|Soils, Crops and Fertilizer Use: A Field Manual for Development Workers (Peace Corps, 1986, 338 p.)|
|Chapter 10: Fertilizer guidelines for specific crops|
This chapter gives specific guidelines for using organic and chemical fertilizers on the following crops:
Cereals belong to the Grass Family (Gramineae). Although relatively low in protein (7-14%) compared to pulses (20-39%), cereals supply about half the protein in the typical Third World diet, since they're eaten in large amounts. Per capita cereal consumption in the developing countries averages about 450-500 grams a day. Of the cereals listed above, millet is the most heat- and drought-tolerant, followed by sorghum.
NOTE: For more specific information on cereals, consult the PC/ICE Traditional Field Crops manual M-13.
Basic Facts about Maize
Mature, dry maize kernels contain about 9% protein. Yellow varieties also contain significant amounts of carotene which humans and animals can convert into vitamin A.
Depending on the variety and temperature, maize reaches physiological maturity (the stage where the kernels have ceased accumulating dry matter like starch and protein) in about 90-130 days after seedling emergence when grown in the 0-1000 m zone in the tropics. At elevation" above 2200 m, the growing period may be as long as 8-12 months.
The main difference between an early (90-day) and late 130-day) variety is in the length of the vegetative period (plant emergence to tasseling) which will vary from about 4270 days. The reproductive period (tasseling to maturity) for both types is fairly similar (about 50-58 days). The tassel emerges about 1-2 days before it begins to shed pollen. The silks emerge from the ears 2-3 days after pollen shedding has started. Pollen shedding lasts 5-8 days, and most silks are pollinated the same day they emerge. Maize is oross-pollinated (i.e. 95% or more of the kernels on an ear are pollinated by other maize plants). Under favorable conditions, all the the maize ears will have silked within 3-5 days. Shortage of pollen is rarely a problem; poor ear fill or skipped kernels are almost always caused by delayed silk emergence or by ovule abortion, both of which are caused by drought, overcrowding, or a shortage of N or P.
Maximum nutrient and water uptake occurs during the period from about 3 weeks before to 3 weeks after pollination. By silking time, maize has taken up 65% of its N, 50% of its P, and 75% of its K. Pollination is A very critical time and is readily affected by stress. Just 12 days of wilting during this period cuts yields by up to 22%, and 6-8 days by up to 50%.
At physiological maturity, the kernels still contain about 30-35% moisture which is too wet for spoilage-free storage (except in the form of dehusked ears stored in a crib). Most small farmers allow the maize to continue drying in the field on the stalk for several or more weeks before harvesting.
A large ear of maize may have 1000 kernels, but 500-600 is normal. Any shortage of water, nutrients, or sunlight during the first few weeks of kernel development usually affects the kernels at ear's tip first, making thee shrivel or abort.
Most tropical and subtropical maize varieties commonly produce 2-3 useful ears per plant under good conditions.
In contrast, most U.S. Corn Belt types are single-eared. One advantage of multiple-eared varieties (often called prolifics) is that they have some built-in buffering capacity in the event of adverse conditions and cay still be able to produce at least one ear.
Yields: Average yields of shelled grain (14% moisture) under varying conditions are shown in Table 10-1.
Top farmers in U.S. Corn Belt
Average for Third World
Feasible yield for small farmers using improved practices with adequate moisture
Fertilizer Response of Maize
Maize responds well to both organic and chemical fertilizers. However, since it's usually a staple crop grown on larger fields, most farmers are unlikely to have enough organic fertilizer to meet maize's nutrient needs. Chemical fertilizer can give excellent returns if used as part of an appropriate package of practices.
Evaluating fertilizer response: When starting from a low yield base of 1000-1500 kg/ha, yields of shelled maize should increase by about 25-50 kg for each kg of N applied up to a yield of about 4000-5000 kg/ha. Above this, the response usually drops below this ratio. The response formula applies to rates within the "low-medium-high" ranges of the Table 94 in Chapter 9. Such yield boosts will be obtained ii:
· Other nutrients like P and K are supplied as needed, soil moisture is adequate, a responsive variety is used, and there are no serious limiting factors such as insects, diseases, weeds, poor drainage, or excessive soil acidity.
· The fertilizer is applied correctly and at the right time.
EXAMPLE: In 1975/76, 168 small farmers in Zaire took part in the Programme National Mais (PNM). Yields averaged 4700 kg/ha using a fertilizer rate of 64-45-30 (kg/ha of N-P2O5-K2O). Given that average yields without fertilizer were about 1500 kg/ha, did the farmers get a good response?
SOLUTION: According to the response formula above, 64 kg of N should produce a yield increase of about 1600-3200 kg/ha for a total yield of 3100 (1600 + 1500) to 4700 (3200 + 1500) kg/ha. Since farmers averaged 4700 kg/ha, each kg of N increased the yield by 50 kg, a very good response.
N-P-K Needs of Maize: Use Table 9-4 in Chapter 9 as a guide. Maize and other Grass Family crops are more efficient K extractors than most other crops. On many loamy to clayey soils of volcanic origin, little or no K may be needed, but check to make sure. Research has shown that maize can effectively utilize up to 60 kg/ha of P2O5 when a localized placement method is used (band, hole, half circle).
Secondary Nutrients: Sulfur deficiencies in maize are very uncommon but most likely to occur in sandy, volcanic soils under high rainfall or in cases where low sulfur fertilizers have been used for several years (see Chapter 9). Magnesium deficiencies are also unusual except in very acid soils (below pH 5.5). Calcium deficiencies are very rare but can occur in extremely acidic soils.
Micronutrients: Except for zinc, maize isn't especially susceptible to micronutrient deficiencies (zing, copper, iron, manganese, boron, molybdenum). Except for Mo, they're most likely to occur above a pH of 6.8 or in sand, or organic soils (peats). Large applications of P may lower zinc uptake below the critical level in low zinc soils. To confirm a zinc deficiency, spray 10-20 plants with 6 cc of zinc sulfate dissolved in 4 liters of water plus 3-6 cc of liquid dishwashing detergent as a spreading agent (wetting agent). If zinc is lacking, new leaves will be a normal been when they emerge. (See Chapter 9 for suggested zinc rates.)
Hunger Signs in Maize: See Appendix E.
Chemical Fertilizer Application Guidelines
Apply about 1/3-lJ2 of total N at planting time, along with all the P and K. Sidedress the remaining N at knee-high stage (4-6 weeks after seedling emergence). Where leaching losses are likely to be high (heavy rainfall, sandy soils), it's best to split the total N into 3 applications: 1/3 at planting, 1/3 at knee high, 1/3 at tasseling. Under such conditions, leaching losses of K can also be a problem, so it may be advisable to split the K dosage into 2 applications (at planting and at knee-high stage).
First application: Use an NP or NPK fertilizer with a ratio that allows all the P and K to be applied, but only 1/3-1/2 of the total N. Apply the fertilizer at planting time, using one of the localized placement methods covered in Chapter 9. Don't broadcast the fertilizer. If furrow irrigation is used, be sure to place the fertilizer below the high-water mark (see Fig. 9-1 in Chapter 9).
NOTE: If the NP or NPK fertilizer is band-applied, low to moderate rates can be placed in the same furrow right along with the seeds. Don't apply more than 200-250 kg/ha of 1620-0 or 14-14-14 (or their equivalent); nor more than 100-125 kg/ha or 18-46-0 or 16-480 (all-ammonium phosphate; it releases some free ammonia which can injure seeds if placed to close).
Nitrogen Sidedressing Recommendations for Maize
· The remaining N can be applied in one or two sidedressings, depending on the potential for leaching. Under heavy rainfall or on very sandy soils, 2 sidedressings are best. If one sidedressing is Bade, it's best applied when the plants are knee-high (about a month after emergence in warm weather). If needed, the second sidedressing should be made at tasseling time.
· Use a straight N fertilizer like urea (45% N), ammonium nitrate (33% N), or ammonium sulfate (21% N). (These 3 are compared in Chapter 9.)
· Deep placement of the sidedressed N isn't necessary and and might also cause injurious root pruning. Try to get it down 2-3 cm deep, which is enough to prevent ammonia loss (especially a problem with urea) or wash-out by heavy rainfall on sloping soils. It can be banded right down the row middles, because roots fro. adjacent rows have already met and crossed each other by knee-high stage. (If the row middles are heavily compacted, root growth may not extend to them; in this case, place the band about 30 cm out from the row.
· If labor or time is short, every other row can be sidedressed at double the rate.
· If furrow irrigation is used, follow the special placement guidelines mentioned above.
Some Guidelines for Plant Population and Spacing
Overly high populations cause increased lodging (tipping over), barren stalks, unfilled ears, and small ears. Overly low densities will lessen fertilizer response. The ideal plant population varies with the variety and growing conditions (especially available moisture). Check with the local ag extension service for population recommendations. Table 10-2 provides some suggested population guidelines.
TABLE 10-2 Suggested Plant Population for Maize
Recommended Plant Populations*
Low fertility and/or moisture
Adequate fertility & moisture
Adequate moisture, high fer-
tility (100+ kg/ha N), top management, and use of a responsive variety adapted to high populations
*To achieve these densities, overplant by 15-20S to allow for the actual germination percentage of the seed and for plant mortality.
Maize has a lot of buffering capacity as far as yield response to plant population. A density 40% below optimum may lower yields by only 15-20%, because the plants respond to the greater amount of space by producing more or larger ears.
Ear size is a good indicator of how adequate the population was for the particular growing conditions. Dry, dehusked ears weighing more than 280-300 grams usually mean that the population was too low and that yields could have been about 10-20% higher. Ear size of prolifics (multiple-ear varieties) won't vary as much with population changes; instead, the number of ears per plant will decrease as population is raised.
Plant spacing: Many small farmers using hand planting will sow 4-6 seeds per hole with the holes a meter or so apart. Although this reduces planting time and labor, yields suffer due to the competition for water, sunlight, and nutrients within a small area. A good compromise is to plant 2 seeds every 45-50 cm or 3 seeds every 60-68 cm with a between row spacing of 80-90 cm. This gives a final stand of about 37,000-45,000 plants/ha. It usually isn't worth the extra effort to drill-plant the seeds (one seed very 22-25 cm) if planting by hand.
Basic Facts about Sorghum
Mature, dry sorghum seeds contain about 8-13% protein. As with maize, varieties with a yellow endosperm (the major portion of the seed aside from the germ) contain significant amounts carotene (converted to vitamin A by humans and animals).
Adaptation: Sorghum tolerates a wide range of climatic and soil conditions. It's considerably more heat and drought-tolerant than maize and also withstands periodic waterlogging. without much damage.
Grain vs. forage VB. dual-Purpose sorghums: Most sorghums grown exclusively for grain are very short (80-150 cm). They have had "dwarf" genes bred into them to reduce plant height for more manageable machine-harvesting and a better ratio of grain to leaf. In contrast, forage sorghums are much taller and have smaller seeds and a higher ratio of stalk and leaves to grain. In much of the Third World where farmers need both grain and livestock feed, dual-purpose types are used that are intermediate in their characteristics. The stalks are also used for fencing and building materials.
Forage sorghums will yield several harvests by producing new stalks and leaves from the base of the plant after being cut. Even most grain sorghum varieties have this ability (called ratooning) and can produce 2 (sometimes 3) grain harvests in warm climates where the rainy season is long enough. A new root system develops in sorghum after each harvest. Grain yield of the ratoon crop is usually about half that of the first crop.
Leaves and stalks of young sorghum plants or drought-stunted ones under 60 cm contain toxic amounts of hydrocyanic acid (HCN or prussic acid). If livestock feed on such plants, fatal poisoning may result. The HCN content decreases as plants mature and is never a problem with the seed.
Growth Stages of Sorghum
Depending on the variety and temperature, grain sorghum reaches maturity in about 90130 days within the 0-1000 m elevation zone in the tropics. However, some traditional, daylength-sensitive varieties can take up to 180-200 days, due to delayed flowering. As with maize, the main difference between a 90 day and a 130 day variety is in the length of the vegetative phase (seedling emergence to flowering). The reproductive phase (pollination to maturity) is about 30-40 days for all types. Sorghum plants initially grow much slower than maize during the first 3 weeks.
Yields: Grain sorghum has better yield stability over a wider range of growing conditions than maize. Grain yields are about the same as for maize when moisture is adequate, although high humidity increases the chance of fungal head mold. Under low moisture, sorghum will considerably outyield maize. Of the international research institutes, ICRISAT (see Appendix G) is the one most involved in sorghum production research.
Forage sorghum yields can be impressive. For example, small farmers in El Salvador can harvest 3 cuttings of forage sorghum (or sorghum-sudan crosses) during the 6 month rainy season. Total wet-weight yields of 100,000 kg/ha have been obtained. The best time to harvest in terms of the trade-off between nutritive value (higher when plants are young)) and yield (higher when plants are near maturity) is from the earl, heading stage up until the seeds have reached the soft dough stage (i.e. when they have the consistency of soft dough).
Fertilizer Guidelines for Sorghum
As with maize, sorghum responds well to either organic or chemical fertilizers, as long as moisture isn't too liaised and responsive varieties are used.
Sorghum has the same nutrient needs a. maize; however, as far as micronutrients, sorghum is most susceptible to iron deficiency rather than zinc. Unless special chelated fores of iron are used, soil applications are seldom effective. Deficiencies should be treated by spraying plants with a solution of 2.0-2.6 kg of ferrous sulfate in 100 liters of water with sufficient wetting agent to assure uniform leaf coverage. Begin spraying as soon as symptoms appear; several applications may be needed where deficiencies are severe (most likely at soil pH's above 6.8).
Sorghum seeds and seedlings are more sensitive to fertilizer burn than maize, 80 avoid fertilizer contact with the seed. Follow the same NPK guideline. as for maize. Ii more than one grain harvest is to be taken iron one planting,
apply all the P and K at planting, along with about 30-60 kg/ha of N; sidedress another 3050 kg/ha of N about 30 days later; after the first harvest, apply 30-50 kg/ha of N, followed by another sidedressing of 30-50 kg/ha 26-30 days later. Remember that actual N inn's the same as actual fertilizer (i.e. 50 kg actual N = 150 kg of 33-0-0).
Millets are a group of small-seeded annual grasses grown for grain and forage and are the main staple food grain in regions of Africa and Asia, especially where hot and semiarid. Pearl millet (Pennisetum typhoides; syn. P. glaucum) is the type most widely grown and is the focus of this section.
Pearl millet is even more drought resistant than sorghum and will outyield other cereals (including sorghum) under high temperatures, marginal rainfall, low soil fertility, and a short rainy season. It is less susceptible than sorghum to stem-boring insects, but share's sorghum's vulnerability to bird feeding losses. It lacks sorghum's tolerance to flooding but withstands soil salinity and alkali conditions fairly well (see Chapter 12).
Pearl millet varieties are grouped into early (76-100 days) and late (120-200 days) types. The late types are very daylength-sensitive and won't head (flower) until near the end of the rains; this allows them to escape serious fungal head mold and insect damage.
Average Billet yields in West Africa range fro. 300-700 kg/ha and tend to be low due to marginal growing conditions and a relative lack of research efforts which have only recently begun. Under improved management, traditional varieties have yielded 1000-1500 kg/ha, and improved varieties up to 2000-3500 kg/ha. Of the international research institutes, ICRISAT (see Appendix G) i" the one most involved with millet production research.
Fertilizer Response on Millet
Low soil moisture is a major factor limiting fertilizer response, and traditional varieties also tend to be less responsive. Studies in India by ICRISAT (Internal. Crops Res. Instit. for the Semi-Arid Tropics) showed that improved pearl millet varieties responded to N rates up to 160 kg/ha under adequate moisture, but that traditional types seldom responded well above the 40-80 kg/ha range. Follow the application methods for maize.
In terms of production, rice vies with maize as the number two cereal in the world after wheat. White rice (Billed to remove the bran layer) contains about 6.7% protein, while brown rice contains about 7.4%, along with 4-5 times more beneficial fiber. The milling process also removes 70-80% of the levels of 22 vitamins and minerals, only 4 of which are replaced in the "enriched" brands.
Lowland (Flooded) vs. Upland (Dryland) Rice
Thanks to a very efficient air transfer system from the shoots to the roots, rice can grow under flooded conditions. It can also be grown without flooding on clayey soils with slow drainage where a high moisture content can be maintined. Flooded rice is usually grown with a 5-10 cm layer of water over the field, and yields are often 50-60% higher than dryland rice for several reasons:
· Flooding provides a more ideal
temperature environment for the roots.
· It increases the availability of certain nutrients, especially P.
· It helps control weeds.
On the other hand, flooded rice production requires level land, plenty of water, a system of canals and dikes, and soils impermeable enough to prevent excessive water loos.
Transplanting vs. Direct Seeding
In the Third World, flooded rice is usually started out in a nursery seeded and then transplanted to the field about 1030 days later. Transplanting gives the plants a jump on weeds, and the confined nursery conditions "eke it easier to care for and raise healthy seedlings. Transplanting also results in better spacing and survival in the field. On the other hand, direct planting hastens maturity by 7-10 days and eliminates the labor of caring for and transplanting the seedlings. However, direct planting allows more opportunity for rat and bird damage and makes weeding more difficult if the seed is broadcast (i.e. a hand-pushed rotary weeder couldn't be used).
Stages of Growth for Rice
Rice reaches maturity in about 110-150 days, though some native varieties that are daylength-sensitive may take 6 months or more. Here is a summary of growth stages:
· Nursery stage (for transplanted rice): 9-30 days depending on weather and type of nursery.
· Vegetative phase: The period from transplanting to 50-60 days after. The plants tiller during this stage, each plant producing 3-30 or more, depending on variety and spacing. About 50-75% of the tillers eventually produce productive heads of grain. (Tillers are additional shoots produced from the base of She plant.)
· Vegetative-lag phase: Occurs in late-maturing, daylength-sensitive types; some tillers die back. Much of the difference in time to maturity between early and late varieties occurs here.
· Reproductive phase: About 35 days and includes the period from Panicle initiation to flowering. Panicle initiation occurs about 60-70 days after seeding in the case of a 130 day variety; at this stage, the panicle (grain head) is just a millimeter long and is inside the stem.
Ripening phase: From flowering to maturity; lasts about 30 days.
Nutrient Needs of Rice
High-N response vs. low-N response varieties: Nearly all native tropical rice varieties are low-N response types which are tall growing (over 1.5 m) and leafy. They respond to increasing N rates by growing still taller and producing more tillers (stems from the same plant>. This leads to lodging (tipping overt plus a mutual competitive shading by the added tillers which reduces the number of seed heads. These varieties seldom respond well to more than 30 kg/ha of N.
Most of the temperate zone rice varieties are high-N responders and are short-strawed (90-120 cm) with a high percentage of seed-producing tillers. Many of the improved tropical varieties first developed during the Screen Revolution in the 1960's are crosses between the two types. They may show a profitable response to up to 100 kg or more of N per hectare.
N-P-K Needs: The N needs of rice depend a lot on whether a low-N or high-N variety is used. The P needs of flooded rice are unusually low compared with other grain crops, because flooding increases the soil's available P. Rates on flooded soils seldom exceed 4045 kg/ha of P2O5. Responses to added K are most likely to occur on sandy soils. The rice straw itself contains about 80-90% of the plant's total K, so returning crop residues to the soil is a good way to recycle K. (The same is true for the residues of other crops.)
Secondary and micronutrient deficiencies are uncommon, although iron deficiency is occasionally found above a soil pH of 7.0.
Hunger signs in rice: See Appendix 8.
Organic Fertilizer Possibilities for Rice
Rice responds well to compost, manure, and green manure crops (see Chapter 8). The large volume of rice straw produced can be mixed with manure and composted. In the Philippines, IRRI (Internal. Rice Research Institute) has obtained good results using 2 legumes (Sesbania sesban) and Phaselous lathyroides) as green manure crops, plowing the. under at flowering stage about 3 weeks before tranaplanting rice seedlings. This 3 week interval should be observed, since freshly incorporated green manure crops release toxic decomposition products (especially under flooded conditions) that can injure the rice seedlings. (For more specifics on these 2 legumes, refer to Appendix F.)
Blue-green algae (cyanobacteria): Free-living blue-green algae can thrive in flooded soils and fix significant amounts of N. In Egypt, India, and Burma, rice soils are often purposely innoculated with this algae.
Azolla is a low-growing, aquatic fern that has a symbiotic relationship with a type of bluegreen algae called Anabaena azollae that lives in its leaves and fixes N. It also has a high protein content, making it a potential feed source for fish and animals. Azolla forms a dense eat and can be grown either as a green manure or intercropped with flooded rice. Farmers in China and Viet Nam have used Azolla in their rice fields for centuries. Recently, it has been tried in other rice zones in Asia and Africa. Trials have shown that, where adapted, Azolla can provide from 30-100% of the N needs of rice, depending on the yield goal. To be successful, Azolla needs a high level of phosphorus, plentiful water, and temperatures not much above 30°C (86°F). It can be seriously attacked by insects, especially in the tropics. The use of Azolla is labor-intensive, because the fern produces no seeds; it must be continually maintained as a vegetatively-growing plant and transferred to rice paddies in this fore.
Guidelines for Applying Chemical Fertilizers on Rice
Dryland Rice: Apply an NP or NPR fertilizer at or shortly before planting. If applied before planting, it can be broadcast and harrowed into the soil, although P rates may have to be several times higher to get the same effect as from localized placement. Deep placement of broadcast P isn't as necessary with rice, since many of the roots are found near the surface. Apply about 1/3 of the N at planting and sidedress the rest about 50-60 days later. Total N rates can go as high as 110 kg/ha when using improved varieties.
Transplanted, flooded rice: Most seedling nurseries aren't fertilized. I! P is needed, an NP, or NPK fertilizer should be broadcast and harrowed in shortly before transplanting. Urea or an ammonium N fertilizer should be used to avoid losses by denitrification or leaching (see below). IRRI recommends applying half the N before transplanting and the other half about a week before panicle initiation (about 60-70 days after seeding for a 130 day variety). On sandy soils' the N should be applied in 3 equal doses: 1/3 before transplanting, 1/3 20-30 days later, and 1/3 at panicle initiation.
Placement of N under Flooded Conditions: This is very important to understand in order to avoid heavy N losses:
· Flooded rice soils have two distinct layers: one with oxygen (aerobic layer) and one without oxygen (anaerobic layer). The aerobic layer is confined to the top 0.5-1.0 cm of soil, with the anaerobic layer beginning below it (see Fig. 10-1J.
FIGURE 10-1: Diagram of a flooded rice soil showing the fate of applied nitrogen.
When chemical fertilizer N is needed, choose urea or an ammonium fore of N, and be sure it's applied about 5 cm deep. This greatly reduces N losses fro. two sources: denitrification (conversion to N gas) and leaching. (Refer to Fig. 9-4.)
Ammonium N and urea (which is converted to ammonium) can be held and retained by clay and humus particles. (See Chapter 6.) Nitrate N is readily leached.
If ammonium N or urea are placed shallowly, they will be in the aerobic layer where there's enough oxygen for soil bacteria to change the fertilizer into leachable nitrate. The nitrate then moves down into the anaerobic zone from where it either continues leaching or is converted to N gas by oxygen-hungry bacteria and is lost to the atmosphere. (This conversion of nitrate to N gas is called denitrification and is discussed in Chapter 6 in the section on N).
However, if the ammonium N or urea is originally placed in the anaerobic zone, it will remain as ammonium and be safe from leaching or denitrification.
Numerous studies have shown that broadcasting N fertilizer over the flooded soil is only about half as effective as deeper placement in the anerobic layer.
N Application Methods for Flooded Rice Soils
When making the initial N application on flooded rice, the soil should be flooded within a couple days to prevent excessive conversion of ammonium to nitrate.
Urea is mobile for the first day or two after application until it's been converted to ammonium. Since this conversion requires oxygen, delay flooding the soil for 2 days after application to allow the conversion to occur.
Avoid off-and-on flooding of the field. Once the anaerobic layer begins drying out, it becomes aerobic.
Topdressed N (N broadcast over the water) can be worked into the soil with a handpushed rotary weeder.
Don't topdress with N when the leaves are wet. Granules that stick to the leaves will cause burned spots, and their N will be wasted if no rain occurs.
As you can see, fertilizer use on flooded rice is especially complex, so be sure to consult reliable sources of information in your country.
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).
Common beans (Phaseolus vulgaris), including kidney beans, navy beans, black beans, and pinto beans.
All varieties of soybeans.
Pea and Vetch Group
Garden peas (sweet peas), field peas, lentils, broad beans (fava beans), vetches.
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 26°C (79°F) 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.
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.
Basic Facts on Cassava
Cassava is a drought-resistant tuber crop known for its adaptation to poor soils. It's the 4th most important en;ergy source in tropics after rice, maize, and sugarcane. Though its roots are very low in protein, they are an excellent calorie source. In many areas, the leaves are also consumed (cooked) and are rich in protein (about 30% on a dry weight basis), vitamin A (as carotene), vitamin C, and folic acid. They are also a fair source of iron and calcium. Two cassava leaves provide a child with enough vitamin A for a day and cook down to a volume of only 15 cc (1 tablespoon).
The tubers and the leaves contain varying amounts of toxic hydrocyanic acid (HCN, prussic acid). Varieties are grouped into "bitter" (high HCN) and "sweet" (low HCN) types. Even the tubers of the sweet varieties must first be detoxified by peeling (most of the HCN is in the peel), followed by cooking, roasting, or sun drying. The bitter varieties are often used for commercial starch and alcohol production since they tend to be better yielders.
Cassava roots are ready for harvest from 8-1S months after planting cut sections of the stem about 25-30 cm long. For pure stands, a density of about 10,000 plants/hectare seems to be best. Although the tubers store well in the ground if harvesting is delayed, they spoil within a few days, once dug. Harvesting of the leaves will decrease tuber yields which average about 9600 kg/ha. Experimental yields of 80,000 kg/ha or more have been obtained. On poor soils, under low-moisture conditions, yields drop to about 1000-2000 kg/ha.
Cassava is unusual in that it has no critical period after establishment where drought will greatly lower yields. Surprisingly the bulk of the plant's roots are quite shallow but have the ability to proliferate in response to moisture stress. It is considered to be potentially one of the most efficient carbohydrate (energy) producers under adverse farming conditions in tropical areas and is also a relatively low-management crop.
Fertilizer Needs of Cassava
Cassava has an unusually good tolerance for very acidic soils with their high levels of soluble aluminum which would injure other crops. Although it is well adapted to lowfertility soils, it responds very well to organic and chemical fertilizers. Some agronomists feel that cassava extracts large amounts of nutrients (esp. K) from the soil, which nay lead to fertility exhaustion after several years of intensive cropping, unless nutrient additions are made. However, research has shown that, except for K, cassava actually uses fewer soil nutrients than other crops per unit of dry Batter produced. It is believed that mycorrhizae root fungi (see Chapter 1) play an especially important role in aiding the cassava plant's uptake of P on low-P soils.
NPK Needs: Excessive N will favor leaf production over tuber growth, so recommended rates fall in the range of 50-120 kg/ha. P is the nutrient most likely to be deficient in much of Latin America, but N and K shortages are more common in Africa and Asia. Recommended rates of P2O5 range from 60-130 kg/ha. Cassava has one of the highest K needs of any tropical crop, and rates range from 60-150 kg/ha of X20. Split applications of K are often recommended, especially where leaching potential is high.
Application Methods: Apply all the P and K, along with about 1/3-1/2 of the N at planting. The band or half-circle method can be used; avoid broadcasting on soils with a high P tieup capacity. The rest of the N can be sidedressed in 1-2 applications between 1 and 3 months after planting. (Under high rainfall, the K dosage should be divided into 2 applications.)
Basic Facts on Potatoes
Worldwide, potatoes rank 4th in total production after wheat, maize, and rice, although production in the tropics is often restricted by high temperatures. Potatoes are often erroneously maligned as a low-quality, "fattening" food. While not a rich protein source, they have twice the protein content of cassava or sweet potatoes, and the amino acid quality rivals that of meat. They are very low in fat (1%) and are a fair source of vitamin C. A 140 g potato (5 oz.) has about 100 calories compared with 270 for an 85 g (3 oz>) hamburger. This portion will provide about 4-5% of the daily calorie needs for an adult, along with 6% of the protein, 35% of the vitamin C, 10% of the iron, 20% of the vitamin Be, and a number of other nutrients.
In recent years, potato production has made a rapid expansion into tropical and subtropical areas for several reasons. Potatoes produce more edible energy per unit of time than almost any other crop, including maize and cassava. They are in high demand throughout the Third World and co- and a good price.
One limiting factor is the high cost of production (about $1000/hectare), largely due to the volume and cost of the required "seed" (whole or cut potatoes). Usually 1000-2400 kg/ha are needed. Recent innovative research led by the CIP (Internal. Potato Center in Lima, Peru) has devised new methods of propagation such as true potato seed (TPS), tuberlets produced from TPS or leaf bud cuttings, and tissue culture from stem cuttings.
Adaptation: Potatoes prefer cooler temperatures and will withstand light frosts. The best yields are usually obtained in areas where the mean daily mean temperature (average of high and low) doesn't exceed 20-21 °C (68-70°F) during the tuber formation period. Higher temperatures depress tuber production, since the plants tend to respire (burn up) much of the starch they produce instead of storing it. (Higher temperatures are OK during early growth). The yield-depressing effects of high daytime temperatures can be partially offset by cool nighttime temperatures. Potato varieties vary a lot in their heat tolerance. Recent breeding work spearheaded by the CIP has led to the development of more heat-tolerant varieties.
Growth Stages: Most varieties mature in about 100-125 days after planting the seed pieces. Emergence occurs about 2-4 weeks after sowing, and tuber formation begins about 3 weeks later (it has nothing to do with flowering). Full maturity is often not attained due to defoliation by fungal leafspot diseases like early and late blight. Potatoes require more skill and care to grow than most other field crops and are prone to many leaf and tuber diseases.
Fertilizer Needs of Potatoes
Potatoes are heavy feeders and respond very well to organic and chemical fertilizers, especially since their root system is small and tubers develop over a relatively short time period. They prefer a soil pH of 5.0-6.5 and are fairly tolerant of acidity. One way of controlling potato "cab disease (a soil fungus; Streptomyces scabies) is to maintain the pH below 5.5.
Nitrogen: Overfertilization with N favors top growth over tuber growth, but most improved varieties will show a good response up to 110 kg/ha of N or ave. Recommended rates range from about 50-80 kg/ha for Third World small farmers.
Phosphorus: P increases the number rather than the size of tubers, shortens maturity, and improves quality. Rates as high as 100-200 kg/ha of P2O5 are recommended for low-P soils and should be banded, not broadcast.
Potassium: Potatoes have especially heavy K needs, and even high K soils may become depleted after a few years of potato growing. Rates for medium K soils run about 50-100 kg/ha of K2O. with even higher amounts for low-K soils. At K,0 rates much above 50-60 kg/ha, potassium sulfate should be used instead of potassium chloride (muriate of potash), because excess chloride lowers the starch content ~nd quality of the tubers.
Magnesium deficiencies are sometimes a problem in acid soils below pH 5.5. When liming, use dolomitic limestone. Epsom salts (magnesium sulfate) can be applied to the soil at 200250 kg/ha, or plants can be sprayed with a solution of 2.0-2.5 kg of epsom salts in 100 liters of water.
Hunger Signs in Potatoes: See Appendix B.
Application Methods for Chemical Fertilizer: Apply 1/3-l/2 of the N and all of the P and K at planting in a band about 5-7.5 cm (3-4 fingers-width) to the side of the seed row and 6 cm below its depth. Sidedress the regaining N about 40 days later in a band about 25-30 cm out from the row. The N can be worked into the ground a bit by combining the sidedressing with a weeding or hilling-up operation.
Sweet potatoes are an excellent energy source and are also low in fat like other root crops. The orange-fleshed varieties are very high in vitamin A (as carotene]. One average sweetpotato (5 cm x 12.5 cm) provides about half the daily adult requirement of vitamin C, along with twice the vitamin A needed (if orange-fleshed). In many areas, the leaves are also consumed either fresh or cooked and are good sources of vitamin A, vitamin C, folic acid, iron, calcium, and potassium; they also contain a fair amount of protein.
Unlike Irish potatoes, sweet potatoes are a ware-weather crop; the roots are ready for harvest in about 4-5 months. ID the tropics, planting is usually done with vine tip cuttings about 30-40 cm long. About 2/3 of the cutting's length (at least 4 nodes) should be covered with soil and the remaining third left exposed. (Tubers originate from the buried nodes.) Cuttings are hardy and begin rooting in just 2-3 days. A recent study in Puerto Rico showed that pre-rooting the cuttings by holding them for 2 days before planting increased the number of tubers and the yield; however, removing the the leaves from cuttings to reduce transpiration decreased yields. Plantings can also be started by planting "slips" (young plants produced by planting tubers densely in a nursery bed). Vine tip cuttings have the advantage of not spreading soil-borne sweet potato diseases.
Fertilizer Needs of Sweet Potatoes
Both organic and chemical fertilizers give good responses. Excessive amounts of N will favor top growth over root growth, so rates of 50-80 kg/ha are recommended. Phosphorus rates range from about 40-70 kg/ha of P2O5 5. Sweet potatoes are heavy K feeders; on low-K soils, rates of 80-130 kg/ha of K2O are recommended. Boron deficiency sometimes occurs and can be treated by mixing 5-6 kg/ha of borax (11% boron) with the NPK fertilizer; this equals only 0.5-0.6 grams of borax per sq. meter. Higher rates may cause plant injury.
Apply 1/3-1/2 of the N and K at planting tine, along with all of the P. Sidedress the remaining N and K in 1-2 applications about 1-2 months after planting. Use the band method at planting. If planting is done on high ridges, the NPK fertilizer can be applied in a band running right below the future plant row; the ridge can then be built right over it and will separate the fertilizer with a enough soil HO that burning won't occur.
Hunger Signs in Sweet Potatoes: N deficiency causes the leaves to turn light green to yellow, and the vines become deep red. P hunger causes dark green leaves that have a purpling over the veins on the backside of the leaves. K hunger begins with a yellowing and bronzing of the leaf tips and margins which gradually moves toward the center. Bee also Appendix E.
Most vegetables are very low in calories but have a high nutrient density in terms of vitamins and minerals. The dark-green leafy vegetables like kangkong, bok choy, amaranth, and collards are excellent sources of vitamin A (as carotene), Vitamin C, B vitamins, calcium, iron, and potassium. (However, amaranth, spinach, Swiss chard, and beet greens contain oxalates which bind up much of their iron and calcium; they can be partially deactivated by cooking). Dark-green leafy vegetables also provide significant protein.
The deep-yellow and orange vegetables like cantaloupe, carrots, and Hubbard squash are excellent sources of vitamin A, vitamin C, and potassium. For example, one large carrot contains twice the adult daily requirement of vitamin A. Aside from preventing vitamin A deficiency which leads to blindness and death from infections, carotene is now known to play an important role in preventing several type" of cancer.
The Asian Vegetable Research and Development Center in Taiwan is the international research center most involved with tropical and subtropical vegetable production. The AVRDC has developed a number of heat-tolerant varieties of cool-season vegetables like cauliflower and also work" on disease resistance and general production practices. (Bee Appendix G for the address.)
General Fertilizer Needs of Vegetables
Since most Third World small farmers grow vegetables on small plots, this is an ideal situation for using organic fertilizers (see Chapter 8), and responses are excellent. However, in cases where organic fertilizers are in short supply, chemical fertilizer can be used if resources permit and will usually be very cost-effective on well--managed plots.
NPK Needs: The kind and amount of fertilizer needed varies a lot with the soil, the vegetable, and other key factors covered in Chapter 9. Table 10-4 gives a range of NPK rates from a number of research and extension sources worldwide.
TABLE 10-4 Co "on NPK Rates for Vegetables
Susceptibility of Vegetables to Secondary Nutrient Deficiencies
Cabbage, eggplant, pepper, tomato, cucumber, watermelon
Asparagus, onions, and the Crucifer family (cabbage, collard, broccoli, turnips, bok choy, kale, cauliflower)
Response of Vegetables to Micronutrients When Soil Levels are Low.
CHEMICAL FERTILIZER APPLICATION GUIDELINES FOR VEGETABLES
The band method of application is very suitable for direct-seeded vegetables like turnips, radish, mustard, bok choy, leaf lettuce, spinach, Chinese cabbage, and okra. The halfcircle method works well with cucurbits (cucumber, squash, etc.) and transplants. Apply all the P and K along with 1/3-1/2 of the N at planting; sidedress the remaining N in one or more applications, depending on time to maturity and harvest method. For example, leafy greens, like leaf lettuce, spinach, Swiss chard, and bok choy, can be harvested either all at once or picked a few leaves at a time over a month or two (new leaves keep emerging from the center). In the latter case, 2 or 3 sidedressings can be made at 3-week intervals.
Tomatoes, peppers, eggplant, cabbage, collard, broccoli, cauliflower, head lettuce, and onions usually do better if first started out in a nursery seedbed, seedbox, or small containers and then transplanted to the field 3-6 weeks later.
Nursery Seedbed or Seedbox: In most cases, manure or compost will supply enough nutrients for the nursery stage. (See Chapter 4 for how to prepare a nursery seedbox mix.) However, there are 3 cases where chemical fertilizer might be needed:
· If the soil has been heat sterilized before planting, there may not be enough beneficial bacteria left to convert the organic N in the manure or compost into available N. However, fresh manure has a good amount of available N.
· If the manure or compost is of poor quality due to poor storage and exposure.
· If plants become N deficient while in the nursery. Watering is often high enough to cause lots of leaching.
If NPK fertilizer is needed, broadcast the equivalent of 60-80 grams/sq. meter (600-800 kg/ha) of 10-20-10 or 10-30-10 and mix it thoroughly into the top 10-15 cm of soil. Don't exceed 60-80 kg/ha of N or plants may become overly succulent and more prone to damping-off fungus disease. Fertilizers with a 1:2:1 or 1:3:1 ratio work best since they allow you to apply sufficient broadcast P without exceeding N or K rates.
NOTE: Disregard the amount of NPK applied in the nursery when calculating NPK rates needed' fro. transplanting onward.
N Deficiency in the Nursery Seedbed: If the plants turn yellow from N deficiency, dissolve a straight N fertilizer in water and apply it over the bed at the rate of 30 kg/ha of N which equals:
15 grams ammonium sulfate (21-0-0) per sq. meter
10 grams ammonium nitrate (33-0-0) per sq. meter
7-8 grams urea (45-0-0) per sq. meter.
Water plants with plain water afterwards to wash off any fertilizer solution from the leaves. If plants are being "hardened" in preparation for field setting (usually done the last 7-10 days before setting), N fertilizer will be counterproductive.
Using a Starter Solution for Transplants: Pouring a liquid starter fertilizer solution around the base of the plants after setting them will help get things off to a good start. Manure tea (see Chapter 8) or chemical fertilizer can be used for this. If using chemical fertilizer, here are the guidelines:
· Since P is the nutrient most involved in stimulating root regeneration and development, choose a formula with a good ratio of P in it such as 12-24-12, 18-46-0, or 10-30-10. Some N is helpful too, since it helps in the uptake of N.
· Except for a few like 18-46-0, most granular NPK fertilizers dissolve poorly in water. Grinding or mashing is helpful and will improve solubility.
· Dosage: Mix up 8-15 cc of fertilizer per liter of water and apply about 1 cup (240 cc) around the base of each transplant after setting.
· The starter solution only supplies enough nutrients for the first week or so of growth; additional organic or chemical fertilizer will be needed.
· NOTE: As in the case of fertilizer applied to a nursery seedbed, this starter fertilizer application is not counted when calculating overall NPK totals.
Applying Solid Fertilizer at Transplant Time: Use an NPK fertilizer that supplies 1/3-1/2 of the total N and all the P needed. If leaching is likely to be high, only about 1/3-1/2 of the K should be applied. The half-circle method works very well for transplants and should be made about 7.5-10 co (about 4 fingers-width) out from the stem and 5-10 cm deep.
How to Sidedress Nitrogen: Review the sidedressing guidelines in Chapter 9 before proceeding. Here are some more specific suggestions for vegetables:
· Long-duration crops like indeterminate tomatoes, eggplant, and peppers may require 3-4 or more sidedressings at 3-4 week intervals.
· Medium duration crops like broccoli and cauliflower will normally need 1-2 at 3-4 week intervals.
· Apply about 30 kg/ha of actual N per sidedressing as a rough figure. It's more accurate to subtract the at-transplanting dosage from the total N and then divide the result by the number of sidedressings needed.
· Apply the N in a band or half circle about 20 cm out from the plant; cover it lightly with soil. (This can be done by weeding with a hoe following the application.)
(Banana, Mango, Papaya)
Fruit crops are often a very casual or neglected part of agriculture but can play several useful roles on small fares and in gardening projects:
· Nutrition: Fruits can be valuable sources of energy, vitamin, and minerals. Some like mango and papaya are rich sources of vitamin A (as carotene) and vitamin C. Citrus fruits and guava are high in vitamin C. Nearly all fruits provide large amounts of potassium, an important body electrolyte. Even the leaves of some types like jujube and papaya are eaten and provide vitamins A and C, 8 vitamins such as folic acid, and minerals such as calcium and iron.
· Income: Fruit crops can be a good income producer and merit inclusion in most gardening projects.
· Other functions: Some like cashew and jujube (Zyziphus mauritiana) can be part of a living fence or windbreak.
Basic Facts on Bananas
An average size banana has about 100 calories and is about 70% water. Bananas are a fair source of vitamin C and are very high in potassium; they are excellent as a carbohydrate source but are low in protein.
Bananas vs. Plantains: Plantains are close relatives of bananas but larger and with a much lower sugar content when ripe. They should be cooked before eating.
The banana plant's stem is called a pseudostem, since it's really formed from rolled-up leaves growing out of a true stem located underground in the con (i.e. a core is an underground stem). A new feat appears every 10 days until the terminal bud (flower) emerges at 7-8 months; harvest follows in about 80-90 days.
Most of the plant's roots are found in the top 15 OD of soil, though some penetrate 60 to 90 c.. The roots may grow out laterally as far as 5 meters. Lateral roots grow out fro. the main roots and are the only ones that absorb nutrients and water. Since these feeder roots are scarce close to the stalk, fertilizer should be applied about 60 cm or more out from the base.
Established plantings regenerate themselves by producing several "suckers" per mother plant; the mother plant produces just one crop. In establishing new plantings, either corms from suckers or the suckers themselves are used. "Sword" suckers which have slim narrow leaves are preferred for propagation; "water" suckers (broad, wide leaves) are considered to be inferior, due to smaller corms.
Adaptation: Bananas prefer a warm, moist climate with about 1500-2500 mm of rainfall fairly well distributed. The, prefer full sun but have a slight tolerance to shade. Good drainage is important; the plants can tolerate only a da' or two of flooding. HiBh winds (above 65 km/hr) cause considerable damage by tearing leaves and uprooting plants. Although tolerant of a soil pH ranging from 4.5-8.0, bananas do beet at about 6.0-7.5. Very low pH can promote Panama disease (a soil-borne fungus; Fusarium oxysporum).
Yields: A good bunch will contain 8 hands (fruit clusters) with 15 fingers (fruits) each and weigh about 20 kg. Yields range from about 10,000-30,000 kg/ha when planted as the sole crop.
Fertilizer Needs of Bananas
Bananas use high amounts of N and K, though their P needs are moderate. Bananas benefit from high levels of soil organic matter. The planting hole can be partially filled with rotted manure or compost. Locating a compost pile adjacent to banana trees will provide shade, and any leached nutrients from the pile will benefit the plants. Mulching around the plants is very beneficial.
Feasibility of Chemical Fertilizers: When bananas are grown in the back yard or as part of a mixed garden (see Chapter 8), there is seldom any need for applying chemical fertilizers. Compost and manure can easily satisfy the nutrient needs of the plants.
Nitrogen: N-deficient plants have a pale yellowish-green color. N stimulates faster growth, earlier flower emergence and maturity, greater leaf area, and increased fruit size. N recommendations range from about 150-350 kg/ha applied in 3-10 applications, depending on leaching potential. About 80 grams actual N per plant is considered the minimum for commercial plantings, and often 100-200 grams actual N is used. All the N should be applied before flowering, since it's important to stimulate early rapid growth. Research has shown a good correlation between the area of the third leaf and total bunch weight. Later N applications seem to promote ''openhandedness" of the bunch. Where regular spraying with fungicides is done, N can be supplied foliarly using urea (600 grams urea per 100 liters water for plants 1-2 months old and up to 3 kg/100 liters on older plants. One study showed that 65% of the urea was absorbed through the leaves in just 25 minutes.
Phosphorus: P needs are relatively low compared with N and K. Most recommendations are in the range of 50-85 kg/ha of P2O5 or about 50-100 grams P2O5 per plant. P can be applied in one application at or near planting or at various times as part of an NPK fertilizer. P deficiency causes a premature drying of the lower leaves.
Potassium: Where deficient, added K greatly increases yields and pseudostem growth, improves fruit quality and storage life, and promotes disease resistance. Moderate K deficiencies cause yellowing around the outer edges of the leaves; more severe hunger causes the leaf tips to turn reddish-brown and die. K hunger is also associated with the disorder called "premature yellowing" of the leaven. Most recommendations range from 80250 grams of KsO per plant or about 110-380 kg/ha of K2O. K can be applied in 3 or more applications, depending on leaching potential.
Magnesium: Deficiencies are common in acid soils, especially where high amounts of K are used. Applying 200250 grams of dolomitic limestone per plant will cure deficiencies. Mg hunger produces a broad bend of yellowing along the edges of the lower leaves.
Iron. Zinc. and Manganese deficiencies can occur at soil pH's above 6.8.
Molybdenum deficiency has occurred in Honduras on highly-lesched acid soils. Raising the pH is often effective at controlling Mo deficiency if the soil is very acid; otherwise, Mo should be applied.
Application Methods for Chemical Fertilizers: Young plants should have the fertilizer applied in a 30-50 co wide band around the plant, starting about 30-40 cm from the stem. The band can be moved out to about 60-90 cm fro. the stem as the plants grow. Cover the fertilizer with about 3-5 co of soil, but be careful not to injure the shallow roots.
Associated Growing Practices for Bananas
Much banana growing by small farmers is done on a very casual basis. Diseases, nematodes, insects, and overcrowding are co. on. Don't count on fertilizer alone to boost yields under such conditions. Some possibly appropriate improved practicer are listed below:
Proper selection and preparation of planting materials. Trimming the cores and sterilizing then with hot water or chlorox and water will help control neeatodes and diseases and prevent their spread to new ground.
· Mulching to suppress weeds, conserve water, and add organic matter.
· Pruning out the excess suckers.
· A spray program for insects and diseases.
· Cutting off the terminal bud and dipping the cut in a fungicide solution to prevent decay. This can add about a kilogram to bunch weight.
· Covering maturing fruits with clear polythene baga with air holes; it can speed up maturity by 2 weeks and increase yield up to 20%.
Mango is a widely-adapted tropical and subtropical evergreen tree that can grow as tall as 15-25 m with a spread of up to 15 m or more (smaller dwarf varieties are also available). It is related to cashew, pistachio nut, and poison ivy. Mango does well on a variety of soils as long au drainage is good. It prefers an annual rainfall of at least 450-1000 mm distributed over at least 6 months but likes a pronounced dry season for flowering and fruiting. It does well within a pH range of 5.5-7.5. Mango has fair drought-resistance, thanks to a very deep taproot.
One medium mango (200 grams) supplies more than twice the daily adult requirement of vitamins A and C along with about 150 calories of energy. The fruit can be eaten fresh, juiced, or made into preserves and chutney
Most of the world's mangos are grown from aced, but the fruit tends to be stringy and variable in quality. The best varieties are produced by budding or grafting to diseaseresistant rootstocks. Grafted varieties begin bearing at 4-5 years of age (seedling mangos take longer), and the fruit is ready for harvest about 100-140 days after flowering. They have an economic life as long as 40-80 years. An average yield under good management is about 400-ff00 fruits/tree.
Fertilizer Needs of Mango
Mango responds well to organic or chemical fertilizers. Mulching around the trees is a very beneficial practice. Nitrogen helps stimulate flowering and vegetative growth and lessens the tendency of alternate bearing (fruiting every other year).
Where chemical fertilizer is used, yearly rates per tree run about 0.5-1.6 kg N, 1.5-3.2 kg P2O5, and 0.5-1.0 kg K2O. The N should be split into 4-8 applications depending on rainfall; K should also be split where leaching potential is high (sandy soils, high rainfall). If more convenient, an NPK fertilizer can be applied in split applications. Micronutrient sprays of copper, zinc, iron, and manganese are applied where needed.
Application method: For young trees, chemical fertilizer is spread uniformly over the root area from near the trunk to 60-90 cm beyond the edge of the leaf canopy (called the drip line). To avoid root damage, work it on no deeper than 3-4 cm, and apply it evenly. P is utilized fairly well with this method. Unlike uniform broadcasting over the entire soil surface, this method confines it to a small area, somewhat like a localized placement method (see Chapter 9).
Papaya is a short-lived, fast-growing perennial tree about 4-6 meters tall. It does well on most soils as long as drainage is good; it won't tolerate flooding for more than 48 hours. Papaya needs a minimum of 1000-2500 mm annual rainfall fairly well distributed; otherwise, supplemental watering is needed. It has a weak, hollow stem which makes it susceptible to wind damage. Papaya prefers a soil pH of about 6.0-7.0. It is susceptible to nematodes.
There are 3 kinds of papaya trees: male, female, and hermaphroditic, but flowers may change fro. female to male under stress. Male trees seldom produce, and their fruit is misshapen and of poor quality. Fruit production on female trees requires the presence of a male tree for pollination. Hermaphroditic varieties such as the Solo group are selfpollinating. (Solo papayas produce grapefruit-size fruit and are popular for the export market; however, all Solos except the Cariflora variety are very susceptible to ring spot disease and several other viruses which are co on in Central America and the Caribbean. Cucurbits such as squash and cucumber are an alternate host for these viruses.)
Papaya is propagated from seeds which sprout in about 10-15 days and can be sown in pots or directly in the ground. Flowering occurs about 5 months later, and fruit is ready to harvest at about 9-11 months of age. Yield per tree is about 60-90 kg/year. Trees have an economic life of about 3-4 years.
Papaya fruits vary in size, shape, and color; the most co "on flesh colors are yellow or reddish. The outside shin ripens to a golden color, starting from the stem. One medium papaya (300 grams) supplies 100% of the daily adult requirement of vitamin A and 3-5 fold the daily vitamin C needs. It can be eaten raw or preserved. The leaves and the pulp of unripe fruit contain a good amount of an enzyme called papain, useful as a meat tenderizer and digestive aid. The leaves can also be eaten and have all the benefits of other dark green leafy vegetables, being rich in vitamins and minerals (including calcium) and also in protein. In some areas, papaya leaves are used as a diarrhea remedy.
Like most crops, papaya will respond well to organic fertilizers. However, farm manure shouldn't be mixed with the soil in the planting hole since it is known to favor the development of Pythium root rot. Papaya responds well to a continuous supply of nitrogen; P and K help promote rapid growth and early flowering; K is especially important after flowering.
In Australia, 8-12-6 fertilizer is recommended at 700 grams/tree the first year and 9001350 grams/tree in the following years. The dosage is split into 4 applications per year. In South Africa, 100 grams actual N is recommended per tree the first year and 200 grams per year after that; P2O5, is applied once at about 100 grams per tree (about 450 grams of 0-20-0).
Application method: The fertilizer should be broadcast in a wide band from near the trunk outward about 60-90 cm during the first 6 months, expanding to 1.5 meters as trees grow. Work it in shallowly 2-3 cm to avoid root damage. The P applied in this manner isn't subject to as much tie-up as with regular broadcasting, because it's still confined to a relatively small area.
Some hunger signs in papaya: Yellowing of the bottom. leaves may indicate N deficiency. P deficiency produces dark-green leaves with a reddish-purple discoloration of the leaf veins and leaf stalks.
During the wet season, well-managed tropical pastures can provide enough feed for normal growth of calves and beef cattle and for production of 1-2 gallons (3.75-7.5 liters) of milk daily per cow. Supplemental feeding with high-energy sources like maize, molasses, etc. will be needed for higher milk production or more rapid fattening. Fro. 2.5-5 460 kg cattle or 3.75-7.5 275 kg stock (or about the same number of dairy cattle) can be carried per hectare. Once the dry season sets in, both the amount and feed value of the pasture seriously declines, and even well-managed pastures can usually satisfy only the maintenance requirements of cattle (no growth or milk production). Under irrigation or well-distributed rainfall, tropical pastures should be able to produce about 550-1100 kg of live-weight gain per hectare yearly without supplemental feeding.
Tropical grasses like elephant (rapier), guinea, pangola, bermuda, pare, and star give excellent responses to fertilizer, especially N. However, if overall management of the pasture and animals is low, it's questionable whether chemical fertilizer would be costeffective.
N is the most important nutrient in terms of amount, and rates up to 300 kg/ha or more yearly may be profitable under good management and well-distributed rainfall (or irrigation). Aside from increasing the pasture yield, N also increases the protein content to varying degrees, depending on the amount applied, type of grass, rainfall, and stage of maturity at which the pasture is grazed.
To reduce leaching losses, N should be applied in several applications. In humid areas without a pronounced dry season' N is usually applied 4-6 times a year. In areas with a dry season, 3-4 applications should be made, all of thee during the wet season, unless irrigation is used. Work in Puerto Rico has shown that applying 110 kg/ha of N to recentlygrazed pastures 6-8 weeks before the start of the dry season will greatly increase the amount and nutritive value of the grass carried over into the dry season. With this method, grazing should be deferred immediately following the N application until the dry season begins. Guinea grass produce. an especially good standing hay with this method.
If urea is used, up to 30-35% of its N may be lost as ammonia gaff (refer to Chapter g); this may be partly offset by urea's typically lower price compared to other N sources; if urea is applied within a few hours before rainfall or irrigation, ammonia losses will be minimized.
Phosphorus: P can be applied once a year, since it won't leach. Rates of 60-90 kg/ha of P2O5 are common.
Potassium: Up to 220 kg/ha of K2O may be needed on low-X soils under intensive management and year-round grass growth. Grasses tend to take up K in excess of their needs, so it's a good idea to split the dosage into 2 or more applications to avoid this "luxury consumption".
Sulfur: A sulfur-bearing fertilizer should be included in the fertilizer program, especially on sandy soils under high rainfall. Ammonium sulfate, single superphosphate, and ammonium phosphate sulfate (16-20-0) are good 8 sources. It's a good idea to supply about 20 kg/ha of sulfur per year (60 kg. sulfate).
Calcium and Magnesium: Remember that urea or ammonium sources of N have an acid effect on the soil. Liming may be needed after a few years of continued N applications. Soils with a low exchange capacity (negative charge) will drop more quickly in pH. Lime can be broadcast over the pasture. Use dolomitic limestone, or supply magnesium in another for. to avoid deficiencies. Cattle are very sensitive to Mg deficiencies which can be caused by applying high rates of K without supplemental Mg. In cases where both the soil and the liming material are low in Mg, it may be necessary to apply about 100 kg/ha of magnesium oxide or 400 kg/ha of magnesium sulfate ((epsom salts) yearly in 2 applications.
Micronutrients: Deficiencies aren't likely, except in very leached or sandy soils or at pH's above 7.0 (except for molybdenum).
Value of "Self-fertilization" of Pastures by Cattle
Roughly 80% of the NPK and other nutrients in the feed are returned in the manure, which would seem to make fertilizers largely unnecessary for pastures. However, animals do a poor fob at uniformly distributing the manure over the pasture; research has shown that only about 15% of the pasture is actually covered per year under typical stocking rates. A good deal of the N is lost as ammonia gas or by leaching.
Grass-Legume Pastures in the Tropics
Temperate-zone legumes like alfalfa and most clovers aren't well adapted to tropical humidity or very acid soils. Unlike temperate-zone pastures, few tropical pastures contain legumes. Legumes can significantly improve the feed value of a pasture, because they're higher in protein than grasses; they also decline more slowly in feed value as they increase in height between grazings. Legumes can also supply all their own N as well as that needed by the grasses with which they're grown in association.
Relatively little research has been done with tropical pasture legumes, but things are improving. One problem is that most tropical legumes have trouble competing with the rapid growth rate of most tropical grasses and get shaded out. Some are sensitive to overgrazing or aren't very palatable. However, tropical kudzu (Pueraria phaseoloides), Centrosema pubescens, siratro (Siratro atropuroureus), and several others have been grown successfully in combination with tropical grasses like guinea, star, and molasses grass. Townsville stylo (Stylosanthes humilis) is a self-regenerating annual (it reseeds itself) that can be easily established and maintained with a variety of grasses. Leucaena (ipil-ipil, Leaucaena leucocephala) is a perennial tree/shrub that can be grown in rows in a pasture and used for browsing. (These and other pasture legumes are described in Appendix F; leucaena is also discussed in the agro-forestry section in Chapter 8.) Consult with a pasture specialist concerning recommended grass-legume mixes for your area.
Fertilizing Grass-Legume Pastures: Since the leguee fixes enough N for itself and the grass, no N fertilizer is needed. In fact, adding N will favor grass growth and eventually shade out the legume. Adequate P and K as well as sulfur are needed to maintain a good proportion of legume to grass. Compared to grasses, legumes are weak K extractors and are also susceptible to molybdenum and boron deficiencies.
USE THE "PACKAGE" APPROACH FOR PASTURE MANAGEMENT
It takes much more than just fertilizer for successful beef and milk production. Other practices like good grazing management, good stock, disease control, weed control, supplemental feeding, and worming are just as important. Some of these are summarized below.
NOTE: The following data is not designed to make you qualified to work with cattle but, rather, to give you some initial background in this area to facilitate further investigation and discussion cattle and pasture specialists in your country.
As grasses regrow after being grazed or cut, they decline in feed value as they nature, especially in protein. Tropical conditions favor rapid grass growth and maturity, and most grazed grasses may be unable to supply enough protein after only 4-5 weeks, even when fertilizer N is used.
Under low-management conditions, cattle are usually continuously confined to one pasture at a low stocking rate. The pasture's rapid growth outstrips the cattle's ability to consume the grass before it's become overly mature and low in quality. For example, a study in Trinidad showed that the crude protein content of pangola grass dropped from 15% 10 days after grazing began down to 4.2% 42 days later (dry-weight basis).
Rotation grazing consists of dividing up the pasture into 4-6 paddocks and putting all the cattle in one paddock at a time. Each paddock should be of a adze that allows the cattle to graze down the grass in 4-7 days before they are coved to the next one. About 3 weeks rest is needed between grazings to obtain sufficient regrowth. Longer periods may be needed during cooler weather and shorter periods during more rapid growth. Guinea grass should be grazed down to about 20 cm, and pare, elephant, and pangola down to 1015 cat N fertilizer can be applied after each grazing. Over-grazing will use up stored food reserves in the roots and weaken the stand.
Dry Season Feeding: Hay and Silage
Forage quantity and quality decline disastrously during the dry season. Cattle often lose a good part of their wet season weight gains during the dry months and may take 4-6 years to reach slaughter weight (360-550 kg). It's possible to reduce this to 2-3 years, largely through the use of hay or silage for supplemental dry-season feeding. Most low management cattle raisers in the tropics have too few animals per hectare to fully utilize all the lush wet season growth, but too many in terms of the scant amount of dry season forage. Making hay or silage out of the wet season surplus growth is one answer. Silage making is usually more feasible than hay making during the wet season. (About 2000 kg of water has to be evaporated from freshly cut grass in order to make 1000 kg of hay!) Peace Corps Volunteers in El Salvador helped to establish a successful silage program with small cattle-growers, using sorghum-sudangrass. Yields averaging around 100,000 kg/ha have been obtained from taking 3 cuttings during the 6-month rainy season. They have also made good-quality pangola, stargrass, and jaragua hay at the end of the rainy season.
Provide Minerals for Cattle
Except for salt, cobalt, iodine, and copper, livestock can usually get all their essential minerals from well managed pastures. Salt licks containing trace minerals should be supplied. Young cattle need about 20 grams of salt per da' and older ones about 30 grams. kidding 10 grams copper sulfate and 10 grams cobalt sulfate per 16 kg. of iodized salt will provide a satisfactory mineral mix for cattle grazing on fertilized pastures.
Control Weeds: Weeds compete for "pace, water, light, and nutrients with pastures; some may be poisonous as well. Broadlead weeds are the most common. Herbicides nay be needed, but first se sure the particular chemical is registered for use on pastures; follow label instructions closely to avoid injury to the pasture or contamination of meat and milk.
Keep Animals Healthy: Cattle raisers should follow the recommended vaccination schedule for their area; brucellosis, anthrax, blackleg, and others may be needed. Periodic worming is also essential, as well as tick control.
FOR FURTHER INFORMATION: Several international research institutes such as Winrock and CIAT are involved in research/extension efforts with pasture and cattle management in the Third World. See Appendix a for their addresses.