Important facts on the plant nutrients

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

MACRONUTRIENTS

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

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.

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.