| Soils, Crops and Fertilizer Use |
|Chapter 9: Using chemical fertilizers|
The section gives "how to" instructions on the following fertilizer application methods and compares their use:
• Localized placement (band, hole, half circle)
• Special placement considerations for furrow irrigated soils.
• Application through the irrigation water
• Foliar applications
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 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.
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.
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.)
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:
Additional N to apply over 3 sidedressings.
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.
• 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.
* 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:
Maize Yield Per Hectare
N + P
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.