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Chapter 6: Soil fertility and plant nutrition simplified

A Practical Approach

For many farmers and development workers, soil fertility management is one of the more mysterious aspects of crop production. How and when do you apply fertilizers? What kind and how much? What about chemical vs. organic fertilizers? Part II of this manual will cover all of these concerns.

Let's Make a Deal

Soil fertility may not seem like a very stimulating topic, and you may not feel like reading over 100 pages on it. But, look at it this way. Soil fertility management is a vital part of successful crop production, yet an area that's often misunderstood and prone to faulty management. If you're willing to read through Part II and learn to use it as a field reference, you'll know at least as much about the practical aspects of soil fertility as most agronomists. But more important, you'll be a more effective as extension worker in terms of your knowledge; you'll also find that many of these principles can be readily understood and applied by farmers who have little or no formal education. After all, one goal of true development work is to "empower" the poor, to give them more control over their circumstances. Knowledge of this sort is a step in the right direction.

Before covering how to use organic and chemical fertilizers, there are some very useful soil fertility fundamentals we should go over. That's the purpose of this chapter.

How plants grow

Plants grow by enlarging their cells and by developing new ones at their shoot and root tips.

Photosynthesis: How Plants Make Food and Tissue

Plants produce food for energy, tissue building, and storage by a process called photosynthesis which takes place in the green, chlorophyll-containing cells found mainly in the leaves. These cells take carbon dioxide from the air and combine it with water (taken in by the roots) to make simple sugars, using chlorophyll and sunlight as catalysts. As shown below, oxygen is also a byproduct:

Sunlight

Carbon dioxide + water -> Sugar + Oxygen Chlorophyll

This sugar is the real "food" of plants, and here's what they do with it:

• It's used for energy in a process called respiration in which the plant digests it much like we do and releases carbon dioxide.

• Sugar is also storied "as is" in varying amounts (i.e. maize contains a small amount, but sugarcane has a lot).

• It's used to make cellulose and other types of fiber that hold cells and plants together. Plants are the only source of fiber in our diets.

• Sugar can be converted to starch, the main component of most seeds and other starchy crops such as bananas, potatoes, cassava (manioc), and other root crops. It is also converted into fat, a principal constituent of some crops such as coconut, soybeans, peanuts, and avocados.

• Sugar, when combined with nitrogen, forms protein.

Photosynthesis governs the rate of plant growth and is the biggest factor affecting crop yields. It's encouraged by:

• Adequate sunlight. Cloudy weather lowers the rate of photosynthesis.

• Adequate moisture.

• Favorable temperatures, which vary with the type of plant.

• Adequate mineral nutrients like nitrogen, phosphorus, etc.

• Good insect and disease control which prevents the destruction of green tissue.

• Adequate carbon dioxide. Normal air contains enough. Some greenhouse growers try to raise the level.

A Note on "C4" Plants: Some crops such as maize, sorghum, amaranth, and sugarcane have an unusually efficient type of photosynthesis that functions best under high temperatures, full sun, and low-humidity conditions. They're called C4 plants.

So Where Do the Plant Nutrients Fit In?

The plant mineral nutrients like nitrogen are supplied by the soil and supplements of fertilizer. They are absorbed by the root hairs (tiny, delicate protrusions on the roots) in the form of ions (molecules with a + or - charge) from the soil water and perform many functions. Some, like potassium, are used in sugar and starch formation, while nitrogen is used for making protein and chlorophyll.

Available vs. unavailable forms of mineral nutrients

Aside from water and carbon dioxide, plants need about 14 mineral nutrients: NITROGEN, PHOSPHORUS, POTASSIUM, CALCIUM, MAGNESIUM, SULFUR, IRON, MANGANESE, COPPER, ZINC, BORON, MOLYBDENUM, SODIUM and CHLORINE (these last two are rarely deficient).

Each of these mineral nutrients occurs in both available and unavailable forms in the soil. For instance, only about 1-2% of a soil's potassium is actually available to roots; most of the other 98-99% is tied up as part of rock fragments or clay particles and is very slowly released over time.

Likewise, only about 1-2% of a soil's nitrogen is readily usable by plants. The rest is in the organic form as dead leaves, roots, and crop residues in various states of decomposition; organic nitrogen doesn't become usable until soil bacteria have converted it into ammonium or nitrate ions.

The same applies for each of the other nutrients to varying degrees, depending on soil conditions. As we'll see, soil pH can have a big effect on nutrient availability.

Soil negative charge and nutrient holding ability

Some soils have much higher leaching losses than others. (Leaching occurs when downward-moving water carries nutrients with it out of the root zone).

Factors Affecting Leaching Losses

Soil texture, negative charge, and amount of rainfall or watering determine a soil's leaching potential:

• Sandy soils are very susceptible to leaching losses for 2 reasons. First, they tend to have a low negative charge. which means little ability to hold on to plus-charged nutrients. Second, a given amount of water will penetrate more deeply than on finer textured soils with higher water-holding capacity (see Chapter 2).

• Clayey soils and those high in humus have lower leaching losses, due to higher negative charge and better water-holding capacity (up to twice as much as sandy soils. Remember, however, that "tropical" clays (e.g. kaolin and hydrous oxide clays) have a very low negative charge.

• The higher the rainfall, the higher the leaching losses.

• The "worst-case" scenario for leaching would be a sandy soil low in humus, under high rainfall.

How Negative Charge Helps a Soil Hold Nutrients

Clay and humus particles have a negative charge. The available forms of plant nutrients exist as ions which are molecules with a positive (plus or +) charge or negative (minus or -) charge. The minus-charged clay and humus particles act like little magnets to attract and hold those plus-charged ions like potassium (K+), calcium (Ca++), and magnesium (Mg++), which gives them some resistance to leaching. The nice thing is that the plus-charged ions (called cations) are available to roots even when held by the clay and humus particles. Cations will still leach somewhat, but not nearly as much as the anions.

Unfortunately, the minus-charged nutrient anions like nitrate (NO3-) and sulfate (SO.--) aren't so lucky. Since like charges repel, they're not held by the minus-charged clay and humus particles; instead, they end up floating around freely in the soil water, which makes them very prone to leaching in most cases.

TABLE 6-1 Some Common Plant Nutrients and Their Susceptibility to Leaching

+ Charged Nutrients (Cations) (fairly resistant to leaching)

- Charged Nutrients (Anions) (easily lost by leaching)

Ammonium nitrogen (NH4+)

Nitrate nitrogen (NO3)

Potassium (K+)

Sulfate (SO)

Calcium (Ca++) (Mg++)

NOTE: Leaching losses of potassium can be a problem on sandy soils under high Magnesium rainfall.

What about Phosphorus?: It's an exception. Even though its 2 soil ionic forms (H2PO4-, HPO4--) have a minus charge, they hardly move at all in the soil, because they readily form insoluble, immobile compounds with iron, aluminum, calcium, and magnesium. While this keeps phosphorus from leaching, roots have trouble absorbing it in this form. This "tie-up" is called phosphorus fixation and can be a serious problem on many soils, especially when phosphorus fertilizers aren't applied correctly.

NOTE: Don't confuse phosphorus fixation with nitrogen fixation (the process by which rhizobia bacteria associated with legumes convert atmospheric N into usable form for these plants).

How Soil Negative Charge is Measured: Cation Exchange Capacity (C.E.C.)

The exchange capacity of a soil ( also called cation exchange capacity or C.E.C.) is a measure of its negative charge or the amount of plus-charged nutrients it can hold against leaching.

A soil's C.E.C. depends on its clay and humus content, since they're the only 2 soil particles with a minus charge. Soils with a low C.E.C. are especially prone to leaching and have poor nutrient-retaining ability. Even soils of the same texture can vary markedly in C.E.C. due to variations in humus content and the type of clay minerals they contain (see Table 6-3).

Table 6-2 illustrates the marked variations in C.E.C. between humus and clay, as well as among different types of clay. Note the very high charge of humus, which is why it can easily account for the major portion of the C.E.C. in many soils, even when present at typical normal levels of just 2-4% (by weight). These differences explain why C.E.C. varies so much among soils (even those of the same texture), as shown in Table 6-3.

TABLE 6-2 The Relative Cation Exchange Capacity of Clay and Humus Particles

 

C.E.C.*

Humus

150-200

"Temperate" -type

15-100

clay minerals

 

"Tropical" -type

2-15

clay minerals

 

TABLE 6-3 Typical Variations in Cation Exchange Capacity among Soils

Soil Name

C.E.C. of the Topsoil*

Hilo clay (Hawaii)

67

Cecil clay (Alabama)

4.8

Susquehanna clay (Alabama)

34.2

Greenville sandy loam (Alabama)

2.3

Colma sandy loam (California)

17.1

*Don't worry about what the actual numbers mean; it's the comparison that counts. (If you're familiar with chemistry, C.F.C. is measured in terms of milk-equivalents of cations per 100 grams of soil.)

Soil pH and how it affects crops growth

Soils can be acid, neutral, or basic (alkaline), and this is measured in pH units. The pH scale runs from 1 (maximum acidity) to 14 (maximum alkalinity) with 7 being neutral. Most soils fall in the range of 5.0-7.5 with extremes from 4.0-9.0 as shown in Figure 6-1.


FIGURE 6-1 pH Ranges of Soils

Acidity is caused by hydrogen ions (H+), and alkalinity by hydroxyl ions (OH-). A pH of 7.0 is neutral, meaning that there are equal numbers of H+ and OH- ions. As pH drops below 7.0, the H+ ions begin to outnumber the OH- ions, and acidity increases.

The pH scale is logarithmic: This means that a soil with a pH of 4.0 is 10 times more acid than one with a pH of 5.0, and 100 times more acid than one with a pH of 6.0. A pH of 3.9 is 1000 times more acid than a pH of 6.9. Likewise, a pH of 8.5 is 10 times more alkaline than a pH of 7.5 but 10 times less alkaline than a pH of 9.5.

Why Does pH Vary so Much Among Soils?

Climate, amount of leaching, parent material, and farming practices all affect soil pH:

• Climate and Leaching: As illustrated by Figure 6-1, soils of high-rainfall regions are likely to be on the acid side. That's because a good deal of calcium and magnesium (the main bases in the soil) have been leached away by rainfall. It's a slow process since they're both plus-charged ions, and here's how it works. Acid-forming H+ ions are produced by the breakdown of organic matter and acid parent rocks like granite, and also by using most chemical fertilizers containing nitrogen. Being cations, too, these H+ ions can "bump" some of the adhered calcium and magnesium ions off the clay and humus particles and take their places. Once back in the soil water, the Ca and Mg are prone to leaching. Where rainfall is high, this results in more acid soils.

In contrast, soils of drier regions like the Sahel are more likely to be alkaline or only slightly acid, because less leaching of Ca and Mg occurs where rainfall is lower.

Soils of drier regions aren't always alkaline, nor are soils of wet regions always acidic. Parent rock and farming practices also have an influence.

• Parent Rock: Soils formed from basic rocks like limestone and basalt tend to be less acidic (or more basic) than those developed from acidic rocks like granite and sandstone. However, even soils formed from limestone may be somewhat acidic if leaching has been intense.

• Farming practices: Liming the soil will lessen acidity (raise pH). Manure, compost, and most chemical fertilizers containing nitrogen will gradually increase soil acidity when used over the years.

Why Worry about Soil pH Anyway?

Soil pH can have a big effect on crop growth and yields. Most crops will produce satisfactory yields within a pH range of about 5.5-7.5, with a pH of about 6.3 being ideal for most. Some crops like pineapple, coffee, potatoes, and sweet potatoes are especially tolerant of soil acidity. (For more information on crop tolerance, see Table 11-1 in Chapter 11.)

How Soil pH Affects Crop Growth

• The tie-up (fixation) of phosphorus is greatly affected by soil pH. Phosphorus is most available within a pH range of 6.0-7.0. (See Chapter 11.)

• Very acid soils can be toxic to plants. Aluminum, manganese, and iron become more soluble as soil acidity increases and can actually injure plant roots at pH's below 5.0-5.5, depending on the soil and type of plant.

• Soil pH affects the availability of micronutrients to plant roots. Except for molybdenum, the other 5 micronutrients (iron, manganese, copper, zinc, boron) become increasingly available to plants as acidity decreases (i.e. as pH rises). Iron and manganese are the most affected and may become so insoluble at pH's above 6.5 that plants can suffer deficiencies (see Figure 6-2).

• Most beneficial soil microbes can't thrive in very acid soils. (Beneficial soil bacteria and fungi are described in Chapter 1).

Salinity and alkalinity problems occur at pH's of 8.0 and above where sodium and other salts are present at levels high enough to be toxic to plants ( see Chapter 12).


FIGURE 6-2: Influence of soil pH on plant nutrient availability in mineral soils. Widest parts of the shaded areas indicate maximum availability of each nutrient.

How can You Measure Soil pH?

Soils labs determine pH as a routine part of soil testing. You can also make fairly accurate readings right in field with a liquid indicator kit or portable electric tester. See Chapter 11 on liming for details.

How Can You Change Soil pH?

Liming will lessen soil acidity and raise the pH. Some common liming materials are limestone (calcium carbonate), dolomitic limestone (a mixture of calcium and magnesium carbonates), and burned lime (calcium oxide). Sulfur and aluminum sulfate can be used to further lower the pH of acid soils "sometimes done for acid-robing crops like blueberries). Gypsum (calcium sulfate or chalk) has no effect as a liming material but can be used to remove sodium (a powerful base) from very alkaline soils, thus lowering the pH toward neutral.

For more information on changing soil pH, see Chapter 11 on liming and Chapter 12 on salinity and alkalinity problems.

Important facts on the plant nutrients

The plant mineral nutrients can be grouped into 2 classes: MACRONUTRIENTS and MICRONUTRIENTS.

MACRONUTRIENTS

Primary Macronutrients

Secondary Macronutrients

NITROGEN (N)

CALCIUM (Ca)

PHOSPHORUS (P)

MAGNESIUM (Mg)

POTASSIUM (K)

SULFUR (S)

MICRONUTRIENTS

 

IRON (Fe)

ZINC (Zn)

MANGANESE (Mn)

BORON (B)

COPPER (Cu)

MOLYBDENUM (Mo)

Macronutrients vs. Micronutrients

The 6 macronutrients make up about 99% of a plant's diet. N,P,and K account for about 60% and are definitely the "BIG 3" of soil fertility in terms of quantity needed and likelihood of deficiencies.

TABLE 6-4

Amount of Nutrients Needed to Produce 4000 kg of Shelled Maize

Macronutrients

Kg

Micronutrients

Kg

Nitrogen

112

Iron

3.0

Phosphorus (P2O5)

43

Manganese

0.7

Potassium (K2O)

89

Zinc

0.2

Calcium

21

Copper

0.05

Magnesium

18

Boron

0.05

Sulfur

17

Molybdenum

0.0054

This doesn't mean that the secondary macronutrients or the micronutrients are any less essential. Although their deficiencies usually aren't as common, they can have just as serious an effect on crop yields when they occur.

THE "BIG 3": N, P, and K

NITROGEN (N)

Role of Nitrogen

N is the most commonly deficient nutrient in most cultivated soils. It plays several important roles:

• It's an essential part of chlorophyll, needed for photosynthesis.

• Plants combine N with sugars to make protein. All protein contains about 16% N.

• It promotes vegetative growth (leafy growth).

• It promotes plumpness of grain kernels.

Crops Vary in their N Needs

Crops with High N Needs

• Crops making lots of vegetative growth have high N needs, as long as there's sufficient water for high yields.

• Cereals, leafy vegetables (lettuce, cabbage, etc.), fruit-type vegetables ( tomatoes, peppers, etc.), pasture grasses, sugarcane, and bananas. However, most of the traditional, taller-growing varieties of rice and wheat are likely to lodge ( tip over) at high N rates.

• Legume crops also have high N needs but are a special case because of their N-fixing ability.

Crops with Moderate N Needs

• Most root crops such as turnips, beets, carrots, tropical yams, potatoes, sweet potatoes, cassava (manioc), and taro have lower N needs than those above. Too much N may favor leaf production over tuber growth. However, some of the newer potato cultivars (varieties) respond well to high levels of N.

What about the N Needs of Legumes?

Legumes are partly to wholly self-sufficient in meeting their own N needs due to their symbiotic relationship with rhizobia bacteria (Rhizobium sp.) that live in nodules on their roots. The rhizobia convert the unavailable nitrogen in the soil air to a usable form for the plant; this process is called nitrogen fixation. As explained below, legume. vary in their N-fixing ability. (For more information on N fixation, refer to the section on pulses in Chapter 10.):

• Some pulses "legumes producing edible seeds), such as soybeans, cowpeas, peanuts, mungbeans, pigeonpeas, winged beans, and vining (tropical) types of lima beans, can meet all their own N needs if the right strain of rhizobia is present.

• Field beans (navy beans, black beans, kidney beans, pinto beans; botanical name = Phaseolus vulgaris), field peas (Pisum arvense), garden peas (Pisum sativum), and the bushy varieties of lima beans have less efficient types of rhizobia and can meet only about half their N needs through fixation.

• Pasture legumes, such as clovers, tropical kudzu, and stylo, are wholly self-sufficient and can even produce enough extra N to satisfy the needs of any pasture grass that might be intermixed with them.

The Effects of too Much N

Too much N may have an adverse effect on crop growth, especially if other nutrients are deficient. It may:

• Delay maturity, though not always.

• Lower disease resistance by making growth overly succulent and more easily penetrated by disease organisms.

• Discourage tuber or fruit formation in favor of vegetative growth.

• Increase lodging (tipping over of stems), especially in the traditional, taller-growing varieties of rice and wheat.

HOW NITROGEN BEHAVES IN THE SOIL

Available vs. Unavailable Forms of N

Only about 1-2 percent of a soil's native N is actually available to plants and exists in the inorganic (mineral) form as ammonium (NH.+) or nitrate (NO3-). The other 98-99 percent is bound up in the unavailable organic form as part of humus or crop residues which soil microbes gradually convert to ammonium and nitrate (see Fig. 6-3). Most soils are too low in organic matter to supply available N at a rapid enough rate, so that's why N fertilizer (organic or chemical) is needed.

(Unavailable)

(Available)

(Available)

ORGANIC N à

AMMONIUM N (NH4+) à

NITRATE N (NO3-)

 

(weeks, months)

(days, weeks)

FIGURE 6-3: Soil microbes convert organic N to available forms.

Available N can be Easily Lost by Leaching

The ammonium (NH4+) form of N has fairly good resistance to leaching (except on low C.E.C. soils) because of its plus charge. However, nitrate N (NO3-) is more readily leached because of its minus charge.

Lowering leaching losses: If using chemical fertilizers, you might think that leaching could be avoided by choosing the ammonium form of N. The problem is that, in warm soils, soil bacteria will convert nearly all the ammonium into leachable nitrate in just 7-10 days! In cooler soils, the conversion is much less rapid. For example, if soil temperature averages 11°C (52°F), only about 50 percent of the ammonium will be converted to leachable nitrate in 5 weeks. In fact, farmers in the U.S. Corn Belt can apply ammonium N fertilizers in the fall (5-6 months before planting) with little or no leaching losses, thanks to very low winter soil temperatures.

Under warm conditions, the most practical way to reduce leaching losses is to "spoon feed" N when its applied as a chemical fertilizer or to use organic sources like compost and well-rotted manure which are slow-release sources of available N (organic N does not leach). This is covered in Chapters 8 and 9.

Denitrification: Another Way that N is Lost

In poorly-drained soils where there's little air, much available N can be lost by denitrification. What happens is that certain kinds of anaerobic bacteria (those that can function without oxygen) convert any nitrate (NO3-) into nitrogen gas which escapes out of the soil and is lost. tosses can be very high if the soil is flooded for even a day or two after a heavy rain.

Even soils that appear well-drained at the surface may have serious denitrification losses taking place in the subsoil if it's poorly drained. Improving drainage is the best way to control these losses (see Chapter 2). Flooded rice soils require special fertilizer management to avoid large denitrification losses of N. (See Chapter 10.)

Temporary N Tie-up by Crop Residues

Available soil N can become temporarily tied up by bacteria if crop residues or organic conditioners like rice hulls are worked into the soil. Below is an explanation of this type of N tie-up and how to prevent it:

• The soil bacteria that decompose crop residues use carbon for energy and nitrogen to make protein for growth and multiplication. Most non-legume residues such as maize stalks have plenty of carbon but too little N. Rice hulls, peanuts hulls, millet hulls, and sawdust even less.

• The bacteria make up for the shortage of N in their "food" by borrowing nitrate N from the soil itself. A crop growing in such a soil may suffer a temporary N deficiency until the bacteria have completed most of the initial "digesting". As the residues are converted to humus, bacterial activity, decreases and the "borrowed" N again becomes available as many bacteria die off. The temporary N deficiency may last several weeks.

• NOTE: Stalk and leaf residues of legumes like beans, cowpeas, and peanuts are usually high enough in N to avoid these tie-up problems. However, peanut hulls (shells) are low in N.

This type of N tie-up can be prevented in 3 ways:

• If possible, turn under low-N residues at least a month before planting to give them time to rot. However, little decomposition will occur if the soil is dry or very cold.

• At planting time, be sure to add enough N (either from chemical fertilizer or from an organic source high in readily available N such as fresh manure) to sustain the crop during the tie-up period. This will take roughly 30-60 kg/ha of actual N or the equivalent of 75150 kg/ha of urea fertilizer (45-0-0). The fertilizer doesn't have to be mixed into the entire soil area, either. In fact, less N is needed if it's placed near the crop row where the plants have good access to it.

• The residues can be collected and composted before returning them to the field. This is done in parts of S.E. Asia with rice straw residue but is laborious and requires the addition of high-N materials such as fresh manure to encourage the breakdown of the lowN straw. (For more information on composting, refer to Chapter 8.)

PHOSPHORUS (P)

Role of Phosphorus

Phosphorus plays many roles in plant growth and exerts a beneficial effect on:

• Root formation and early growth.

• Flowering, fruiting, and seed formation.

• Crop quality, especially in vegetables and forage crops.

• Resistance to some diseases.

Phosphorus Deficiencies are Widespread

As with N, most soils are deficient in P for several reasons:

• Most soils are low in total P.

• Much of a soil's natural P is tied up and unavailable to plants.

• Much of the P applied in chemical fertilizer form can become tied up also.

Phosphorus Tie-up (Fixation)

Only about 5-20 percent of the P you apply as chemical fertilizer to an annual crop like maize or vegetables will actually be available to it. In acid soils, much of the P gets "fixed" (tied up) by reacting with iron, aluminum, and manganese to form insoluble compounds. In basic soils, the added P has a similar reaction with calcium and magnesium.

The amount of P immediately available from an application of chemical fertilizer depends on the amount applied but even more so on the application method used.

Some of the 80-95 percent of the P that becomes "fixed" will eventually become available again to crops over the years. There's a saying that applying fertilizer P is like putting money in the bank and living off the interest. The amount of future interest you get depends a lot on the type of soil. Some soils, especially very acid, red soils high in "tropical" clays, can have an extraordinary P fixation ability and may tie up 95-99 percent applied fertilizer P in a virtually irreversible, unavailable form.

The P in organic fertilizers like compost and manure is much less subject to fixation.

NOTE: Don't confuse P fixation with N fixation!

Temporary P tie-up by decomposing crop residues: As with N, some soil P can become temporarily tied up when low-nitrogen crop residues (i.e. those from non-legumes) are worked into the soil. The bacteria that break down the residues need P as well as N for their growth and multiplication and end up borrowing both from the soil as explained in the previous section on nitrogen. Such tie-up can last for several weeks or more, but can be compensated for by applying P fertilizer near the row. Legume residues break down quickly enough so that tie-up isn't a problem.

How to Minimize P Tie-up Problems

Application method is vitally important: In most cases, chemical fertilizer P should not be broadcast ( spread) but applied in a band, hole or half-circle to concentrate it near the plant row. (Refer to Chapter 9.)

Maintain a good level of soil organic matter: Decaying organic matter produces humus and organic acids that form complexes with iron and aluminum; this can considerably reduce their ability to tie up P.

• Lime overly acid soils: P fixation problems are more serious at very low pH's. Likewise, pH's above 7.5 increase P tie-up too. P is most available within a pH range of 6.0-7.0.

• N helps encourage the plant's uptake of P, so applying N and P at the same time is helpful (if N is needed).

Some Good News: P Doesn't Leach!

Unlike nitrate N, P is pretty immobile in the soil, and leaching losses are virtually nil, even on sandy soils. This means there's no need to "spoonfeed" fertilizer P by splitting the dosage into two or more applications; all can be applied at transplanting or planting.

POTASSIUM (K)

Role of Potassium

• It promotes starch and sugar formation. Crops such as bananas, sugarcane, and starchy root crops like potatoes, cassava, and taro have especially high needs.

• It favors root growth, stalk strength, disease resistance, and general plant vigor.

K Deficiencies are Less Common

• Unlike N and P, deficiencies of K are lees likely, but don't automatically assume that K isn't somewhat deficient in your area.

• Soils of volcanic origin tend to be especially high in K.

Relative K Needs of Crops

• Starch and sugar crops have the highest requirements.

• Cereal crops and other grasses have a better ability to extract K from the soil than broadleaf plants.

"Luxury Consumption" of K

If high rates of potassium are applied, plants have a tendency to take up more than they need. Some soil specialists feel that "luxury consumption" is aggravated by shortages of other nutrients. Others feel that this problem is over-exaggerated. At any rate, limited resource farmers are unlikely to apply high enough rates of X to promote luxury consumption.

K Tie-up Problems are Usually Minor

Only about 1-2 percent of a soil's total K is in the available form, but even this is often enough to supply the needs of some crops. Tie-up of added K is usually not a problem. Some soils high in the 2:1 temperate clays such as montmorillonite can temporarily tie up some added K. (Clay types are covered in Chapter 2.)

Leaching Losses of K are Usually Minor

Available K is a cation (K+) and is therefore somewhat resistant to leaching on most soils. However, leaching losses can be substantial on sandy soils (or others that have a low C.E.C.) where rainfall is high. In this case, it's best to "spoonfeed" K by making 2-3 applications if chemical fertilizer is used. Acidic soils lose more K by leaching than limed ones.

Recycling of K

Unlike N and P which accumulate mainly in the seed or grain, about 2/3rds of the K that plants take up remains in their leaves and stalks. Returning crop residues to the soil is a good way to recycle K.

The Potassium/Magnesium Balance: High applications of K can provoke magnesium deficiencies in some crops. For example, overuse of K in grass pastures has caused Mg deficiencies in both the grass and the livestock.

THE SECONDARY MACRONUTRIENTS: (Ca, Mg, S)

CALCIUM

• Calcium is not only an important plant nutrient but is also used as a liming material to lessen acidity.

• Even very acid soils usually have enough calcium to fulfill plant needs, although pH may be too low for good

crop growth. Peanuts have unusually high Ca needs and often require gypsum applications.

• Available calcium has a plus charge and therefore has some resistance to leaching.

MAGNESIUM

• Magnesium deficiencies are most likely to occur in sandy, acid soils (usually below pH 5.5).

• Like calcium, Mg is a cation (Mg++) is also fairly resistant to leaching, compared to nitrate N (NO3-).

The calcium/magnesium ratio: Mg deficiencies can be provoked if the ratio of Ca to Mg in the soil becomes too high, even though the soil contains enough Mg. This is more often a problem on sandy soils (or other low C.E.C. soils) where it's easy to upset the nutrient balance. When liming, it's a good idea to use dolomitic limestone (a mix of Ca and Mg).

Potassium-induced Mg deficiencies: Refer to the section on K above.

SULFUR

• Sulfur is used in protein synthesis and by the N-fixing rhizobia bacteria. It also forms part of several vitamins and is used in oil (fat) formation.

• Crucifer (Brassica) Family plants (cabbage, broccoli, turnip, etc.), onions, and asparagus have especially high S needs, followed by tobacco, cotton, and legumes.

• S deficiencies aren't common but are most likely to occur in highly leached soils (sandy, low C.E.C., high rainfall).

• Volcanic soils tend to be low in S; farmland near industrial areas usually receives more than enough S from the air.

• The high-analysis grades of chemical fertilizers are low in sulfur and may lead to deficiencies if used as the sole source of fertilizer continually.

Leaching Losses of Sulfur

The available form of sulfur is the sulfate ion (SO4-) which is readily leached, especially in sandy soils under high rainfall. A good part of the soil's sulfur is in the unavailable organic form which bacteria convert to available sulfur. Organic sulfur is an important reservoir of this nutrient, since it doesn't leach in this form. As with N and P, sulfur can become temporarily tied up when large amounts of low-nitrogen crop residues (i.e. those from non-legumes) are plowed under, because the decomposition bacteria need sulfur as well.

Sulfur retention: Appreciable amounts of available sulfur can be retained against leaching in subsoils high in tropical-type clays; plant roots can utilize this source.

THE MICRONUTRIENTS

(Iron, Manganese, Copper, Zinc, Boron, Molybdenum)

• The micronutrients perform many vital functions, but are needed in very small amounts.

• The difference between toxic and deficient levels is often small. As little as 75 grams of Mo per hectare may cure a deficiency for several years, but 3-4 kg might severely injure plants. Boron is another touchy one.

Where to Suspect Micronutrient Deficiencies

Although less common than macronutrient deficiencies, macronutrient deficiencies can be just as serious when they occur and are favored by:

• Highly leached, acid, sandy soils.

• Organic soils (peats or those soils containing at roast 20% humus by weight). Copper deficiencies are especially common on these soils.

• Soil pH's above 6.8-7.0, except in the case of Mo which doesn't become less available as pH is increased.

• Intensively cropped soils fertilized with macronutrients only.

Susceptible Crops: Vegetables, legumes, and tree crops are more prone to micronutrient deficiencies than cereal Brains and pasture grasses. However, sorghum is very sensitive to iron deficiencies as maize is to zinc deficiencies. Table 10-5 in Chapter 10 lists the susceptibility of specific vegetables to micronutrient deficiencies.

Micronutrient Toxicities

Iron and manganese can become toxic to plants in very acid soils below pH 5.0-5.5 when they become too soluble. Poor drainage also promotes this problem. Boron and molybdenum can become toxic if over-applied.

How to Correct Deficiencies or Toxicities

• Adiusting pH: Molybdenum deficiencies can often be more effectively corrected by raising soil pH if very, acid. Raising pH is effective in alleviating iron and manganese toxicities (aluminum too), and improving drainage will also help.

• Soil applications of micronutrients: Effectiveness varies. Iron and manganese are very readily tied up when applied to soils where they're deficient. Special chelated forms are available which are less subject to soil tie-up. (See Chapter 9).

• Foliar applications: Since such small amounts are needed, it's practical to spray plant foliage with a very diluted micronutrient solution. This also avoids soil tie-up problems. Several applications may be needed. In some cases, foliar fungicides like Maneb (containing manganese), Zineb (containing zinc), and Cupravit (containing copper) are used to supply deficient micronutrients to vegetable and tree crops in conjunction with control of foliar fungal diseases.

Application rates for micronutrients See Chapter 9.