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2. Agroforestry systems

Agroforestry systems involve growing woody herbaceous species and perennials in association with food crops and livestock on the same piece of land. Agroforestry systems have been described extensively in several reports (i.e., Kang et al., 1981, 1989, 1990; Harwood, 1987;
Nair. 1989; Szott et al., 1991). They are known to increase ecological diversity within a landscape unit and optimize the use of limited resources through the integration of complementary components. There are three principal types of agroforestry systems (Fig. 19).

Table 36 Effect of mulching and fertilizer on yield of plantain and banana on an acid soil in eastern Nigeria

* Most plants broke.

Numbers in the same column followed by the same letter are not significantly different at 5%.

(IITA. 1981)

Table 37 Comparative effects of fertilizer and mulch on plantain yield on an acid soil in eastern Nigeria

(IITA. 1981)

(i) Agrisilvicultural: This system involves simultaneously growing crops and trees on the same piece of land. Some commonly used agrisilviculture systems include alley cropping (Plate 38) and hedgerow cropping.

Fig. 18 Synergistic effects of using fertilizers in combination with returning crop residue

* N₀P₀K₀, = control with no fertilizer. (a) years 1 4: N₁ = 80 kg N/ha, N₂ = 160 kg N/ha; (b) years 5 8: Ni = 100 kg N/ha, N₁ = 200 kg N/ha; (c) years 9 10: Ni = 75 kg N/ha, N₂ = 150 kg N/ha. P₁ = 30 kg P/ha. P₂ = 60 kg P/ha, K₁ = 40 kg K/ha. K₂ = 80 kg K/ha.

(Kang, 1993)*

Fig. 19 Types of agroforestry systems

(ii) Silvopastoral: This system involves raising livestock on improved pastures grown in association with trees. Some commonly used systems are alley farming and live fences (Plate 39).

Table 38 Commonly recommended species for agroforestry systems in the humid tropics

(NRC, 1993a)

Table 39 Merits and limitations of agroforestry systems

(iii) Agrisilvopastoral: This system involves a three-way mixture based on a combination of crops, trees. and animals. Such a system requires skillful management, and can be sustainable even in harsh environments and fragile soils.

A wide selection of tree species and woody shrubs can be used for agroforestry systems (Table 38). Some of these trees are suited for acid soil conditions and others for erosion control, some are more appropriate as forage trees, and still others are useful for pruning to be used as mulch. The choice of appropriate species is critical to the success of agroforestry systems. In addition to the intended use, the choice of tree and associated crop species also depends on cultural and ethnic factors of social importance.

The merits and limitations of agroforestry systems are shown in Table 39. A principal advantage of these systems is the reduction in the length of the fallow period and the potential for continuous and intensive cropping. Agroforestry may facilitate intensive land use for multiple uses on relatively fertile soils. It may also enable relatively more intensive use on steep lands and marginal soils, which cannot be used otherwise. A major advantage of agroforestry systems on sloping lands is erosion control. Closely spaced contour hedgerows of suitable woody perennials and shrubs can drastically reduce the risks of runoff and accelerated soil erosion. The data in Table 40 indicate large reductions in runoff and soil erosion with hedgerows of Gliricidia and Leucaena established at 2 and 4 m intervals.

Table 40 Alley-cropping effects on runoff and soil erosion from maize-cowpea rotation measured in 1984

(Lal, 1989a)

Table 41 Net primary production of biomass for commonly recommended multi-purpose tree species in the humid tropics

(NRC, 1993a)

Table 42 Nutrient composition of foliage of some trees and woody perennials grown with agroforestry systems in the humid tropics

(FAO. 1990)

Another benefit of growing woody perennials and trees in association with crops is the large quantity of biomass produced. The net primary production of biomass for perennials ranges from 1.5 to 6 Mg/ha/yr (Table 41). This biomass is a valuable resource for small land holders of the humid tropics. In addition to its use as forage, fuel, and staking material, the biomass can also be returned to the soil as mulch for soil protection and nutrient recycling. The nutrient content of the foliage of some of these species ranges from 2% to 4% for N. 0.2% to 0.3% for P. and 1% to 3% for K (Table 42). Consequently, a substantial amount of plant-available nutrients can be added to the soil by returning 3 to 7 Mg/ha of biomass. The data in Table 43 show that nutrients contributed by the biomass returned to the soil from Leucaenu can be 275 to 440 kg/ha of N. 15 to 49 kg/ha of P. 133 to 264 kg/ha of K, 74 to 195 kg/ha of Ca, and 17 to 52 kg/ha of Mg. Nutrients contributed vary widely among different species (Table 44). Rather than prunings, even the litter fall from some trees can add substantial amounts of nutrients (Table 45). Not all the nutrients recycled in the biomass, however, are available to crops. The nutrient use efficiency may be only 20% to 30% or less.

Table 43 Amount of nutrients in Leucaena leucocephala prunings

(Hauser and Kang, 1993)

Table 44 Estimated nutrient addition through prunings of four woody shrubs grown at 4 m intervals on an Alfisol in western Nigeria

(Kang and Wilson. 1987)

Table 45 Biomass and nutrient addition by litter fall of a three-year-old plantation of Cassia siamea

(Ghuman and Lal. 1990)

Table 46 Grain yield of maize alley cropped with Leucaena leucocephala hedgerows and in plots with no hedgerows (control) on an Alfisol in southwestern Nigeria, and absolute and relative yield advantages of alley cropping

(Kang., 1993)

Table 47 Effects of agroforestry systems on relative grain yields of cowpea a tropical Alfisol

* Relative yield is calculated as a ratio of actual yield to average yield of all seasons and all treatments. expressed in percent. No fertilizer was applied.

(Lal 1989a)

Fig. 20 Grain yield as affected by position of the crop row from the perennial hedgerow

Table 48 Grain yield of rice from an experiment conducted at Yurimaguas, Peru, with three species of shrubs

(TROPSOIL.1987)

Because of added nutrients and other favorable soil physical factors, total production can be more with agroforestry than with simple crop-based, tree-based, or animal-based systems. However, the yield of individual components may be decreased. The data in Table 46 show that the yield of maize was the same or better with alley cropping than without. However, the data in Table 47 on cowpea yield show a significant yield reduction with alley cropping, probably due to allelopathic effects. The reduction in average cowpea yield was as much as 11% by Leucaena at 4 m intervals and 38% by Leucaena at 2 m intervals. Yield reduction may also happen due to competition for nutrients between perennials and annuals. An example of the competition is shown by the data in Table 48 on an acid soil at Yurimaquas, Peru. Rice crop yields increased with wider spacing between hedgerows, and with increase in distance from the hedgerow (Fig. 20). The grain yield of the row next to the hedge of Inga or Erythrina was about 1300 kg/ha. The yield was about 1500 kg/ha or less for the fourth spacing The grain yield increased to more than 2000 kg/ha for the eighth row from the hedgerow. The grain yield of the row next to the hedgerow decreased by about 40% regardless of the hedgerow species. In addition, the high labor requirement is another limitation of agroforestry systems. The system is labor-intensive and complete mechanization is often difficult and not feasible to achieve.

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