|Soils, Crops and Fertilizer Use: A Field Manual for Development Workers (Peace Corps, 1986, 338 p.)|
|About this manual|
|Chapter 1: Down to earth - Some Important Soil Basics|
|What is soil, anyway?|
|Why do soils vary so much?|
|Topsoil vs. subsoil|
|The mineral side of soil: sand, silt, and clay|
|Distinguishing ''tropical'' soils from ''temperate'' soils|
|Organic matter - a soil's best friend|
|The role of soil microorganisms|
|Chapter 2: Trouble-shooting soil physical problems|
|Getting to know the soils in your area|
|Soil water-holding capacity|
|Chapter 3: Basic soil conservation practices|
|Chapter 4: Seedbed preparation|
|The what and why of tillage|
|Common tillage equipment|
|The abuses of tillage and how to avoid them|
|Making the right seedbed for the crop, soil, and climate|
|How deep should land be tilled?|
|How fine a seedbed?|
|Some handy seedbed skills for intensive vegetable production|
|Chapter 5: Watering vegetables: When? How Often? How Much?|
|It pays to use water wisely|
|Some common watering mistakes and their effects|
|Factors influencing plant water needs|
|Ok, so get to the point! how much water do plants need and how often?|
|Some methods for improving water use efficiency|
|Chapter 6: Soil fertility and plant nutrition simplified|
|Let's Make a Deal|
|How plants grow|
|Available vs. unavailable forms of mineral nutrients|
|Soil negative charge and nutrient holding ability|
|Soil pH and how it affects crops growth|
|Important facts on the plant nutrients|
|Chapter 7: Evaluating a soil's fertility|
|Plant tissue testing|
|Using visual ''hunger signs''|
|Chapter 8: Using organic fertilizers and soil conditioners|
|What are organic fertilizers?|
|Organic vs. chemical fertilizers: which are best?|
|Some examples of successful farming using organic fertilizers|
|How to use organic fertilizers and soil conditioners|
|Chapter 9: Using chemical fertilizers|
|What are chemical fertilizers?|
|Are chemical fertilizers appropriate for limited-resource farmers?|
|An introduction to chemical fertilizers|
|Common chemical fertilizers and their characteristics|
|The effect of fertilizers on soil pH|
|Fertilizer salt index and ''burn'' potential|
|Basic application principles for N, P, and K|
|Fertilizer application methods explained and compared|
|Troubleshooting faulty fertilizer practices|
|Getting the most out of fertilizer use: crop management as an integrated system|
|Understanding fertilizer math|
|Chapter 10: Fertilizer guidelines for specific crops|
|Pulses (grain legumes)|
|Tropical fruit crops|
|Chapter 11: Liming soils|
|The purpose of liming|
|When is liming needed?|
|How to measure soil pH|
|How to calculate the actual amount of lime needed|
|How and when to lime|
|Chapter 12: Salinity and alkalinity problems|
|How salinity and alkalinity harm crop growth|
|Lab diagnosis of salinity and alkalinity|
|Appendix A: Useful measurements and conversions|
|Appendix B: How to determine soil moisture content|
|Appendix C: Spacing guide for contour ditches and other erosion barriers*|
|Appendix D: Composition of common chemical fertilizers|
|Appendix E: Hunger signs in common crops|
|Appendix F: Legumes for green manuring and cover-cropping in tropical and subtropical regions|
|Appendix G: Some sources of technical support|
|Appendix H: A bibliography of useful references|
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.