfates, and nitrates. Some of these dissolve very readily in the soil water after application. If too high a salt concentration accumulates near the seed or roots, they become unable to absorb enough moisture and show many of the symptoms of drought. If you've taken a biology course, you may remember the principle of osmosis, which seeks to equalize the salt concentration of 2 solutions separated by a permeable membrane (in this case, the seed coat or root hair surface). This is what causes most fertilizer burn. The difference in salt concentration between the inside of the seed or roots and the soil water outside them creates an osmotic "pull" that either prevents water from being taken in or actually draws it out of the plant tissues.

How to Spot Fertilizer Burn

Here are some symptoms that can indicate fertilizer burn.

• Poor seed germination (poor seedling emergence): However, this can be caused by many other factors like low-viability seed, disease, lack of soil moisture, etc.

• Seedlings begin to wilt, and then become yellow and eventually brown and dead, starting at the leaf tips. This can also be caused by other factors such as drought, insects, and diseases. Even established plants can suffer fertilizer burn if a nitrogen sidedressing is applied too close or at too high a rate.

Other Types of Fertilizer Burn

• Some fertilizers like urea and all-ammonium phosphate can also cause burning by releasing free ammonia gas if placed too near seeds, seedlings, or established plants.

• Fertilizer granules containing N and K will cause burn spots on plant leaves if spilled on them.

• Foliar applications will burn the leaves if too strong.

Fertilizers Vary in their Burn Potential

Fertilizers vary in their soluble salt contents. Those containing N and K have the highest salt ratings and are much more likely to cause burn than straight P fertilizers like superphosphate. Some fertilizers like urea and all-ammonium phosphate release free ammonia gas which can also cause burn when placed too close.


TABLE 9-2 Relative Burn Potential of Common Fertilizers

How to Prevent Fertilizer Burn

As shown by Table 9-2, those fertilizers containing N and K are much more likely to cause burn than those that contain only P, such as superphosphate.

Fertilizer burn is more likely when a localized placement method is used (band, hole, half circle). Follow the distance guidelines carefully that are given in the fertilizer application methods section farther along.

Fertilizer burn is more likely to occur on sandy soils, since salts and free ammonia move more readily.

• Fertilizer burn is gore likely under low-moisture conditions.

• Avoid placing fertilizer in contact with seeds or directly under the seed furrow, even if separated by a few centimeters of soil. Salts can move upward an the soil dries out and can reach the seeds. However, superphosphate can be banded directly under the seed furrow if separated by soil.

• If sidedressing plants by broadcasting an N fertilizer, avoid applying it when the leaves are wet or else the granules may cling to the leaves and cause burn spots.

• If applying an N sidedressing by hand-watering plants with N fertilizer dissolved in water, be sure to wash off the leaves afterwards with plain water.

How to Treat Fertilizer Burn: If water is available, use liberal amounts to flush away the salts from the needs or seedlings. If not, hope for rain!

Basic application principles for N, P, and K

Before covering the specific application methods for chemical fertilizers, let's go over some important principles that affects how N, P, and K can be best applied.

NITROGEN APPLICATION PRINCIPLES

Remember that nearly all chemical fertilizer N is mobile and leachable in the soil, because ammonium N is rapidly converted to mobile nitrate in warm soils. The sandier the soil and the higher the rainfall, the greater the potential leaching losses.

How to Combat Leaching Losses of N

If all the N is applied at planting or transplanting, much may be lost by leaching, especially since young plants have relatively small N needs. For annual crops, such as maize, tomatoes, and cabbage, it's far better to "spoonfeed" the N by applying only 1/3-1/2 of the total (but no less than 30 kg/ha actual N) at planting or transplanting, usually as part of an NP or NPK fertilizer. The remaining 1/2-2/3 is applied in one to several sidedressings along the crop row, starting about 4 weeks after the initial NPK application. Sidedressings usually consist of a straight N fertilizer like urea or ammonium sulfate.

Guidelines for Sidedressing N

The number of sidedressings over which the remaining N is divided depends on 2 factors:

• The potential for leaching losses as influenced by texture and rainfall.

• The length of growing period for the crop.

Here are some examples:

Maize: Usually needs one sidedressing around knee-high stage (about 4 weeks after planting in warm areas). Under high rainfall, especially on sandy soils, 2 sidedressings are recommended: one at knee high, one at tasseling.

Vegetables: A very short season crop like radishes doesn't need a sidedressing. Leafy vegetables such as lettuce, pak choy, and amaranth may get one to several sidedressings (at 3-4 week intervals), depending on whether the whole plant is harvested at once or picked a few leaves at a time over a longer period. Short-term cucurbits like summer squash and cucumber can use 1-2 sidedressings, while longer-tare ones like melons and winter squash might need 2-3. Tomatoes will need from 2 to as many as 6 or more, depending on leaching conditions and length of production. A good interval between sidedressings is 3-4 weeks.

Where to Place Sidedressed N: We'll cover this under application methods in a few pages.

How Deep to Place N: Since N is so mobile, it doesn't have to be placed deep in order to reach the roots, but just enough (2-5 cm deep) to avoid being washed away by rain or losing N as ammonia gas (refer to the section on N fertilizers).

PHOSPHORUS APPLICATION PRINCIPLES

The yield response obtained from applying fertilizer P to P-deficient soils depends a lot on how and when it's applied. Learn these important guidelines:

• Apply P early: Young seedlings need a high concentration of P in their tissues for early growth and root development. One study showed that young maize seedlings take up 22 times more P per unit of length than plants 11 weeks old. P should be applied at planting or transplanting time.

• Remember that applying P in combination with N (if needed) helps stimulate P uptake.

• Application method has a big influence on the soil's ability to tie up applied P. Broadcasting (spreading) fertilizer P usually results in far more tie-up than using a localized placement method (band, hole, or half circle) since it maximizes the contact of each fertilizer granule with soil particles than can cause tie-up. These methods will be explained in the upcoming section on fertilizer application

• Place broadcast P deep: It should be thoroughly mixed into the topsoil with a plow or hoe, except when spread around tree crops (this will be explained farther along under application methods).

• Don't "spoonfeed" P: Depending on application method, the mobility of P varies from nothing to very moderate. Leaching is never a problem, so all of the P can be applied in one application. There's no advantage to making sidedressings as growth proceeds unless P hunger signs develop.

POTASSIUM APPLICATION PRINCIPLES

K ranks midway between N and P in terms of mobility and leaching. As with P, all the K can usually be applied at planting or transplanting as part of an NPK fertilizer or as a straight K fertilizer. Where leaching losses are likely to be high, split applications of K may be needed. Split applications are also recommended for pastures to avoid "luxury consumption" of K. (Refer the section on potassium in Chapter 6.)

Fertilizer application methods explained and compared

The section gives "how to" instructions on the following fertilizer application methods and compares their use:

• Broadcasting

• Localized placement (band, hole, half circle)

• Special placement considerations for furrow irrigated soils.

• Application through the irrigation water

• Foliar applications

BROADCASTING

Broadcasting refers to spreading the fertilizer evenly over the soil surface with or without working it into the soil. Localized placement refers to applying fertilizer in a band, hole, or half-circle near the seed row or plants.

NOTE: For convenience, this manual will often refer to these 2 methods by their initials, "BC" and "LP".

Advantages of Broadcasting

• It gives a more even distribution of nutrients throughout the root zone than the LP method, allowing more roots to come in contact with the fertilizer. It's usually the best method for obtaining maximum yields.

• There's less danger of fertilizer "burn" since the fertilizer is more diluted.

• It may give better distribution of labor by allowing the initial NPK application to be done before planting.

Disadvantages of Broadcasting

• It maximizes the tie-up of fertilizer P: Broadcasting requires from 3-10 times more P to produce the same yield increase compared to using an LP method.

• Although broadcasting may produce higher yields if enough extra P is applied to make up for increased tie-up, it's doubtful whether limited-resource farmers should be aiming for maximum yields. Most of them face several yield-limiting factors ranging from marginal land to insufficient capital.

• It also promotes more K tie-up than the LP method on soils where this is a problem (i.e. those high in 2:1 temperate-type clays such as illite; see Chapter 2).

• It feeds the weeds as well as the crop.

• It's difficult to spread fertilizer evenly by hand.

• Any fertilizer containing P needs to be worked well into the topsoil when broadcast (see below). Not all farmers have the time, labor, or equipment to do this.

• It's not well suited to crops with less extensive root systems (e.g. carrots, lettuce, and potatoes) unless they're spaced very close together. (See the section on intensive gardening in Chapter 4.)

Why Broadcasting P is Usually not a Good Idea

With a few exceptions, chemical fertilizers containing P should not be broadcast over the soil, even if they are plowed or hoed into the topsoil. Broadcasting spreads out the fertilizer too thinly, exposing each granule to full soil contact, which maximizes the opportunity for P fixation (P tie-up). (Review the section on P tie-up in Chapter 6 if this concept isn't clear). Remember that some soils high in tropical-type clays have especially high P fixation capacities. It takes about 3-10 times (or even more) P to provide the same yield boost when broadcast compared to when locally placed.

The "LP" method greatly reduces the opportunity for P tie-up by minimizing the soil's contact with the fertilizer. It also results in a high enough concentration of P to overcome the tie-up ability of the soil immediately surrounding the fertilizer.

Should P Ever be Broadcast?

Farmers with adequate capital and whose soil has only a moderate P tie-up capacity will sometimes make large broadcast applications of P to build up the "oil's P status. Such applications may be effective for several years and are often combined with localized placement of smaller amounts near the seed row at planting or transplanting to stimulate early growth. Few limited-resource farmers will be able to afford such large broadcast applications which are better suited to very high yield goals, good capital availability, and soils low in P fixation ability.

However, there are several situations where broadcasting a P fertilizer may be appropriate, even for limited-resource farmers:

• Nursery seedbeds: Given the dense spacings used in beds for producing transplants, the "LP" method isn't practical. Also, nursery seedbeds are very small, so enough extra P can be applied without excessive cost.

• Other small Plots where the extra high rates needed can be applied at a reasonable cost, especially those where the seeds have been broadcast, making an LP method difficult. The main rationale for broadcasting would be to avoid the labor of banding fertilizer on directplanted vegies; where transplants are being set, broadcasting has much less justification, because the plants are set far enough apart to be quickly fertilized by an LP method like a half circle or a hole.

• Flooded rice fields: While flooded, a soil's P tie-up capacity is considerably reduced, so fertilizer P can be broadcast and still have good availability in rice paddies.

• Tree crops: Broadcasting P fertilizer in a broad band (30-40 cm wide) around a tree is an effective application method. It doesn't result in as much P tie-up, since the fertilizer is still fairly concentrated compared to uniformly broadcasting it over the whole field.

• Pastures: Topdressing (broadcasting fertilizer and leaving it on the surface) is the only practical method for applying fertilizer to an established pasture. Even though the P isn't worked into the soil, it's still utilized, since grass roots grow very close to the surface. There's also less P tie-up near the surface due to the high humus level promoted by the pasture.

Broadcast P Must be Worked into the Soil

Broadcast P is virtually immobile due to P tie-up in the soil. It won't reach the roots unless it's thoroughly worked into the topsoil with a hoe or plow. A rake or a disk harrow won't move it down deep enough. Leaving P fertilizer on the soil surface is a common mistake and results in much less yield response. On soils that have been heavily mulched with rice straw, etc., root development can be quite good near the soil surface (it doesn't dry out as much), and surface broadcasting may be feasible where moisture is adequate to keep the surface continually wet.

AN EXCEPTION: When applying fertilizer P in a wide band around established tree crops, it should be worked in shallowly (2-3 cm) to avoid pruning tree roots which grow close to the surface.

How to Broadcast Fertilizer Evenly

When broadcasting fertilizer by hand, a more even distribution can be obtained by first dividing the dosage into 2-3 parts. Apply the first part while walking lengthwise down the field or plot; apply the 2nd part while walking at right angles to the first pass, and so on.

Hand-cranked fertilizer broadcasters are Also available in some areas, as are tractor drawn spreaders.

LOCALIZED PLACEMENT METHODS: Band, Hole, Half-Circle

The "LP" methods are usually the best ones for limited-resource farmers whose capital, management, and level of other limiting factors point toward using low to moderate rates of chemical fertilizers (when organics are lacking>. As you'll see below, the pros outweigh the cons:

Advantages of the LP Method

• Low to moderate rates of fertilizer (especially P) are more efficiently used than if broadcast. This provides the maximum return per dollar spent, something small farmers should usually be aiming for.

• It minimizes the tie-up of P (and also of K in the less common case where this is a problem).

• It provides an early growth stimulation, especially in cooler soils where plants have trouble taking up enough P. This doesn't always lead to higher yields but helps the crop compete with weeds.

• It doesn't feed the weeds as much.

• It's especially good for crops with less extensive root systems like potatoes, onions, lettuce, and cabbage.

Disadvantages of the LP Method

• It's difficult to produce maximum yields with the LP method alone on low fertility soils. But, maximum yields aren't likely to be the best strategy for most small farmers, anyway, as already mentioned.

• It can be more laborious and time-consuming than the BC method; however, working broadcast fertilizer into the soil may require equal or greater labor.

Depth isn't as Important with ''LP" Placed P

Although broadcast P is immobile and needs to be worked well into the topsoil to reach the roots, LP-applied P doesn't always have to be placed as deep. Recent research has shown that fertilizer P will move down to the roots when an LP method is used. That's because there's sufficient concentration of P in the band, hole, or half-circle to overcome the surrounding soil's tie-up ability enough for some downward movement to occur. LP-placed P will reach the roots, even if applied near the soil surface, as long as there's enough rainfall or watering for good plant growth and for moving the P down to the roots.

Distance and Depth Guidelines for LP Application

Here are specific guidelines for the 3 LP methods of fertilizer application: BANDING, HALF-CIRCLE, and HOLE.

NOTE: Liquid "starter" fertilizer solutions that are applied around a transplant when it's set in the ground are also a type of LP method and are covered in Chapter 10 under vegetables.

BANDING

Banding refers to placing the fertilizer in a continuous narrow strip running parallel to the crop row and fairly close to it. Of the 3 LP methods, banding is the best suited for closely-sown row crops like spinach, lettuce, turnips, and drill-planted (one seed per hole) maize. It can also be used on crops with wider in-row spacings, but the half-circle and hole methods may be more convenient. Studies have shown that only one band along the row is needed rather than 2 (one on each side).

NOTE: The banding guidelines below apply to at-planting applications of N, P, and K. Sidedressing N on growing crops will be covered farther on.

Distance from the Seed Row for Band Applications

• When banding fertilizer at planting time, the band should be placed about 5-7.5 cm (3-4 fingers-width) out from the seed furrow. Closer placement may cause burning. More distant placement may prevent the roots from reaching the fertilizer early enough.

• Don't place a fertilizer containing N or K directly under the seeds, even if separated by a few centimeters of soil. Salts from the N and K compounds will move upwards as the soil dries out between waterings or rains and will injure the seeds or young roots.

• With maize, which has fairly good resistance to fertilizer burn, it's possible to place the fertilizer and the seed in the same furrow under certain conditions (see Chapter 10 under maize). With other crops, it's possible to make a single furrow that's wide enough to accomodate a separate fertilize band and seed row.

How Deep to Make the Band

It can be anywhere from on the surface to 10 cm deep, depending on several factors:

• Where there's enough rainfall or overhead irrigation for good growth, there will be enough water to move the banded N, P, and K down to the roots, even if the band is is placed at or near the surface. This is true even for P; although immobile when applied broadcast, LP-applied P is mobile if there's enough water for downward movement.

• If on a slope, the band should be a few centimeters deep to prevent fertilizer loss from water runoff.

• To avoid N loss as ammonia gas, don't leave fertilizers containing the ammonium form of N on the soil surface if the soil pH is above 7.0. Urea (45-0-0) releases ammonia at any soil pH.

• Where rainfall is unreliable and there's no irrigation, try to make the band as deep as 7.5-10 cm (about 4 fingers-width to a palm's width) deep, which will place the fertilizer where soil moisture and root growth are more plentiful.

How to Make a Fertilizer Band

Here are 4 methods:

• By hand: This works well on small vegetable plots if soil is soft. Use your fingertips.

• By hoe: Use the hoe blade on edge to make a "V"-shaped furrow.

• An animal-drawn wooden plow or cultivator tine can be used to make a furrow.

• Fertilizer band applicators: Hand-pushed, animal-drawn and tractor-drawn models are available. Some animal-drawn planters and most tractor-drawn planters have accessory band applicators that can be purchased as an option.

Surface Banding: A New Technique

Farmers in the U.S. have recently been trying a new method called surface banding with some success. It's based on the fact that P will move downward to reach the roots when an LP method is used (given that there's enough moisture to move the P downward). As explained below, surface banding is mainly suited to field crops (maize, sorghum, beans, etc.) and can save considerable labor compared with normal banding. Here are the main features of surface banding:

• An NP, NPK, or P fertilizer (depending on soil needs) is applied in bands 50-75 cm apart before or after tillage. The bands run the same way as the future rows will. There's no need to purposely align the plant rows near the bands, because most field crops have extensive root systems. It's also possible to make the bands soon after plant emergence, instead.

• Even if the bands are applied before plowing or hoeing, the fertilizer still ends up being mixed with only 10-15% as much soil as would occur with broadcasting. Therefore, surface banding results in much less P tie-up; however, it's less effective in this respect than normal banding if the surface band is spread out by tillage.

There are several situations where surface banding may not be advisable:

• Sloping land, raised beds, or ridges: Surface-applied fertilizer may be lost by runoff from rainfall or overhead watering unless the bands are worked into the soil.

• Furrow irrigation: Surface banding requires overhead moisture (rainfall, sprinklers, or hand-watering) to move the P downward. Furrow irrigation won't allow surface-applied fertilizer to move downward.

HALF-CIRCLE METHOD

This consists of applying the fertilizer in a semi-circle around the plant, seed, or seed group. It's the best of the 3 LP methods for transplants like tomatoes, pepper, eggplant, and cabbage because of their wider in-row spacings. It also works well with "hill"-planted (cluster-planted) maize and other field crops where spacing between plant groups is wide. A half circle is as effective as a full circle.

Distance from the seeds or plants: For seeds, young seedlings, and newly-set transplants, place the half circle about 10 cm (a palm's width) out.

Depth: Follow the same guidelines as given for banding.

HOLE METHOD

This method consists of placing the fertilizer in a hole near the seed, plant, or plant group. It's likely to be the least effective of the 3 LP methods, because it confines the fertilizer to a very small area, making it available to fewer roots. However, it's still much better than using no fertilizer at all on a poor soil. It's best suited to "hill" planted field crops on large areas, especially where farmers use minimal land preparation and plant with planting sticks. (These can also be used to make the fertilizer hole.). However, where there's enough moisture to move fertilizer downward, it would probably be more effective and quicker to use surface bands or surface half circles if slope isn't a problem.

Distance of the hole from the seeds: About 7.5-10 cm (4 fingers-width to a palm's width).

Depth: Where rainfall is unreliable, the hole should ideally be made 10-15 cm deep, but this may not always be practical.

SOME SPECIAL ADVICE FOR FURROW-IRRIGATED SOILS

When using LP methods on furrow-irrigated soils, make sure that the farmer places the fertilizer below the level that the irrigation water will reach in the furrow (see Figure 911. This places the fertilizer below the "high water mark" and enables mobile nutrients like nitrate N and sulfate to move sideways and downward towards the roots. If placed above the high water line, upward capillary water movement will carry these mobile nutrients to the soil surface where they can't be used. (Upward capillary water movement is the same process that enables kerosene to "climb" up the wick in a lamp.)


FIGURE 9-1: Fertilizer application on furrow-irrigated soils. Fertilizer in row A was placed above the high-water mark and will be carried upward away from the roots. Fertilizer in row B has been correctly applied below the high-water mark and will move downward to the roots.

SIDEDRESSING NITROGEN

The reasons for sidedressing N and the number of sidedressings needed were covered a few pages back under N application principles.

Guidelines for Placement of N Sidedressings

There are several ways to sidedress N:

• For close-sown crops, like lettuce and Chinese cabbage, the N fertilizer can be applied in a continuous band parallel to the row and 10-20 cm out from it.

• For vegetables with wider in-row spacings, like tomatoes, eggplant, and cabbage, the halfcircle method works well. Place the half circle about 16-20 cm out from the stem. Closer placement may cause injury. Banding can be used instead if more convenient.

• For maize, sorghum, and millet, N can be sidedressed in a band running right down between each row, even if the rows are a meter apart. That's because these cereals have a very extensive root system. By the time these crops are knee high, the roots from adjacent rows have already crossed each other in the row middles.

Depth to sidedress N: If rain or watering will be adequate to move the N downward, the fertilizer only has to be placed deep enough to prevent it being carried away by water runoff or from losing N as ammonia gas. A depth of 2 cm is fine. Much deeper placement may prune roots if the crop is well along.

Combining sidedressing with a weeding: This can be time and labor saving, since the weeding will cover up the N fertilizer the same time.

MIXING FERTILIZERS WITH WATER

There are 3 ways of applying fertilizers by mixing them with water:

• Making up a starter solution by dissolving an NP or NPK fertilizer with water. (See the section on vegetables in Chapter 10.)

• Mixing an N fertilizer like urea or ammonium nitrate with water and watering it over plants such as those in a nursery seedbed. (See the section on vegetables in Chapter 10.)

• Soluble forms of NPK fertilizer can be applied through drip-irrigation systems. Research has shown that P applied in this way will move downward to the roots. This is because drip irrigation is essentially an "LP" method of applying water and fertilizer. A typical drip system will concentrate water and fertilizer on 20% or less of the soil surface.

• Where sprinkler irrigation is used, soluble N fertilizers like those above can be injected into the pipeline This can be wasteful where water application is uneven, however. (To avoid the possibility of fertilizer burn, be sure to irrigate with plain water for a few minutes afterwards.)

• Soluble N sources can also be dissolved in furrow-irrigation water, but this is too wasteful a method.

FOLIAR FERTILIZER APPLICATIONS Micronutrient Applications

Foliar applications are best suited for applying micronutrients. Since very small levels are needed to treat a deficiency, they can be easily applied in one or two applications without causing "burning". This method is especially well suited to i _ and manganese, since it bypasses soil tie-up of these nutrients.

NPK Foliar Applications

Soluble powders or liquids containing NPK nay be sold in your area for mixing with water and spraying on crops. Some soluble granular fertilizers like urea can be used for this purpose too. Although sellers of foliar NPK fertilizers often claim very profitable yield increases, here are some facts to consider:

• Numerous trials have shown that NPK foliar applications usually will "green up" the leaves; however, significant yield increases usually don't occur, as long as enough NPK is being applied to the soil.

• On the other hand, a 1976 trial by CIAT in Colombia obtained a 225 kg/ha yield increase on field beans by spraying them 3 times with a 2.5% solution (by weight) of 11-48-0 (monoammonium phosphate), even though 150 kg/ha of P2O5 had been added to the soil. The spray contributed only 10 kg/ha of P2O5. However, this soil had a very high P tie-up capacity.

• The soluble powder and liquid foliar fertilizers are much more expensive per unit of nutrient, compared to standard fertilizers.

• Numerous applications may be needed to supply a meaningful amount of NPK through the leaves without burning them.

• Some NPK foliar fertilizers have micronutrients too, but the amounts are usually too small to prevent or cure a deficiency.

• Although foliar applications take effect quickly (1-3 days), they have much less residual value than soil applications.

DETERMINING FERTILIZER RATES

HOW RATES ARE GIVEN

There are 2 basic ways of stating a fertilizer dosage. You'll probably run into both of these:

1. Kg of actual fertilizer needed per hectare

Example: Apply 300 kg/ha of 10-20-10 to maize at planting, followed by 100 kg/ha of urea (45-0-0) at knee-high stage.

This type of dosage is very straightforward, since it tells you the kind and amount of actual fertilizer needed. However, you'll still need to calculate how much fertilizer to buy for the field's size and how much to apply per plant or per meter of row length; we'll cover this under fertilizer math farther along.

2. Kg of N, P2O5, and K2O needed per hectare

Example: Soil test results recommend the following fertilizer dosages for a tomato field:

 

Kg/hectare

 

N

P2O5

K2O

At transplanting

40

80

40

Additional N to apply over 3 sidedressings.

75

   

This way of stating fertilizer dosages is more complicated since it's up to you and the farmer to determine the amount of fertilizer needed per hectare to satisfy the recommendation. (We'll cover this under fertilizer math) This method is often preferred over #1 above, because the types of fertilizers available may vary a lot from one area to another.

What is the most profitable type and amount of fertilizer for limited-resource farmers?

You may have seen boxes of fertilizer labelled "Tomato Fertilizer" or "Rose Fertilizer" in garden shops; the label may even give dosage rates. Unfortunately, it's not that simple. There's no one type of fertilizer or fertilizer rate that's best for one crop. These depend on several factors:

• Nutrient status of the soil which is best determined by a soil test. (See Chapter 7.)

• Type of crop (legume vs. non-legume, etc.).

• Feasible yield goal as determined by:

•• Limiting soil, weather, moisture, and pest factors

•• Farmer management level

•• Capital available for needed inputs

• Expected cost/return based on costs, likely yield, and estimated price. The latter 2 are especially difficult to project for vegetable crops.

What to Do Where Reliable Recommendations Aren't Available

As you saw in Chapter 7 on evaluating soil fertility, it's not always possible to obtain reliable soil test results or recommendations that are geared to the special circumstances of limited-resource farmers. Nonetheless, you can still develop fairly appropriate recommendations by using this manual and doing some local investigation. Here's how:

• Start by checking at the local extension office to see if reliable results are available for soil tests or fertilizer trials conducted on the same type of soil on nearby farms.

• Check to make sure that the Ministry of Agriculture hasn't already developed appropriate fertilizer recommendations for the soil involved, based on soil tests or fertilizer trials.

If such information isn't available, you'll have to start from scratch, beginning with this very useful guideline:

LIMITED-RESOURCE FARMERS SHOULD USUALLY AIM FOR MAXIMUM RETURN PER DOLLAR SPENT. THIS MEANS USING LOW TO MODERATE FERTILIZER RATES BECAUSE OF THE LAW OF DIMINISHING RETURNS.

Fertilizer Response and the Law of Diminishing Returns

Figure 9-2 and Table 9-3 show that the yield response to fertilizer follows the Law of Diminishing returns which has especially important consequences for limited-resource farmers.


FIGURE 9-2: Graph illustrating the Law of Diminishing Returns and its effect on response to fertilizer.


TABLE 9-3 AN EXAMPLE OF THE LAW OF DIMINISHING RETURNS

• As a farmer's capital situation improves, she can afford to become less efficient in terms of maximum return per dollar and begin to aim more toward maximum total profit by applying more fertilizer per hectare (as long as investment in other appropriate practices isn't sacrificed). This is similar to a large supermarket that makes less return per dollar (due to discount pricing) but makes more total profit than a small grocery because of much higher volume.

• By using low to moderate rates of fertilizer, a limited-resource farmer will be able to fertilize more land and, hopefully, have capital left over to invest in complementary improved practices.

To help clarify things, suppose that Table 9-3 applies to a limited-resource farm family with 2 hectares of maize. Let's say they can only afford to buy 80 kg of N and still have enough capital left to invest in other complementary practices. If they applied all 80 kg to one hectare, they would harvest a total of 1672 kg of maize off the 2 hectares (1372 + 305). If they applied 40 kg of N to each hectare, they would harvest 2744 kg of maize, or 1072 kg more than in the first case.

Substitution of fertilizer for land: It can be argued from the above example that it takes more labor to fertilize 2 hectares instead of one. However, the other side is that fertilizer use can reduce the amount of land (and, therefore, labor) needed to produce a given amount of crop, thus cutting costs and allowing for more diversity of production.

SOME GUIDELINES FOR LOW, MEDIUM, AND HIGH NPK RATES

Table 9-4 gives some "ballpark" figures for low, medium, and high NPK rates, based on the realities of limited-resource farming. Even the "high" rates in the table would be considered on the low side by many farmers in North America and Europe who have access to adequate credit. For example, it's not uncommon for U.S. Corn Belt farmers to apply 200 kg of actual N per hectare on maize. Such rates may produce maximum profit per hectare but at the price of a lower return on capital, a less efficient yield response, and possible pollution of ground water and lakes by excess nitrate.

TABLE 9-4 GUIDELINES FOR LOW, MEDIUM. AND HIOH NPK RATES GEARED TO LIMITED-RESOURGE FARMING*

NOTE: DO NOT USE THIS TABLE WITHOUT CONSULTING THE QUALIFICATIONS LISTED BELOW IT.

 

LOW (kg/hectare)

MEDIUM (kg/hectare)

HIGH** (kg/hectare)

N

35-55

60-90

100+

P2O5

25-35

40-60

70+

K2O

30-40

50-70

80+

* Refers to total NPK for one crop; don't include a nursery seedbed application or the use of a starter fertilizer solution in these totals.

** "HIGH" doesn't necessarily imply "too high".

Qualifications to Table 9-4

1. The P rates in the table are based on localized placement, not on broadcasting. About 310 times more P is needed if broadcast.

2. You must consider the soil's likely fertility status. A soil high in available K, would need little or no fertilizer K. Most soils that have been under cropping for a few years are low in N. Most soils are low to medium in P.

3. You must consider the type of crop:

• The N rates in Table 9-4 are geared to high users like maize, sorghum, rice, leafy vegetables, tomatoes, and improved potato varieties. Most root crops have moderate N needs.

• Legumes vs. non-legumes: Peanuts, cowpeas, soybeans, mungbeans, pigeonpeas, chickpeas, and winged beans are very efficient N fixers and seldom require N as long as the proper strain of rhizobia bacteria is present. Beans (Phaseolus vulgaris) and garden peas (Pisum sativum) are only about half as efficient and can use up to 50-60 kg/ha of N.

• Bananas and starchy root crops like taro, cassava, and potatoes have the highest K needs. Cereals often respond less than legumes to applied K, because they are more efficient K extractors.

• Before using Table 9-4, see if the crop is listed in Chapter 10 where more specific fertilizer guidelines are given.

4. You must also consider limiting factors that may affect the response to fertilizer such as: moisture, pests, diseases, soil problems, weather, farmer management level, etc. These are covered in detail in a section on integrated crop production management farther along in this chapter.

GUIDELINES FOR MAGNESIUM AND MlCRONUTRIENT RATES

The following rates are generalized dosages for curing deficiencies when no locationspecific recommendations exist. In addition, you can look up the particular crop in Chapter 10 to see if more specific recommendations are given. Crops and even cultivars (varieties) vary in their micronutrient needs.

NOTE: A wetting agent (spreader) should be used when making foliar applications to assure uniform leaf coverage; if a commercial one isn't available, you can use a mild liquid dishwashing detergent at 1-3 cc/liter.

MAGNESIUM: 3035 kg/ha actual magnesium which equals 150175 kg/ha (1518 g/sq. meter) of epsom salts (magnesium sulfate) which contain about 20% pure Mg. For foliar applications, apply 1228 grams per liter of water.

IRON: For soil applications, chelated iron (912% iron) should be used at 2040 kg/ha to avoid soil tieup. Ferrous sulfate (20% Fe) is very effective for foliar application as a 12% spray (1020 grams ferrous sulfate per liter of water).

MANGANESE: Manganese sulfate can be banded at 510 kg/ha which helps protect it from tieup (it can be mixed with the NPK fertilizer). Foliar applications of manganese sulfate can be very effective, using a 12% spray (1020 grams manganese sulfate per liter).

COPPER: Copper sulfate pentahydrate (25% Cu) can be broadcast at 2540 kg/ha (2.54 g/sq. meter) on mineral soils and at 100300 kg/ha (1030 g/sq. meter) on peat soils. Foliar applications can be made using copper sulfate pentahydrate at 36 grams/liter.

ZINC: 1040 kg/ha (14 g/sq. meter) of zinc sulfate; band at lower rates, broadcast at higher rates. Foliar applications are very effective using a 12% solution of zinc sulfate (1020 g/liter).

BORON: Borax (11% B) can be broadcast at 1025 kg/ha (1.02.5 g/sq. meter) for legumes and certain root crops like sweetpotatoes; for other crops, try 510 kg/ha of borax (0.51.0 g/sq. meter). Use the lower rates on sandy soils. Boron can easily injure plants or seeds if applied at too high a rate or concentrated too close to the row.

MOLYBDENUM: Mo deficiency is most common on overly acid soils because of tieup; liming will often cure a deficiency. Sodium molybdate (40% Mo) can be broadcast at 5001000 grams/hectare. Treating the seed with sodium or ammonium molybdate is the most common way of treating deficiencies (see the section on soybeans in Chapter 10). Excess Mo applied to forage crops can be toxic to livestock.

IMPORTANCE OF ACHIEVING THE RIGHT NUTRIENT BALANCE

If two or more nutrients are deficient simultaneously (very likely), adding only one may give very disappointing results. For example, look at the results of the fertilizer trial below conducted on a soil where both N and P were low:

Treatment

Maize Yield Per Hectare

Yield Increase

None

240 kg

--

N only

720 kg

480 kg

P only

1120 kg

880 kg

N + P

3250 kg

3110 kg

In other cases, an excess of one nutrient relative to another can cause imbalances:

• A high ratio of potassium or ammonium N to magnesium can cause a deficiency of magnesium in susceptible crops such as tobacco and pasture grasses.

• A high ratio of potassium to calcium may provoke a calcium deficiency in peanuts.

Large applications of phosphorus can cause deficiencies of iron or zinc, especially when an LP application method is used. (On the other hand, P fertilizer improves the availability of manganese; this can be important for crops such as oats, soybeans, beans, and peanuts which are especially susceptible to manganese deficiencies.)

A high ratio of calcium to magnesium can cause a magnesium deficiency. This is common where acid soils are limed with materials that contain calcium only instead of with dolomitic limestone.

Overliming a soil can cause micronutrient deficiencies (except for molybdenum>.

Excess soluble copper and manganese can cause iron deficiecies and vice-versa.

Troubleshooting faulty fertilizer practices

You can see now that chemical fertilizers require much more skill to use properly than organic fertilizers in terms of selection, rates, dosage calculations, and application. Now that we've covered most of this, it's easy to understand why the misapplication of chemical fertilizer is a very common problem, whether in the Third World or elsewhere.

Hopefully, this chapter has given you a solid grounding for using chemical fertilizers appropriately. To help tie together all the principles and practices covered, let's practice troubleshooting some common faulty fertilizer recommendations.

What to Look For

When evaluating a fertilizer recommendation, check the following:

• Type of fertilizer

• Amount of fertilizer

• Application method: BC vs. LP, depth, distance

• Timing of applications

• Proportion of total N applied at planting/transplanting

RECOMMENDATION 1: 250 kg/ha of 14-14-14 broadcast and left on the soil surface a day before planting Chinese cabbage, followed by 150 kg/ha of ammonium nitrate (33-0-0) a month later in a band 20 cm out from the row and 2 cm deep.

WHAT'S WRONG?: This recommendation applier a total of 85 kg N, 35 kg P2O5, and 35 kg K2O per hectare. The N rate falls in the medium category, and the P and K rates in the low category, which is OK as long as the soil doesn't have a severe P or K deficiency. However, broadcasting such a low rate of P is a serious mistake and will result in most of it being tied up and unavailable to the crop. It should be banded. Remember that the fertilizer rate table (Table 9-4) is based on localized placement of P; from 3-10 tines or more may be needed if broadcasting is used. It's also a big mistake to leave the 14-14-14 on the soil surface; broadcast P is immobile and won't reach the roots unless thoroughly worked into the top 15-20 cm with a hoe or plow. The proportion of total N (40%) applied at planting is OK. The N sidedressing is applied correctly and at the right time.

RECOMMENDATION 2: 125 kg/ha of urea (45-0-0) applied when grain sorghum is planted, followed by a sidedressing of 200 kg/ha of 16-20-0 at knee high stage.

WHAT'S WRONG?: It's backwards! NP or NPK fertilizer should always be applied at planting (or transplanting), never as a sidedressing. P needs early application, because young plants need a high concentration in their tissues for good early growth and root development. Besides, applying the urea first puts on far more N (63%) than the 1/3-1/2 that should be applied at planting. What about the NPK dosage? It works out to about 88 kg N, 40 kg P2O5 , and 40 kg K2O per hectare which are all in the acceptable range of the rate table a few pages back. However, some extra K might be needed if the soil has a low level.

RECOMMENDATION 3: 300 kg/ha of 12-24-12 when tomatoes are transplanted, applied in a half circle 30 cm out from the stem and 5 cm deep, followed by a sidedressing of 150 kg/ha of ammonium sulfate (21-0-0) every 4 weeks until harvest is over. The sidedressing is applied in a half circle 10 cm out from the stem and 2 cm deep.

WHAT'S WRONG?: To start with, the half-circle for the initial NPK application is placed too far away (30 cm) from the plants. A palm's-width (10 cm) will allow earlier utilization of the fertilizer. The 5 cm depth of the half-circle is correct, as long as there will be sufficient moisture to move the P downward toward the roots.

Secondly, the N sidedressing is placed too close (10 cm) to the stems and may injure the plants; it should be placed about 20-25 cm from the stems.

The N, P, and K dosages seem reasonable, although more K might be needed on a low-K soil. The monthly sidedressings are appropriate, including the rate of 30 kg/ha of actual N per application. In the case of well managed, vine-type (indeterminate) tomatoes, the production period could continue for up to 6 months or more, requiring 6 or more such sidedressings totalling 180 kg/ha or more of actual N. This night seem excessive, but not when you consider the unusually long production period and the potential yield.

RECOMMENDATION 4: 300 kg/ha of 16-20-0 applied when peanuts are planted, followed by a sidedressing of 100 kg/ha of urea (45-0-0) 30 days later.

WHAT'S WRONG?: Peanuts are a very efficient N-fixing legume which can normally satisfy their entire N needs if the proper strain of rhizobia bacteria is present. Even if the soil lacked the right type of rhizobia, it would be much more economical to innoculate the seed with the bacteria than to buy N fertilizer (see the section on peanuts in Chapter 10). Also, K may be needed

RECOMMENDATION 5: Broadcasting 400 kg/ha of 10-10-20 over a nursery seedbed for raising cabbage transplants and working it into the top 5 cm of soil with a rake. (Assume that manure or compost aren't available).

WHAT'S WRONG?: First, a rake will not move broadcast P deep enough into the soil for good availability to the roots. A good hoeing is needed to work the fertilizer into the top 10 cm of soil.

Second, broadcasting is the only feasible method for applying NPK fertilizer to a nursery seedbed (especially if the seed was broadcast), but the rate of P is far too low this. It's not a simple matter of increasing the amount of 10-10-20 either, because you'll end up applying way too much and especially K in order to put on enough P. The real problem is the fertilizer's ratio (1:1:2). In order to apply the high amount of P needed (remember, up to 10 times more is required when the BC method is used) without overapplying N and K, you want a fertilizer with a ratio of around 1:3:1 or 1:4:1. If this isn't available, you can make your own by mixing the the 10-10-20 with single superphosphate (0-20-0) or triple superphosphate (048-0). This is covered in the fertilizer math section at the end of this chapter.

RECOMMENDATION 6: 100 kg/ha of 16-48-0 applied when maize is planted, followed by a sidedressing of 400 kg/ha of ammonium sulfate (20-0-0) when the tassels and silks emerge.

WHAT'S WRONG?: First, a total of 96 kg/ha of N is being applied in the 2 applications (16 + 80) which is acceptable, as long as moisture is adequate and the crop is well managed. But, remember that 1/3-1/2 of the total N should be applied at planting. In this case, only about 16% of the N was put on at planting, which is too little. As a general rule, at least 30 kg/ha of actual N should be applied at planting, mainly to help avoid temporary tie-up of soil N by the bacteria that are decomposing crop residues. (See Chapter 6 under N.) The problem with 16-48-0 is that its 1:3:0 ratio results in too little N being applied, given the rate needed to supply the 48 kg/ha of P2O5 (a satisfactory amount). One solution would be to add some 20-0-0 to the 16-48-0 to supply the extra N needed at planting.

Second, the N sidedressing is applied too late. In warm climates, maize starts tasseling and silking about 50-70 days after planting. Applying the N this late will give much less of a yield boost, because the plants' need for N begins best time for sidedressing. This is usually when maize has reached the knee-high stage.

Third, the P rate is acceptable, but K may be needed.

RECOMMENDATION 7: Making up a starter fertilizer solution by dissolving 4 cc of urea (45-0-0) in a liter of water and pouring 250 cc around the base of each newly set tomato transplant. No other fertilizer, chemical or organic is applied for the rest of growth.

WHAT'S WRONG?: Urea will have little benefit as a starter solution. What's needed is an NP or NPK fertilizer with a good ratio of P which will help stimulate new root growth. A straight P fertilizer could be used, but N helps promote the uptake of P by the roots. Some examples of fertilizers suitable for making up starter solutions are 12-24-12, 10-3010, 18-46-0, and 16-20-0.

In addition, the starter solution isn't meant to replace the normal NP or NPK fertilizer application nor the sidedressing either. It provides only enough nutrients for the first week or so of growth. (For more information on starter solutions, see the section on vegetables in Chapter 10.)

RECOMMENDATION 8: Applying 10-20-10 to potatoes at planting time by banding it directly under the seed pieces, separated by 3-4 cm of soil. Then applying a sidedressing of urea one month after emergence by sprinkling it on the soil surface along the row.

WHAT'S WRONG?: First, there's danger of burning the seed pieces due to upward movement of fertilizer salts as the soil dries out between rains or waterings. The band should either be applied about 3 fingers-width (7.5 cm) off to the aide or separated vertically from the seed pieces by at least 12 cm of soil.

Second, urea releases free ammonia gas which will cause significant N loss unless the urea is worked into the soil a bit. Rainfall or overhead watering could also wash the fertilizer off the ridges or "hills" on which potatoes are usually grown. Applying it right before the plants are hilled up with soil or weeded is a good way to accomplish this without extra work.

Getting the most out of fertilizer use: crop management as an integrated system

It's true that fertilizer use can be the factor producing the largest yield increase, especially on low-fertility soils. However, it's important to realize that low soil fertility is just one of many limiting factors that can affect crop yields. Many a farmer and extension worker have learned the hard way that chemical fertilizer alone may give disappointing results. As a sole input, chemical fertilizers have a further disadvantage in that they provide none of the additional benefits that organic fertilizers offer (i.e. filth improvement, etc.). In this case, it's particularly important to be sure that other limiting factors are addressed, aside from fertility alone.

The "Package" Approach to Yield Improvement

One essential part of successful farm or garden project management is selecting and implementing an integrated system of complementary practices designed to favor production and control major limiting factors. Trying to overcome several major limiting factors at once usually gives a more impressive yield increase than tackling them one at a time. A well-designed and appropriate "package" of practices will actually lower farmer risk, and the synergistic effect can be remarkable. The results of a farm trial on dryland wheat in in Mexico shown below are a good example:

Treatment

Yield Increase

Fertilizer only

5%

Irrigation only

135%

Fert. + irrigation

700%

Some Possible Objections to the "Package" Approach

In Two Ears of Corn, the widely-respected book on grassroots ag extension, author Roland Bunch points out the possible disadvantages of "package" technologies that involve more than two or three new practices. He favors purposely limiting the technology for several reasons:

• Most of the successful people-centered extension projects are ones that started slowly and on a small scale. Each new practice increases program complexity in terms of research, training (of extensionists and farmers), supervision, and availability of inputs.

• In order for innovations to achieve long-term success, they must be adopted by a significant portion of farmers (roughly 25-45%). This "critical mass" is vital to assure eventual wide-spread adoption. Simple packages create less confusion and allow more people to be reached.

• Simple packages that involve a minimum of purchased inputs are more likely to be affordable to small farmers. High-input packages are income-biased and favor the wealthier farmers.

• Packages that achieve spectacular yield and income increases may promote even more economic disparity in a community and lead to jealousy and resentment. In addition, limitedresource farmers (and most of us, for that matter) may not use such sudden income increases wisely.

It's important to note that a "package" doesn't have to include purchased inputs that involve considerable capital.

When working with limited-resource farmers or gardening projects, there are a number of low- or no-cost practices whose introduction can precede or accompany the use of chemical (or organic) fertilizer such as:

• More timely and thorough weeding . Better crop and variety selection . Better water management (See Chapter 5) . Better plant spacing and thinning . Erosion control (See Chapter 3) . Fencing . Mulching . Improved recordkeeping

SOME LIMITING FACTORS THAT CAN SERIOUSLY AFFECT FERTILIZER RESPONSE

Available Moisture

Crops can't utilize nearly as much fertilizer when moisture is limiting, although low to moderate rates will help improve water use efficiency. For example, in the semi-arid wheat regions of the U.S., fertilizer nitrogen recommendations are often geared to the amount of stored subsoil moisture and outlook for rain.

Drought-resistant crops like sorghum and millet exhibit much less response to fertilizer when grown under low rainfall conditions, compared to when moisture is sufficient for good yields. In fact, fertilizer may not show a profitable return where moisture is seriously limited.

Another moisture-related instance where fertilizer use is unlikely to be cost-effective is recessional agriculture. In this system, crops are planted in the saturated soil of riverbanks and floodplains as water levels recede after the rainy season ends. In this case, applied fertilizer will soon end up in dry soil as moisture levels drop, unless it's feasible to hand-water the crop from the river or other source.

Type of Crop

Given adequate moisture and an appropriate variety, cereal grains, most pulses, most vegetables, bananas, sugarcane, and pastures tend to show more response to fertilizer than coffee, cacao, and most tree crops. Soybeans and peanuts often respond better to residual fertility remaining from applications to previous crops.

Variety

Improved varieties, including hybrids, generally give a better response to fertilizer than traditional ones, though this isn't always the case. For example, during the first years of the Puebla Maize Project in Mexico, some of the native varieties consistently outyielded anything the plant breeders developed. In India, on the other hand, an adapted hybrid yielded 4 times as much as the traditional local variety when both were grown under the same package of practices.

Varieties of the same crop can vary greatly in important characteristics aside from fertilizer response such as draught and heat tolerance, resistance to diseases and nematodes. Each of these can also have a big bearing on yield. Be sure that the variety used is well adapted to the area's growing conditions. Be particularly careful with donated seed from U.S. seed companies that is often distributed by relief agencies.

Planting Date: This is an important consideration in areas where delayed, premature, or unseasonal planting increases the likelihood of yield-limiting stresses such as excessive heat or cold, too much or too little moisture, insects, and diseases.

Plant Density (Plant population)

Too low a plant density is a common cause of poor fertilizer response, especially with cereal grains. Where soil fertility is low, farmers tend to plant fewer plants per hectare so that each plant gets a better share of the limited amount of nutrients. Such low plant populations may not be be able to respond well to added nutrients. The reason is that the individual plant's yield-determining factors, such as number of ears per plant, ear size, or seeds per head, may reach their limit at relatively low NPK rates. If so, it may be necessary to increase plant population when fertilizer is introduced in order to obtain more grain yield. Caution is needed here, since varieties vary in their population tolerance, and overly-high plant densities may encourage lodging (tipping over) in cereal crops or use up scarce soil moisture in drought-prone areas.

Soil Limiting Factors: Poor filth, poor drainage, soil compaction, low water-holding capacity, and pH problems all have an adverse effect on fertilizer response.

Weed Control: Weeds not only compete with the crop for sunlight, water, and nutrients, but harbor insects and diseases as well.

Insects, Diseases, and Soil Nematodes: While a well-nourished plant has better resistance to most diseases and pests, they can still wipe out profits if not controlled.

Management Level: The farmer's or project's willingness and ability to implement the minimum level of needed management is a crucial factor. This includes general management skills such as planning and timeliness, as well as essential practices such as weed and insect control where needed.

Understanding fertilizer math

There's a surprising amount of math devolved in using chemical fertilizers. This section covers the following useful fertilizer math skills:

• Converting fertilizer recommendations from an N-P2O5-K2O basis to the actual kind and amount of fertilizer needed.

• Selecting the most economical fertilizer.

• Mixing fertilizers.

• Determining how much fertilizer is needed per area, per Plant, and Per length of row.

• Converting fertilizer dosages from weight to volume.

USE THE METRIC SYSTEM!: It greatly simplifies fertilizer math and most other calculations. Even if your country doesn't use metrics, it's well worth your while to use it for calculation purposes. Here's how to quickly convert some common non-metric units into metric (see Appendix A also):

lbs./acre X 1.12 = kg/ha; 1 lb. = 0.454 kg = 454 g

1 acre = about 4000 sq. meters (actually 4048 m2) 1 manzana (Latin America) = 7000 sq. meters

4" = 10 cm

8" = 20 cm

12" = 30 cm

16" = 40 cm,

18" - 45 cm

24" = 60 cm

30" = 75 cm

32" = 80 cm,

36" = 90 cm

40" = 100 cm

CONVERTING RECOMMENDATIONS FROM AN N, P2O5, K2O BASIS TO THE ACTUAL KIND AND AMOUNT OF FERTILIZER NEEDED

As explained in Chapter 9, fertilizer recommendations aren't always given in terms of actual kind and amount of fertilizer. Instead, technical brochures and soil testing labs often give recommendations in terms of the amount of N, P2O5, and K2O needed per hectare. In this case, it's up to you and the farmer to determine what kind and amount of actual fertilizer is needed per hectare to match this recommendation. Let's run through a practice problem:

PROBLEM 1: A farmer's cooperative has just received the following fertilizer recommendation for a one hectare tomato field.

 

 

kg/hectare

 

N

P2O5

K2O

At transplanting

40

80

40

1st sidedressing at 30 days

30

   

2nd sidedressing at 60 days

30

   

3rd sidedressing at 90 days

30

   

Suppose the local ag supply store has the following fertilizers available. What kind will be needed, how much of each, and what will the cost be?

Fertilizers Available

Cost per 50 kg Sack

15-15-15

$18

16-20-0

$15

20-10-5

$14

10-20-10

$16

20-0-0

$12

SOLUTION

STEP 1: Let's begin with the 40-80-40 transplanting recommendation. The first thing to do is to look at the ratio of N:P2O5:K2O and then look for a fertilizer with a similar ratio. The 40-80-40 figure has a ratio of 1:2:1. Look at the fertilizer list and you'll see that 10-20-10 is the only one with a 1:2:1 ratio, so it's the one that' 8 needed.

STEP 2: There are 2 ways to find out how much 10-20-10 is needed to supply the 40 kg N, 80 kg P2O5, and 40 kg K2O needed for the hectare:

a. You know that each 100 kg of 10-20-10 supplies 10 kg of N, 20 kg of P2O5, and 10 kg of K2O. Therefore 400 kg would supply 40-80-40.

b. The second way is to divide the percentage of N, P2O5 or K2O in the 10-20-10 into the respective amount of N, P2O5, or K2O needed. Let's do this using N:

40 kg N needed = 40 kg = 400 kg 10-20-10 needed 10% N in the fertilizer 0.10

Note that you would get the same answer using P2O5 or K2O so it's only necessary to do this division once.

STEP 3: Now what about the N sidedressings of 30 kg actual N each? In this case, choosing the right fertilizer is easy, since the 20-0-0 fertilizer is the only one containing just N. To find out how much 20-0-0 will be needed to supply the 30 kg of N needed for a sidedressing, use one of the 2 methods in STEP 2 as follows:

a. You know that each 100 kg of 20-0-0 supplies 20 kg of N. 200 kg would supply 40 kg N. It would take 150 kg of 20-0-0 to supply 30 kg (i.e. 150 X 20% = 30)

b. Divide 20% into 30 kg which gives you 150 kg.

Therefore, 3 sidedressing of 150 kg 20-0-0 each will be needed for a total of 450 kg.

STEP 4: You've determined that 400 kg of 10-20-10 and 450 kg of 20-0-0 are needed for the hectare, so you can now calculate the cost:

400 kg 10-20-10 at $16/100 kg =

$64 450 kg

20-0-0 at $12/100 kg =

$54

$118 TOTAL

You won't always be able to fit a recommendation exactly, because the right type of fertilizer may not be available locally. At any rate, you don't have to be exact, since soil tests and recommendations aren't 100% accurate anyway. But, do try to get within 10-25% of the amounts recommended. There's nothing wrong with having to apply more P than needed in order to supply enough K or vice-versa; P won't leach, and K is fairly immobile. However, avoid putting too much N on at planting or leaching losses may be high.

Let's look at a situation where the fertilizers don't exactly fit the recommendation (Problem 2):

PROBLEM 2: Soil test results recommend that Fatou fertilize her maize field as follows:

 

kg per hectare

 

N

P2O5

K2O

At planting

30

50

40

At knee high stage

50

   

Given the following fertilizers, how much and what kind will be needed per hectare?

Fertilizers Available

12-24-12

13-13-20

14-14-14

18-46-0

33-0-0

SOLUTION

STEP 1: Let's begin with the planting recommendation of 30 kg N, 50 kg P2O5, and 40 kg K2O. That's a 3:5:4 ratio (or 1:1.7:1.3). None of the available fertilizers has this ratio, but 12-24-12 is the closest with a 1:2:1: ratio.

STEP 2: Let's figure out how much 12-24-12 is needed to supply the 30 kg of initial N:

30 kg N needed / 12% N in fertilizer = 250 kg 12-24-12

250 kg of 12-24-12 per hectare would supply 30 kg N, 60 kg P2O5 and 30 kg K2O. This falls short on K2O. by about 25% but runs over on P2O5 about 20% This is still satisfactory. Now what would happen if we tried to supply the exact amount of P2O5 (50 kg) using 12-24-12?:

50 kg P2O5 needed / 24% P2O5 in fertilizer = 208 kg 12-24-12

208 kg/hectare of 12-24-12 supplies 25-50-25 which is about 20% less N and 40% less K2O. than called for.

The 3rd option is to see how much 12-24-12 it would take to supply the exact amount of K2O (40 kg) called for:

40 kg K2O needed / 12% K2O in fertilizer = 333 kg 12-24-12

333 kg of 12-24-12/hectare supplies 40-80-40 which is about 30% more N and 60% more P2O5 than called for at planting. You could adjust for the extra N by applying less in the sidedressing, but there's no way to compensate for the 30 kg extra P2O5 applied. True, some of this excess will be available to future crops, but at the expense of having to buy about 33% more 12-24-12 compared to the 250 kg rate.

Thus, of the 3 options, the first one of applying 250 kg of 12-24-12 is best.

STEP 3: Now for the N sidedressing. The 33-0-0 fertilizer (ammonium nitrate) is the only straight N source, so it's the one to use. Calculate the amount needed to supply the 50 kg of N as follows:

50 kg N needed / 33% N in fertilizer = 150 kg 33-0-0 needed

SELECTING THE MOST ECONOMICAL FERTILIZER

You can't compare a 14-14-14 and 10-30-10 fertilizer on the basis of cost per unit of nutrient for 2 reasons:

• A 1:1:1 ratio fertilizer may be better suited than a 1:3:1 ratio or vice-versa, depending on the soil and the crop.

• N, P2O5 and K2O. don't necessarily cost the same per kg of nutrient.

However, you can compare straight fertilizers having just one of the "Big 3", such as urea (45-0-0) vs. ammonium sulfate (20-0-0), or single superphosphate (0-20-0) vs. triple superphosphate (0-48-0). You can also compare NP or NPK fertilizers having the same ratio, such as 10-20-10 and 12-24-12.

When comparing several sources of the same nutrient as to cost, what counts is the cost per kg of nutrient, not the cost per sack. Let's run through a practice problem:

PROBLEM 3: Which of the 3 N fertilizers below is the most economical source of N, other considerations aside?

Fertilizer

% N

Cost per 50 kg sack

Urea

45%

$18.00

Ammonium nitrate

33%

$15.84

Ammonium sulfate

21%

$11.76

SOLUTION: Although ammonium sulfate has the lowest cost per sack, it's not necessarily the cheapest. The real test is the cost per kg of N. Here's how to calculate it:

UREA: A 50 kg sack contains 22.5 kg of N (50 kg x 45%)

$18.00 / 22.5 kg N = $0.80 per kg of N

AMMONIUM NITRATE: A 50 kg sack contains 16.5 kg of N.

$15.84 / 16.5 kg N = $0.96/kg of N

AMMONIUM SULFATE: A 50 kg sack contains 10.5 kg of N.

$11.76 / 10.5 kg N = $1.12/kg of N

This makes urea the cheapest source of N. Usually, the fertilizer with the highest content of the nutrient will be the most economical due to lower shipping costs per unit of actual nutrient. However, this isn't always the case.

Other factors may be important aside from the cost per kg of nutrient. Although it's the most economical (in this case), urea is a very highly concentrated source of N; farmers unfamiliar with urea may over-apply it and waste money or injure their crops. As for ammonium sulfate, it's often the most costly per kg of N, yet it might be the best choice for a sulfur-deficient soil, unless another sulfur-bearing fertilizer were used at planting time. On the other hand, ammonium sulfate is considerably more acid forming in its longterm effect on soil pH than either urea or ammonium nitrate (see Table 9-1). Ammonium nitrate is a quicker-acting N source than ammonium sulfate or urea, because half of its N is already in the mobile, nitrate form. It might be the best choice where a crop is showing N deficiency symptoms (see Appendix E) or where sidedressing has been delayed.

MIXING DIFFERENT FERTILIZERS

There are cases where it's necessary to mix 2 or 3 different fertilizers together in order to obtain the nutrient ratio needed to suit a recommendation. For example:

PROBLEM 4: Suppose that the extension office recommends the following fertilizer rates for cabbage at planting time:

kg per hectare

N

P2O5

K2O

40

80

40

The local ag supply store has the following fertilizers on hand:

15-15-15

16-20-0

0-45-0 (triple superphosphate)

Is it possible to meet the 40-80-40 recommendation by mixing 2 or more of these together? If so, what proportions are needed, and what is the resulting fertilizer formula?

SOLUTION: The 40-80-40 recommendation has a 1:2:1 ratio. The 15-15-15 provides NPK in a 1:1:1 ratio. What's needed is to increase the amount of P by adding some 0-45-0 fertilizer. The easiest way to calculate the amounts needed is to set up a worksheet as follows:

   

N

P2O5

K2O.

100 kg 15-15-15

=

15 kg

15 kg

15 kg

X kg 0-45-0

=

0 kg

15 kg

0 kg

100 + X kg

=

15 kg

30 kg

15 kg

This worksheet helps visualize the problem. It shows that in order to end up with a 1:2:] N:P2O5:K20 ratio, we need to combine 100 kg of 15-15-15 with enough 0-45-0 to add 15 extra kg of P2O5 To figure out how much 0-45-0 is needed, divide 15 by 45%:

15 kg P2O5 needed / 45% P2O5 = 33 kg 0-45-0

Now let's fill in the worksheet:

   

N

P2O5

K2O

100 kg 15-15-15

=

15 kg

15 kg

15 kg

33 kg 0-45-0

=

0 kg

15 kg

0 kg

133 kg

=

15 kg

30 kg

15 kg

This shows that mixing 100 kg of 15-15-16 with 33 kg of 0-45-0 will produce 133 kg. of a fertilizer with a 1:2:1 ratio.

Determining the true formula of the mix: At first glance, it would seem that the formula of the mixture is now 15-30-15, but it isn't! What you've made is 133 kg of fertilizer containing 15 kg N, 30 kg P2O5, and 15 kg K2O. But, fertilizer formulas are based on nutrient content in percent (i.e. kg of N, P2O5, and K2O per 100 kg of fertilizer). Here's how to derive the true formula:

True formula = 15-30-15 / 1.33 = 11.25-22.5-11.25

CAUTION!: Not all Fertilizers can be Mixed

• Lime in any form should not be mixed with ammonium N fertilizers or urea. It will cause loss of N as ammonia gas.

• Lime should also not be mixed with any chemical fertilizer containing P, because it may convert some of the P into an insoluble, unavailable form.

DETERMINING HOW MUCH FERTILIZER IS NEEDED PER AREA, PER PLANT, OR PER LENGTH OF ROW

Fertilizer recommendations are usually given on a per hectare (or per acre) basis. However, such figures are of little use unless you know how to determine the following:

• How much actual fertilizer is needed, given the size of the particular field?

• If the fertilizer will be applied using an LP (localized placement) method, how much fertilizer is needed per plant if the hole or half-circle method is used, or how much per length of row if it's banded? (These 2 application methods were covered earlier in this chapter.)


TABLE 9-5 FERTILIZER MIXING GUIDE

BLANK BOXES = Fertilizers which can be mixed.

BOXES WITH AN "X" = Fertilizers which can be mixed only shortly before use.

BOXES WITH A "O" = Fertilizers which cannot be mixed for chemical reasons.

EXAMPLES: Ammonium sulfate should not be mixed with lime.

Urea can be mixed with single or triple superphosphate shortly before use.

CALCULATING THE AMOUNT OF FERTILIZER NEEDED PER AREA

For Large Fields: Measure the field's dimensions and calculate the area. If its shape is not rectangular, you may have to divide it up into triangles and rectangles and determine the area of each. (The area of a triangle equals 1/2 the base X the height.)

PROBLEM 5: Suppose soil tests recommended applying 250 kg/ha of 16-20-0 to grain sorghum at planting time. How much is needed for a field measuring 40 x 80 meters?

SOLUTION: One hectare = 10000 sq. meters

The field's size = 3200 sq. meters (40 x 80)

3200 sq. meters X 250 kg/ha / 10000 sq, meters = 80 kg of 16-20-0 needed

For Small Plots: The metric system has some very handy shortcuts. A very useful one is:

100 KG/HA = 10 GRAMS PER SQ. METER

In other words, to convert from kg/ha to g/sq. meter, just drop a zero and change kg to grams!! Here's why it works:

100 kg/ha = 100,000 grams/hectare

100,000 grams / 10,000 sq. meters = 10 grams/sq. meter

PROBLEM 6: If the extension service recommends broadcasting 10-30-10 at 600 kg/ha for nursery seedbeds when no compost or manure are available, how many grams of 10-3010 would be needed for a nursery seedbed measuring 1 X 5 meters?

SOLUTION: 600 kg/ha = 60 g/sq. meter

1 x 5 meters = 5 sq. meters

5 sq. meters x 60 g/sq. meter = 300 grams of 10-30-10 needed

CALCULATING THE AMOUNT OF FERTILIZER NEEDED PER PLANT

NOTE: The calculations below are based on open fields with evenly-spaced rows running across them. Where "intensive" gardening is used (beds with alleyways around them), another factor needs to be considered. We'll cover this after explaining the open-field calculations.

If using the half-circle or hole method of placement, the farmer will need to know how much fertilizer is needed per plant (or group of plants if they're in "hills"). There are several ways of doing this, but most people agree that the following method is the simplest:

PROBLEM 7: Angelita is planning to plant a field of maize with rows 90 cm apart. She'll plant 3 seeds per hole with 60 cm between holes, using a planting stick. The extension office recommends applying 18-46-0 at 150 kg/ha. If she uses the hole method of fertilizer placement, how many grams of 18-46-0 should each seed group receive?

SOLUTION: 150 kg/ha = 15 g/sq. meter

0.9 m X 0.6 m = 0.54 sq. meters of space belonging to each plant group.

0.54 X 15 g/sq. meter = 8.1 grams of 18-46-0 per plant group

NOTE: As you see, it's not necessary to know the field's size in order to arrive at the above answer. All that's needed is the rate per hectare and the in-row and between-row spacings. Of course, you need to know the field's area (or the total number of plants) to figure out how much fertilizer to buy.

PROBLEM 8: A communal garden project has run out of manure and is about to transplant cabbage on a field measuring 20 x 20 meters. The local extension office recommends applying 16-20-0 at 250 kg/ha using the half circle method. How much fertilizer should each plant receive if the rows are 60 cm apart with 40 cm between plants in the row?

SOLUTION: 250 kg/ha = 25 g/sq. meter

0.6 m X 0.4 m = 0.24 sq. meters space occupied by each plant

0.24 X 25 g/sq. meter = 6 grams of 16-20-0 needed per plant.

AMOUNT NEEDED PER METER OF ROW LENGTH

NOTE: The calculations below are based on open fields with evenly-spaced rows running across them. Where "intensive" gardening is used (beds with alleyways around them), another factor needs to be considered. We'll cover this after explaining the open-field calculations.

When banding fertilizer, farmers need to know how much to apply per meter of row length (or per row). As with per-plant dosages, there are several ways of calculating this, but the simplest and quickest method is shown below:

PROBLEM 9: Suheyla is about to apply an N sidedressing to her grain sorghum field. The recommendation is 200 kg/ha of 21-0-0 (ammonium sulfate). The rows are spaced 90 cm apart, and she plans to apply the fertilizer in a band running down the middle of each row. How many grams of 21-0-0 should be applied per meter of row length?

SOLUTION

STEP 1: 200 kg/ha = 20 g/sq. meter

STEP 2: All the fertilizer in a meter of row length will be confined in a band. If you can calculate the area belonging to that one meter of row length, you can figure out the dosage per meter:

Area belonging to 1 meter of row length = 1 meter of length X 0.9 m of width = 0.9 sq. meters

STEP 3: 0.9 sq. meters X 20 g/sq. meter = 18 g of 21-0-0 meter.

ADJUSTING CALCULATIONS FOR THE BED-AND-ALLEY SYSTEM

All the above dosage calculations were based on the open-field system of crop spacing where the rows are spaced equally across the field. (Both systems are explained and illustrated in Chapter 4.) However, if you use the same assumption when calculating dosages for intensively-grown crops (bed-and-alley system) you'll end up significantly shortchanging the plants on fertilizer. Here's why:

• In the intensive system, vegetables are grown under very close spacings within beds (flat, raised, or sunken) that are separated by alleyways used for all foot and equipment traffic.

• Since virtually all root growth takes place in the soil within the beds, no fertilizer (or water) should be applied to the alleys.

• The fertilizer recommendation per hectare is the same for both systems, BUT this means that the dosage per plant, per meter of row length, and per planted area (i.e. beds only) will be higher in the intensive system to make up for the fact that no fertilizer is applied to the alleyways.

• Another way of explaining this is that plants grown under the intensive system are spaced much more closely than when grown on an open-field basis. Because of this, more fertilizer is needed per sq. meter of actual bed. Since alleyways aren't fertilized, the amount of fertilizer per hectare ends up the same for both systems.

NOTE: You may think that water rates need to be similarly increased per sq. meter, but not so. That's because the high plant densities under bed-and-alley cropping shade more of the soil surface and thus lower evaporation losses of water; this helps compensate for the increased usage caused by the higher plant density. Also, the plant leaves shade each other more, which lowers transpiration (actual plant usage).

Let's go over how to calculate fertilizer dosages for bed-and-alley cropping.

PER AREA (Bed-and-Alley System)

In the open-field system, 100 kg per hectare equals 10 grams per sq. meter. Now, in the intensive system you have 2 types of area: bed area (planted area) and alley area. This means that 100 kg/ha of fertilizer doesn't work out to 10 g/sq. meter of actual bed area. If you applied 10 g per sq. meter to all the bed area, you'd end up applying much less than 100 kg/ha, because of not fertilizing the alley area which can equal about 30-40% of the total area.

Let's run through a practice problem on how to adjust for this:

PROBLEM 10: Akbar has 10 beds each measuring 1 x 10 meters; they're separated from each other by alleyways 60 cm wide on all 4 sides. He is told to apply 12-24-12 at 300 kg/ha at planting and wants to know how much fertilizer to buy.

SOLUTION

STEP 1: 300 kg/ha = 30 g/sq. meter of total area (beds + alleys)

STEP 2: Determine the total area (beds + alleys) occupied by Akbar's plots:

It's accurate enough to assume that each 1 x 10 meter bed is separated from another by a 60 cm alleyway on each of its 4 sides.

Therefore each bed along with its associated portion of alley (half the width of each alley) measures 1.6 x 10.6 meters which equals 17 sq. meters.

10 beds x 17 sq. meters (bed + alley) = 170 sq. m

STEP 3: Calculate the amount of 12-24-12 needed for all the beds, based on the total area involved.

170 sq. m x 30 g/sq. m = 5100 g = 5.1 kg of 12-24-12 needed

STEP 4: Calculate the amount needed per bed:

5.1 kg are needed but will be applied only to the beds themselves, not alleys.

5100 grams = 510 grams 12-24-12 needed per bed 10 beds

Now you can see how much difference there is in dosages. If you had based the dosage on 30 g/sq. m and used bed area alone, each bed would receive 300 grams of fertilizer (10 x 30) instead of the 510 grams it really deserves!

PER PLANT (Bed-and-Alley System)

In this case, the easiest way to calculate the upwardly-adjusted rate is to count the number of plants on a bed and then divide that into the amount of fertilizer needed per bed as we did for Akbar's plot above.

PROBLEM 11: Suppose Akbar is planning to transplant cabbage on the beds above. He'll run 3 rows down the length of each bed with 40 cm between rows and 40 cm between plants in the rows. How much 12-24-12 should each cabbage transplant receive if he plans to use the half-circle method and the same rate per hectare (300 kg)?

SOLUTION

STEP 1: Find how how many cabbage plants will fit on each bed:

25 plants will fit in each row (24 spaces with 40 cm between them, with the first and last plant being 20 cm from the bed's end). 75 total plants/bed. (See Figure 9-3.)


FIGURE 9-3: A 1 x 10 m bed can accomodate 75 cabbage plants spaced 40 cm x 40 cm.

STEP 2: Find the dosage per plant:

In the above problem, we determined that Akbar needs 510 g of 12-24-12 per bed.

510 g = 6.8 g 12-24-12 per plant 75 cabbage plants

Now, let's compare this dosage to that obtained from using open-field system math calculations as in Problems 7 and 8 a few pages back:

300 kg/ha = 30 g/sq. m

0.4 m (40 cm) X 0.4 m = 0.16 sq. m of space belong to each plant

0.16 x 30 g/sq. m = 4.8 grams (too little)

If Akbar applied 4.8 g per plant, each bed would receive only 360 g (instead of 510 g), which would work out to about 212 kg/ha instead of the recommended 300 kg/ha. (If each bed occupies 17 m2 (including alleyway area), there would be about 588 beds in a hectare; 588 x 360 g = 212 kg.)

AMOUNT PER METER OF ROW LENGTH (Bed-and-Alley System)

In this case, the simplest method is to find the amount of fertilizer needed per bed as in Problem 10 and divide this by the number of rows per bed.

PROBLEM 12: Suppose Akbar decides to plant leaf lettuce on the 10 beds in Problem 10 g in rows 20 cm apart running the short way (i.e. 1 meter long rows). Using the same fertilizer rate (300 kg/ha of 12-24-12, how much should be applied per meter of row if the band method is used?

SOLUTION

STEP 1: Find out how many rows will fit on each 1 x 10 m bed:

50 rows with 49 row spaces each of 20 cm will fit on a 1 x 10 m bed. Each of the 2 end rows will be 10 cm in from the bed's edge.

STEP 2: Determine the dosage per meter of row (i.e. one row in this case):

From Problem 10, we know that 510 grams are needed per bed, so:

510 grams = 10.2 g of 12-24-12 per 50 rows one meter of row length

Again, let's compare this intensive system dosage with that obtained by using open-field calculations as in Problem 9:

300 kg/ha of 12-24-12 = 30 g/sq. meter

0.2 m (20 cm) X 1 meter = 0.2 sq. meters space belonging to each meter-long row

0.2 sq. meters x 30 g/sq. m = 6 g of 12-24-12 per sq. m (too low)

CONVERTING FERTILIZER DOSAGES FROM A WEIGHT TO A VOLUME BASIS

TABLE 9-6

FERTILIZER WEIGHT TO VOLUME COMPARISONS

 

Grams of Fertilizer Equal to:

 

Fertilizer

100 cc(ml)

1 Level Tablespoonful (15 cc)

Ammonium sulfate

108-120 g

16-18 g

Ammonium nitrate

85 g

13 g

(prilled)

   

Ammonium nitrate

100 g

15 g

(granulated)

   

Urea

75-79 g

11-12 g

16-20-0

98-104 g

15 g

18-46-0

93-108 g

14-16 g

Potassium chloride

100-120 g

15-18 g

(0-0-60)

   

Single superphosphate

109-11, g

16-18 g

Triple superphosphate

100-112 g

15-17 g

Most other NP and

93-110 g

14-16.5 g

NPK fertilizers

   

As shown by the above dosage problems, the amount of chemical fertilizer needed per plant or per meter of row is surprisingly small, usually ranging from 5-30 grams. To assure accuracy and cost-effectiveness, farmers should not attempt to estimate such small amounts. However, since few farmers or gardeners have easy access to accurate scales, it's very helpful if to convert the fertilizer dosage from weight to volume. This doesn't mean simply converting grams to cubic centimeters, either. The dosage should be given in terms of a commonly available volume measure such as a:

• Juice can

• Tuna fish can

• Bottle cap lid

• Match box

• Spoon size commonly used in the area

This can be done by using a gram scale (check the post office or a pharmacy) to measure the densities of the common fertilizers available and comparing them to water (1 gram = 1 cc or 1 ml). Then you can measure the volume of commonly available containers like those above and calculate how many grams of fertilizer they hold.

Fertilizers vary a lot in their density, depending on type, brand, and moisture content. If no scales are available, use Table 9-6.

Here's a practice problem for converting fertilizer weight to volume:

PROBLEM 13: How many grams of urea would a 120 cc juice can hold?

SOLUTION: 100 cc of urea weighs 74-79 grams.

(120 cc / 100 cc) X 74-79 g = 89-95 g of urea in one juice can