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VIII. Nutrient management

A. Chemical fertilizers
B. Reducing nutrient losses
C. Biological nitrogen fixation (BNF)
D. Nutrient cycling

An adequate supply of plant nutrients is essential for efficient crop production on the highly weathered and leached soils of the humid tropics. Effective nutrient management requires two steps: (i) increase nutrient reserves in the soil and their availability to crops; and (ii) assess the nutrient requirements of the crops to be grown. Effective nutrient management involves meeting the nutrient requirements for expected yields from soil reserves and supplemental additions of chemical fertilizers and organic manures. Most soils of the humid tropics (e.g., Oxisols, Ultisols, and Alfisols) are low in nutrient reserves. Therefore, it is necessary to supply nutrients from external sources. There are diverse sources of nutrients, including chemical fertilizers, biological nitrogen fixation, and nutrient recycling.

A. Chemical fertilizers

The use of chemical fertilizers is essential for obtaining high yields in the highly weathered soils of the humid tropics. However, many small land holders and resource-poor farmers cannot afford costly fertilizers. Most soils in the humid tropics are so deficient in primary nutrients that it is imperative that strategies be developed for adding them from outside the ecosystem. Otherwise, sustainable cropping systems cannot be developed.

The most deficient nutrients in the soils of the humid tropics are N. P. and Ca as macro-nutrients and perhaps Zn as a micro-nutrient. There is some potential for enhancing N supply by biological N fixation. Other nutrients have to be supplied. The concept of low external input must clearly be examined in view of the limited nutrient reserves of these soils. However, the requirements for chemical fertilizers can be reduced drastically by decreasing losses, recycling nutrients, and through biological N fixation (Fig. 16).

B. Reducing nutrient losses

1. Soil erosion
2. Leaching
3. Volatilization losses

Nutrient losses occur through accelerated erosion, leaching, and volatilization. Soil and crop management systems must be adopted to minimize these losses:

1. Soil erosion

The magnitude of nutrient loss through accelerated erosion can be very high. The data in Tables 27 and 28 show examples of the magnitude of nutrient loss in runoff and eroded soil for different systems of soil and crop management in western Nigeria. Expectedly, nutrient losses are very high in bare uncropped land. Nonetheless, losses of nutrients are also high in cropped plow-till systems of seedbed preparation. Total nutrient loss in maize-maize plow-till treatment was 32.2 kg/ha/yr in runoff and 35.0 kg/ha/yr in eroded soil. Similarly, nutrient loss in cowpea-maize plow-till treatment was 23.4 kg/ha/yr in runoff and 29.3 kg/ha/yr in eroded soil. In contrast, nutrient loss in runoff was decreased to 1.1 kg/ha/yr with mulch and to 4.0 kg/ha/yr with no-till systems of soil management. Because the use of mulch and no-till systems reduced soil loss to zero, there was practically no nutrient loss in eroded soil. The judicious use of erosion preventive and control measures described in the previous sections can drastically reduce nutrient loss in runoff and eroded soil.

Fig. 16 Strategies for decreasing chemical fertilizer requirements

Table 27 Management effects on nutrient losses in runoff from an Alfisol at 5% slope in 1973

Maize = Zea mays
Cowpea =Vigna unguiculata

(Lal, 1976)

Table 28 Management effects on nutrient losses in eroded soil from an Alfisol at 5% slope in 1973

T = < 0. 1 kg/ha/yr
(Lal, 1976)

2. Leaching

Similar to soil erosion, leaching losses can also be high in the humid tropics. The data in Table 29 from Alfisols in western Nigeria show that losses of nutrients leached out of the root zone with seepage water can be substantial, amounting to 300 to 500 kg/ha/yr of nutrient loss. It is likely that the assessment of leaching losses measured by the Iysimetric technique used in this experiment is biased because it used a confined volume and no provisions were made for runoff disposal. Because surface runoff had no outlet. it accentuated leaching losses.

Leaching losses of plant nutrients can also be minimized by soil, crop, and fertilizer management techniques. In terms of crop management, the best strategy is to maintain an actively growing crop on the soil surface. Also, incorporation of a deep-rooted crop to capture nutrients translocated to the sub-soil is essential to reduce leaching losses. Soil management systems that enhance the water-holding capacity of the root zone can be useful in decreasing leaching losses. In this regard, maintaining high levels of soil organic matter content is an important strategy.

Table 29 Effect of soil and crop management systems on nutrient losses by leaching from Alfisols in western Nigeria in 1985

(Lal. 1992)

Fertilizer management is also important in decreasing leaching losses. Split applications and use of slow-release formulations are some options. However, split applications may be labor-intensive and slow-release formulations are expensive and probably not available to resource-poor farmes of the humid tropics.

3. Volatilization losses

High soil temperatures and moist conditions throughout the year may accentuate volatilization loss of nitrogen contained in the soil and applied in fertilizers and organic amendments. Soil temperatures of 40° to 50°C at 1 cm depth are commonly observed. Several soil and fertilizer management options are available that can decrease volatilization losses. Use of crop residue mulch and no-till systems are useful techniques to regulate soil moisture and temperature regimes. Maintaining continuous ground cover through mixed and relay cropping is another useful strategy. Volatilization losses can also be reduced by the incorporation of fertilizers and organic amendments into the soil rather than broadcast on the soil surface.

There are fertilizer formulations that are less soluble and decrease volatilization losses. Coating nitrogenous fertilizer with material that decreases solubility also decreases volatilization losses. The slow release formulations are effective in reducing losses due to leaching and volatilization. The use of nitrification-inhibiting compounds is another strategy to inhibit oxidation of ammonia into nitrates. These compounds are usually applied at low rates of 0.5 to 1.0 kg/ha.

Weed Control: Effective weed control can be achieved through appropriate measures of soil and crop management. Although weeds compete for limited resources, nutrients absorbed by weeds are temporarily immobilized and remain within the ecosystem. Judicious weed control can be achieved through crop management, soil management and application of herbicides. The soil and crop management techniques of weed control are more appropriate than chemical control measures for resource-poor farmers.

Table 30 Tropical legumes that can be grown as cover crops to procure in situ mulch

(Adapted from Lal. 1990c.-FAO, 1990)

C. Biological nitrogen fixation (BNF)

Augmenting the nitrogen supply to crops through BNF is a viable option for resource-poor farmers of the humid tropics and must be exploited to its fullest potential. The amount of N fixed by legumes can range from 20 to 200 kg/ha/yr depending on the species, soil type, climate, and agro-ecoregion. Some common legumes that can be grown as cover crops to procure mulch and increase BNF are listed in Table 30. Several perennial shrubs and woody species also can be used to enhance the nitrogen status of the soil. These species and their role in nutrient cycling and N fixation will be discussed in the following section.

D. Nutrient cycling

1. Crop residue mulch
2. Agroforestry systems

Nutrient cycling and re-use is an important strategy for sustainable crop production in the humid tropics. It involves returning nutrients removed by crops and animals to the soil for future use. In addition to crops and animals, soil fauna (e.g., earthworms, termites) also play an important role in cycling of several elements, including C, N. P. S. B. Cu. Zn, and Mo. Growing deep-rooted plants is important in order to cycle nutrients from the sub-soil by returning them through crop residue to the surface where the following shallow-rooted crops can use them. Used effectively, recycling can substantially reduce chemical fertilizer requirements. Some important recycling strategies outlined in Fig. 17 include crop residue mulch, deep-rooted perennials, and animal wastes.

Fig. 17 Strategies for recycling nutrients

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