Cover Image
close this book Soils, Crops and Fertilizer Use
close this folder Chapter 10: Fertilizer guidelines for specific crops
View the document Cereals
View the document Pulses (grain legumes)
View the document Root crops
View the document Vegetables
View the document Tropical fruit crops
View the document Tropical pastures

Pulses (grain legumes)

Beans, cowpeas, peanuts, and soybeans are known as pulses, pulse crops, or grain legumes, along with other edible-seeded legumes such as chickpeas, pigeonpeas, mungbeans, winged beans, lima beans, and English peas. As opposed to the cereals which belong to the grass family (Gramineae), pulses belong to the legume family (Leguminosae) whose members produce seeds in pods. Whereas the cereala are monocots, (seedlings emerge with one seed-leaf or cotyledon), the pulses are dicots and emerge from the "oil with 2 seedleavea. In addition, pulses have 2 other notable characteristics:

• Their mature dry seeds contain 2-3 times more protein (20-39%) than the cereals (714%).

• They are partly to wholly self-sufficient in meeting their own N needs, thanks to a process called nitrogen fixation (also referred to as symbiosis).

Nutritional Value of Pulses

One cooked cup of most pulses provides about 15 grams of protein (soybeans have 20), compared to about 5 grams for most cereals. Non-pregnant, non-lactating vegetarian adults can easily satisfy their protein quantity and quality (amino acid) needs by consuming pulses and cereals in a ratio of 1 part pulse per 5 parts cereal (cooked basis). Recent research has shown that cereals and pulses still have a complementary effect on protein quality even if eaten several hours apart.

The mature seeds of pulses are also rich in B vitamins and are fairly good sources of iron. In some areas, particularly West Africa and S.E. Asia, the leaves of certain pulses such as cowpeas and winged beans are also eaten. As with most other dark-green leafy vegetables, pulse leaves are rich sources of many nutrients such as vitamins A and C, folio acid (folacin), calcium, and iron; they also contain a fair amount of protein. However, the leaves of some pulses, such as those of the yam bean (jicama; Pachyrhizus erosus) can be toxic.

Limiting Factors in Pulse Production

When both are grown under similar management, puree yields are usually around half those of cereals for several reasons.

• They are more susceptible to diseases and insects.

• N fixation diverts some energy from the plant. (Don't worry, the trade-off is well worth it.)

• Compared to cereals, legumes use more of their N uptake to increase the protein content of the seed rather than to increase yield.

• Until recently, the amount of research on pulses was a distant second to that devoted to cereals.

Getting the Most out of Nitrogen Fixation

Most legumes have a symbiotic relationship with rhizobia bacteria (Rhizobium sp.) that live in the soil (non-legumes do not). If the correct strain of rhizobia is present, the bacteria will infect the roots soon after seed germination. The plant responds by converting some of its root hairs (tiny, hair-like protrusions from the roots) into nodules (bumps) to to house the bacteria. The rhizobia live off sugars supplied by the plant and "fix" (capture) nitrogen fro. the soil air and convert it to a form (ammonium) that the plant can use. Thanks to nitrogen fixation, legumes are partly to wholly self-sufficient in meeting their own N needs as follows:

• Peanuts, cowpeas, yardlong beans, soybeans, mungbeana, pigeonpeas, chickpeas, winged beans, lablab beans (hyacinth beans; Dolichos lablab), yam beans (Pachyrhizus erosus), and vining (tropical) types of lima beans are wholly self-sufficient if the right strain of rhizobia bacteria is present in the soil.

• Common beans (Phaseolus vulgaris) and English peas have less efficient types of rhizobia and can satisfy only about half of their N needs.

• Legumes for pasture or for green manuring, such as clovers, tropical kodzu, stylo, and leucaena are wholly self-sufficient; the pasture legumes can even supply enough extra N to satisfy the needs of any pasture grass that might be intermixed with the. (as long the, are intimately mixed and the legume makes up at least 40-60% of the mixture>.

Rhizobia Cross-Innoculation Groups: There are a number of different strains of rhizobia. A strain that forms effective nodules on one species of legume won't necessarily do the same on another. Fortunately, there is a good deal of effective cross-innoculation that occurs between a rhizobia strain and different legume species, as shown in Table 10-3.

TABLE 10-3 LEGUME RHIZOBIA GROUPINGS (Includes pulses and other legumes) Cowpea Group

Cowpeas, yardlong beans, peanuts, mung beans, lima beans, pigeon peas, yen bean (jicama; Pachyrhisus erosus), kudzu, crotalaria, velvetbeans, lab lab bean (Dolichos lablab, phasey bean (Phaseolus lathyroides), siratro (Phaseolus atropurpureus), Townsville stylo (Stylosanthes humilis), sesbanias (Sesbania bispinosa, S. grandiflora, S. sesban).

Bean Group

Common beans (Phaseolus vulgaris), including kidney beans, navy beans, black beans, and pinto beans.

Soybean Group

All varieties of soybeans.

Pea and Vetch Group

Garden peas (sweet peas), field peas, lentils, broad beans (fava beans), vetches.

Strain-specific

Each of the following legumes usually require their own specific strain of rhizobia for the most efficient N fixation: winged beans, chickpeas (Cicer arietinum), greenleaf desmodium (Desmodium intortum), silverleaf desmodium (D. uncinatum), centrosema (Centrosema pubescens)), and all varieties of perennial stylo Stylosanthes guyanensis) except Schofield.

A NOTE ON LEUCAENA (Leucaena leucocephala): Although specific rhizobia strains are now available for leucaena, the Nat. Academy of Sciences states that innoculation normally isn't needed as long as other legume trees such as Mimosa, Gliricidia, and Sesbania grow in the area.

When is innoculation necessary?: Under some conditions, the seed of certain legumes should be coated with a commercial innoculant containing the correct strain of rhizobia before planting. On the hand, innoculation usually isn't necessary in cases where a wellnodulated legume belonging to the sane rhizobia group as the intended crop has been grown on the land within the past 2-3 years. This is especially true for members of the cowpea rhizobia group. On the other band, the strain-specific legumes in Table 10-3 almost always benefit from innoculation, particularly when grown on a field for the first time. Where commercial innoculants are readily available, some extension services recommend that farmers innoculate all efficient N-fixing legumes before planting, even when they or others of the same rhizobia group have been grown on the field recently; this is looked upon as cheap insurance.

How to innoculate legume seed: Commercial innoculant is a dark, peat-based powder which contains the living bacteria and comes in a sealed packet (check the expiry date). It should be kept below 26C (79F) or in a refrigerator (but not frozen) until use. Place the seed in a basin and slightly moisten it with water to help the innoculant powder stick. (Adding a bit of molasses helps, too.) Mix the correct amount of innoculant with the seed, and plant it in moist soil within a few hours. Don't expose the innoculant or innoculated seed to direct sunlight for long or it may kill the rhizobia. Some fungicide seed treatments will kill the bacteria, too, as will applying acidic fertilizers like superphosphate in the seed furrow. If commercial innoculant isn't available, try nixing the seed with soil taken from a field that has well-nodulated plants belonging to the crop's rhizobia group.

How to check for proper modulation: Begin checking about 2-3 weeks after planting. Gently remove some plants from the soil (the nodules on some legumes such as soybeans are easily detached), and look for clusters of nodules, especial!, around the taproot. Soybeans and most legumes of the cowpea rhizobia group have round nodules varying in size from BB's (&hot) to small peas. Other legumes may have irregularly shaped nodules. When cut open with a knife or fingernail, the inside of a nodule will be Pink or reddish if actively fixing N. A greenish or white color may indicate an ineffective strain of bacteria. It's normal to find some nodules in a state of decay (each one live. for only a few weeks) with brownish interiors. Incidentally, the pink or reddish color is due to the plant's production of leg-hemaglobin (like our blood's hemaglobin) within the nodule; this compound effectively binds up and inactivates oxygen which would otherwise deactivate the enzyme complex essential for N fixation.

NOTE: Don't confuse rhizobia nodules with root knot nematode galls! Nodules can be detached from the roots; nematode gall" are swellings of the actual root itself and are white and grainy inside.

Troubleshooting inadequate nodulation: The following factors can result in little or no nodulation, even if a commercial innoculant has been used:

• If a commercial innoculant was used, check the precaution mentioned above in the section on innoculation.

• Waterlogging or flooding of the soil may seriously reduce the rhizobia population.

• The rhizobia of soybeans and some other legumes of temperate zone origin, such as alfalfa and some cloves, are sensitive to soil pH's below 6.0.

• Heavy nematode infestations will depress nodulation.

COMMON BEANS (Phaseolus vulgaris)

Basic Facts on Common Beans

Common beans are those types belonging to the botanical classification Phaseolus vulgaris and are grown largely for their dry, edible seeds. The major types are black beans, red and white kidney beans, and pinto beans. The term field beans is a broader one and refers to all types of beans within the genus Phaseolus such as lima beans (P. lunatus) and mung beans (P. aureus), and is sometimes broadened to include those of other genuses like chick peas (Cicer arietinum).

Common beans are best suited to regions with yearly rainfalls of 500-1500 mm, although they will produce good yields with as little as 300-400 mm of rain if it occurs during the crop's growth. Common beans aren't well adapted to high rainfall conditions duere to increased disease and insect problems. Compared with sorghum and millet, beans don't tolerate high heat or limited moisture very well. Good soil drainage is especially important, since they're prone to root rots. They usually grow poorly in very acid soils (below pH 5.6), because they are very sensitive to high levels of soluble aluminum and manganese. (Poor drainage also promotes managanese and aluminum toxicity.)

Spotting aluminum toxicity: Lower leaves of seedlings become uniformly yellow with dead margins; growth is stunted. If severe, plants may die shortly after emergence, but this can be confused with fungal root rot damage.

Spotting manganese toxicity: See Appendix B.

Growth habit and stages: Varieties can be bushy, semi-vining, or vining in growth habit. Time to first flowering varies from about 30-56 days after planting, depending on variety and temperature. In warm weather, early-maturing varieties can produce mature pods in as little as 70-76 days; late varieties take 90 or more days. Bush types usually mature all their pods at about the same time; on the other hand, vining types have an indeterminate growth pattern, meaning that pod maturity is not uniform and that the harvest period is spread out over several weeks or more. Indeterminate varieties can be especially advantageous where moisture conditions are unreliable, since pollination (adversely affected during drought) occurs over a much longer period than for bush types.

Yields: Bean yields in most of the Third World average around 500-700 kg/ha (mature, dry seeds). This compares with a 1600 kg/ha average in the U.S.

Nutrient Needs of Beans

Nitrogen: Beans are among the less efficient N fixers and will usually require some nitrogen; recommended N rates fall in the range of 40-80 kg/ha. Acid-forming fertilizers like ammonium sulfate and urea (see Chapter 9) may increase the likelihood of aluminum and manganese toxicities if banded near the row on very acid soils. In this case, it night be better to spread the N over a larger area.

Phosphorus: Beans have a high P requirement, and this is often the major limiting nutrient, especial!, on soils with high P tie-up ability (see Chapter 6). Rates of 40-80 kg/ha P2O5 are common and should be locally placed. On soils with extremely high P tie-up capacity, rates as high as 200 kg/ha of P2O5 have been applied by banding.

Potassium deficiencies are less conmon in beans.

Magnesium deficiency may occur in very acid soils or those high in Ca and K. See Chapter 9 for recommended rates.

Micronutrients: Beans are most susceptible to manganese, zinc, and boron deficiencies. Varieties vary in their sensitivity (See Chapter 9 for recommended rates.)

When applying an NP or NPK fertilizer at planting, the band method is the most practical, but avoid fertilizer contact with the seeds; beans are rather susceptible to burn.

Like all crops, beans respond well to organic fertilizers when sufficient quantities are available.

COWPEAS (Vigna unguiculata; syn. Vigna sinensis) Basic Facts on Cowpeas

Next to peanuts, cowpeas are the major pulse crop of West Africa in the 500-1200 a. rainfall zone but are grown in many other regions of the world, too. They have better heat and drought tolerance than common beans, but the dry seed doesn't store as well and is very vulnerable to weevil attack. Cowpeas grow well on a wide variety of soils but do require good drainage; they're also more tolerant of soil acidity than common beans. The yardlong bean (asparagus been; Vigna sesquipedalis) is a close relative and is widely grown in Asia and in parts of the Caribbean. Its soil and climatic requirements are similar to those of the cowpea. Both crops can also be used for green manuring.

In some areas, such as West Africa, both the leaves and the seeds are consumed. The cooked leaves are rich sources of vitamin A (as carotene), vitamin C (if not overcooked), folic acid, calcium, and iron.

Cowpeas have much the same growth habit and yields as common beans. Their nutrient needs are also similar, except that cowpeas are very efficient N fixers and seldom require any N.

PEANUTS (Groundnuts) Basic Facts on Peanuts

Peanuts are an important cash and staple crop in much of the Third World. Mature, shelled nuts contain about 28-32% protein and vary in oil content from about 38-50%. While the fat content of most other pulses ranges from about 2-11% of total calories (39% for soybeans d, that of peanuts is a surprising 70%.

Peanuts have good drought resistance and heat tolerance and are especially well adapted to the semi-arid tropics. They can also be grown in wetter climates if leaf fungal diseases like leaf spot can be controlled and if planted so that harvest doesn't coincide with wet weather. Peanuts don't tolerate poor drainage but do grow well in acid soils. A pH around 5.5 is optimum, but peanuts will tolerate soils as acid as pH 4.8. Soils that crust or cake are unsuitable, since penetration of the pegs (see below) is hindered.

Stages of Growth for Peanuts

Flowering begins about 30-45 days after plant emergence and is completed in another 3040 days. The flowers are self-pollinated and wither within Just 5-6 hours after opening. A plant may produce up to 1000 flowers, but only about 15-20% actually produce a mature pod.

The peanuts themselves originate at the tip of pegs which are stalk-like growths containing an ovary (future peanut pod) at their tips. The pegs begin elongating from the flowers after pollination and start to penetrate the soil about 3 weeks later. After reaching a depth of 2-7 cm, the pods begin developing rapidly and reach maturity about 60 days after flowering.

The fruits don't all mature at once, because flowering occurs over 30-40 days. Harvesting can't be delayed until all the pods have matured or heavy losses will result from pod detachment from pegs and from premature sprouting in the Spanish and Valencia types.

Yields: Average yields in the Third World range from about 500-900 kg/ha of unshelled nuts, compared to the U.S. average of 2700 kg/ha. Feasible yields for small farmers using good management are in the range of 1700-3000 kg/ha, depending on rainfall.

Fertilizer Needs and Application Methods for Peanuts

Peanuts tend to give rather unpredictable responses to fertilizer and often respond best to residual fertility from previous applications to other crops.

A special note on organic fertilizers: Organic fertilizers are very appropriate for peanuts. However, in areas where the soil-borne disease white mold (Sclerotium rolfsii) is prevalent, farmers should not leave any organic materials (manure, green manure, crop residues) on the soil surface but work them in thoroughly. Surface organic matter serves as a breeding ground for this white mold fungus.

Soil pH: Peanuts grow best within a pH range of 5. 3-e. 5. Higher pH's increase the likelihood of manganese deficiencies, while very acid conditions favor manganese and aluminum toxicities.

Nitrogen and Nodulation: If the right strain of rhizobia bacteria is present, peanuts can usually satisfy their own N needs with 2 exceptions:

• In low spots that become waterlogged, the rhizobia nay die off and the plants begin to turn yellow. An application of 20-40 kg/ha of N may be needed to carry the plants along until the bacteria become re-established in several weeks.

• In some cases (mainly on light colored, sandy soils), 20-30 kg/ha of N applied at planting has seemed to help the plants along until the rhizobia begin to fix N about 3 weeks after emergence. This isn't widely recommended.

Seed innoculation normally isn't needed if peanuts are sown on land that has grown peanuts, cowpeas, mung beans, or other members of the cowpea rhizobia group within the past 3 years. (Many legume weeds belong to this group, too.) If innoculating, be sure to use the correct strain of rhizobia, Refer to the introductory section on pulses for innoculation instructions.

How to check for proper nodulation: Refer to the introductory section on pulses.

Phosphorus and Potassium: Peanuts have an unusually Rood ability to utilize P and K left over from previous applications and don't often give a good response to direct applications unless soil levels are very low. There is good evidence that high K levels in the podding zone can increase the number of "pops" (unfilled kernels), due to decreased calcium availability.

Calcium: Peanuts are one of the few crops with a very high Ca requirement. Light green plants plus a high percentage of "pops" may indicate Ca deficiency. Calcium doesn't move from the plant to the pods, but each pod has to absorb its own needs. Gypsum (calcium sulfate) is used to supply Ca to peanuts, because it's much more soluble than lime and doesn't raise soil pH. The usual application where deficiencies exist is 600-800 kg/ha of dry gypsum applied right over the row itself (it won't burn) in a band 40-45 c. wide any time from planting til flowering.

Micronutrients: Boron and manganese are the most likely to be deficient. Borax (11% B) can be mixed with fungicide dusts or gypsum at the rate of 5-10 kg/ha of borax. Instead, plants can be sprayed with Solubor (20% B) at 2.75 kg/ha Solubor. Applications above these rates can easily injure plants.

For manganese deficiencies, manganese sulfate (26-28% Mn) can be band applied with the row fertilizer at planting at the rate of 15-20 kg/ha. Plants can be sprayed with soluble manganese sulfate at 5 kg/ha; use a wetting agent.

Hunger Signs in Peanuts: See Appendix E.

SOYBEANS

Basic Facts on Soybeans

Mature, dry soybeans range from 14-24% in oil and about 30-40% in protein. In the Western Hemisphere, soybeans are grown mainly for their oil which is used in margarine, cooking, and industry. The meal remaining after oil extraction is an important high-protein feedstuff used in livestock rations. Raw soybeans have a protein digestion inhibitor (a trypsin inhibitor) which must be deactivated by heating before they can be used for food or feed; this is done during the manufacture of soybean meal.

The largest areas of soybean production are in the U.S., Brazil, Argentina, China, and S.R. Asia. Their reputation as a high-protein crop (35-40%) has tempted many development workers to introduce them. However, be aware of the following potential problems:

• Local pulses may be better adapted to the ares. soybeans prefer a pH of 6.0-7.0 and don't tolerate acid soils well. High rainfall and humidity encourage insects and diseases.

• As with some sorghums and millets, all soybean varieties are very daylength-sensitive and have a narrow range of adaptation north or south of their origins. Flowering and pod formation are stimulated by short day lengths. If a variety is moved to an area of shorter day length (i.e. toward the Equator), flowering and pod formation will begin while the plants are still very small, and yields will be poor.

• While they are a very efficient N fixer, they require a specific strain of rhizobia different from those of other legumes. This strain (Rhizobium japonicum) is unlikely to be present in soils not previously cropped to soybeans; in this case, seed innoculation is needed.

• Soybeans often have acceptance problems as far as taste. However, new preparation methods and innovative recipes have helped overcome this.

Yields: The average soybean yield in the U.S. is about 2000 kg/ha with 2500-3000 kg/ha being common. A realistic yield goal for the tropics would be about 1800-2500 kg/ha.

Fertilizer Guidelines for Soybeans

Soybeans grow best within a pH range of 6.0-7.0. More acid soils depress the activities of its particular strain of rhizobia bacteria and can also cause manganese and aluminum toxicities, as well as molybdenum deficiencies (Mo is also needed by the rhizobia). Above pH 7.0, deficiencies of P and micronutrients (except Mo) are more likely.

Nitrogen: Soybeans can easily meet their own N nee~ds if the right strain of rhizobia is present. Fertilizer N seldom gives an economic response on properly nodulated plants. Some sources recommend applying a small amount of N (25-30 kg/ha) at planting to get the plants off to a good start, but the research evidence is against this.

Seed Innoculation: Soybeans require a very specific strain of rhizobia. Unless soybeans have been grown on the same soil within a year or two and were known to be well nodulated (see under peanuts), the seed should be innoculated with soybean rhizobia called Rhizobium japonicum. (Refer to the beginning of the pulse section in this chapter for information on how to innoculate.)

Phosphorus and Potassium: Soybeans respond well to P and K where soil levels are very low. Like peanuts, response is less likely if soybeans follow a well fertilized crop. Rates of 30-60 kg/ha of P2O5 are common. Soybeans are heavy K users, and rates range from 30100 kg/ha of K2O. P and K can be applied in a band at planting about 5-7.5 cm (3-4 fingerswidth) out from the seed row and 7.5 cm deep. Soybeans are sensitive to fertilizer burn when K is used (P doesn't burn).

Micronutrients: Although sensitive to manganese toxicity in very acid soils, soybeans are also vulnerable to manganese deficiency at pH's above 6.5. Follow the rates given for peanuts. Molybdenum is needed by both the plant and the rhizobia, but deficiencies occur only on acid soils. Liming the soil to a pH of 6.0 will usually correct deficiencies (as well as manganese and aluminum toxicities). Instead of liming, the seed itself can be treated with Mo at the same time it's innoculated. Add 15 grams of sodium or amonium molybdate to one cup (240 cc) of hot water, and then add a bit of molasses or honey. Cool and then mix the solution with 25-30 kg of seed. Mix in the innoculant, and plant ax soon as possible.