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close this bookTraditional Field Crops (Peace Corps, 1981, 283 p.)
close this folderSoil fertility and management
View the document(introduction...)
View the documentDetermining fertilizer needs
View the documentFertilizer types and how to use them
View the documentChemical fertilizers
View the documentBasic guidelines for applving chemical fertilizers
View the documentDetermining how much fertilizer to use
View the documentRecommended fertilizer rates for the reference crops
View the documentFertilizer recommendations for specific crops
View the documentLiming
View the documentWater management

Recommended fertilizer rates for the reference crops

The most profitable rate of fertilizer use for the small farmer depends on management ability, capital available, limiting factors, soil fertility level, type of crop, expected price and cost of fertilizer.

Small farmers should usually aim for maximum return per dollar spent. This means using low to moderate rates of fertilizer because crop yield response is subject to the law of diminishing returns.

Since the efficiency of fertilizer response declines as rates go up, the small farmer with limited capital is usually better off applying low to medium rates of fertilizer. He or she will end up with a higher return per dollar invested, be able to fertilize more land, and have money left over to invest in other complementary yield-improving practices.

As a farmer's capital situation improves, higher rates of fertilizer may be justified as long as investments in other worthwhile practices are not sacrificed. Another factor to consider is that fertilizer can reduce the land and labor needed to produce a given amount of crop, thus cutting costs and allowing for more diversity of production.

Some General Guidelines for Low, Medium and High Rates Of N-P-K

Keeping in mind the many factors that determine optimum fertilizer rates, Table 8 provides a very general guide to LOW, MEDIUM, and HIGH rates of the "Big Three" for the reference crops based on small farmer conditions and using localized placement of P. The "high" rates given here would be considered only low to medium by most farmers in Europe and the U.S. where applications of 200 kg/ha of N are not uncommon on maize and irrigated sorghum.

There are several important qualifications to Table 8:

· YOU MUST CONSIDER THE FERTIILITY LEVEL OF THE SOIL as well as the type of crop. A soil high in available K would need little or no fertilizer K. Most cropped soils tend to be low in N and low to medium in P, but K deficiencies are less common. Peanuts often respond better to residual P and K rather than to direct applications.

· Legumes such as peanuts, cowpeas, soybeans, pigeonpeas, mung beans, and chickpeas are very efficient N fixers if properly innoculated with the correct strain of Rhizobia bacteria or if grown on soils with a good natural population of the correct Rhizobia. In some cases, however, a starter application of 15-25 kg/ha of N has given a positive response by feeding the plants until the Rhizobia begin to fix N (about two to three weeks after plant emergence). Such responses are the exception rather than the rule and are most likely to occur on sandy soils. Beans (Phaseolus vulgaris) are not so efficient at N fixation and can use up to 50-60 kg/ha of N.

· The farmer's management ability is a vital consideration. Farmers should not be encouraged to use a high rate of fertilizer if he or she is not willing or able to use other complementary yieldimproving practices.

Table 8 General guidelines for low, medium, and high rates of N-P-K



LOW (Lbs./acre or kg/hectare)

MEDIUM(Lbs./acre or kg/hectare)

HIGH(Lbs./acre or kg/hectare)

35-55

60-90

100+

P2O5

25-35

40-60

70+

K2O

30-40

50-70

80+