(introductory text...)

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).

(introductory text...)

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

(kg/ha/yr)
Treatment	NO3-N	PO4-P	K	Ca	Mg	Total
Bare	1 0.8	3.4	17.4	36.6	6.8	75.0
Maize-maize (mulch)	0.3	0.03	0.3	0.4	0.1	1.1
Maize-maize (plow-till)	3.1	1.5	11.5	13.2	2.9	32.2
Maize-cowpea (no-till)	0.5	0.2	1.9	1.0	0.4	4.0
Cowpea-maize (plow-till)	2.6	0.8	9.0	9.0	2.0	23.4

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

(kg/ha/yr)
Treatment	Organic carbon	Total N	Bray- P	Ca	Mg	Total
Bare	2317.6	186.2	9.3	108.2	6.0	309.7
Maize-maize (mulch)	T	T	T	T	T	T
Maize-maize (plow-till)	250.0	20.8	1.0	12.2	1.0	35.0
Maize-cowpea (no-till)	T	T	T	T	T	T
Cowpea-maize (plow-till)	187.3	16.7	0.6	11.1	0.9	29.3

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

(kg/ha/yr)
Treatment	NO3-N	NH4-N	PO4-P	Ca	Mg	K	Total
Maize-cowpea	121.4	3.7	1.7	139.0	22.3	36.8	324.9
Mucuna	603.0	7.8	2.4	196.4	22.5	160.0	449.4
Maize-cowpea (after pasture)	68.6	2.3	1.5	97.7	14.3	63.7	248.1
Pasture	131.3	4.7	1.4	256.3	29.1	39.3	462.1
Maize-cowpea	143.9	2.5	0.4	252.3	45.7	16.3	461.1
Forest	57.3	5.9	1.0	376.2	54.8	48.6	543.8

(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

Common name	Scientific name
Calopo	Calopogonium mucunoides
Centro	Centrosema pubescens
Glycine (perennial soybean)	Glycine wighrii
Huban clover	Arachis prostrata
Kudzu	Pueraria phaseoloides
Mucuna	Mucuna utilis
Phasey bean	Phaseolus lathyroides
Pigeon pea	Cajanus cajan
Psophocarpus	Psophocarpus palustris
San hemp	Crotalaria juncea
Spanish clover	Desmodium ucinatum
Stylo	Stylosanthes gracilis
Townsville stylo	Stylosanthey humilis
Velvet bean	Stizolobium deeringianum

Nutrient contents (%)
	N	P	K
Calopogonium spp..	3.02
Desmodium trifolium	2.93	0.14	1 30
Mucuna sp..	2.96	0.32	1.57
Pueraria spp.	2.38	0.25	2 30

(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.

(introductory text...)

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

1. Crop residue mulch

Crop residues contain substantial quantities of plant nutrients. The data in Table 31 show the nutrient composition of the crop residues of some crops grown in the humid tropics. The concentration in oven-dry tissue ranges from 0.58% to 4.0% for N. 0.1% to 1.1% for P. and 0.2% to 3.4% for K. Nitrogen and phosphorus concentrations are generally higher in legumes than in cereals. On a weight basis, the major plant nutrients contained in 1 Mg of crop residue may range from 15 to 60 kg of N. P. and K (Table 32).

The beneficial effects of returning crop residue as mulch on crop yield are well known. These benefits are due not only to the recycling of plant nutrients but also to improvements in soil moisture and temperature regimes, enhancement of soil structure, and erosion control. However, the use of crop residues as fertilizers is especially important to resource-poor farmers. Some examples of the beneficial effects of crop residue mulch on crop yields are shown by the data in Tables 33 through 37. The data in Table 33 show that compared with an unmatched control, crop yields were improved with any mulch material. Rice husks increased maize yield by 0.7 Mg/ha and cassava yield by 12 Mg/ha. The data in Table 34 on yam production on an acid soil in eastern Nigeria show that mulching significantly increased the yam tuber yield. Mulching increased tuber yield by 20% on both ridge till and flat seedbed.

Table 31 Nutrient composition of crop residues of some crops grown in the humid tropics

(kg/ha/yr)
Crop/species	N	P	K	C/N ratio
Cowpea stem	1.07	1.14	2.54
Cowpea leaves	1.99	0.19	2.20
Rice	0.58	0.10	1.38	105.0
Maize	0.59	0.31	1.31	55.0
Oil palm (processed fibre)	1.24	0.10	0.36
Sesbania leaves	4.0	0.19	2.0
Crotolaria spp.	2.89	0.29	0.72
Tephrosia spp.	3.73	0.28	1.78
Water hyacinth	2.04	0.37	3.40	18.0
Azolla spp.	3.68	0.20	0. 15
Typha spp.	1.37	0.21	2.38

(Modified from FAO, 1990)

Returning crop residue as mulch may also have synergistic effects with fertilizer use. The data from the eastern Amazon by Schoningh and Alkamper (1984) showed that crop residue mulches with low C:N ratios had more beneficial effects than those with high C:N ratios (Table 35). On an Ultisol in eastern Nigeria (Tables 36 and 37), the yield of plantain and bananas was drastically improved by residue mulch.

Table 32 Plant nutrients contained in 1 Mg of dry straw

(kg/Mg)
Crop/species	N	P	K	Total
Cowpea stem	10.7	11.4	25.4	47.5
Cowpea leaves	19.9	1.9	22.0	43.8
Rice	5.8	1.0	1 3.8	20.6
Maize	5.9	3.1	13.1	22.1
Oil palm (fibre)	12.4	1.0	3.6	17.0
Sesbania leaves	40.0	1.9	20.0	61.9
Crotolaria spp.	28.9	2.9	7.2	39.0
Tephrosia spp.	37.3	2.8	17.8	57.9
Water hyacinth	20.4	3.7	34.0	58.1
Azolla spp.	36.8	2.0	1.5	40.3
Typha.spp	13.7	2.1	23.8	39 6

(Recalculated from the data in Table 31)

Plantain yield was five times more with mulch than with chemical fertilizers alone. The data in Fig. 18 show that returning crop residue mulch enhanced the beneficial effects of fertilizer application in maize yield (Kang, 1993). Without fertilizer application, residue retention had little effect on maize grain yield.

Table 33 Crop yield response to 22 different mulch materials applied on Alfisols in Nigeria

(Mg/ha)
Mulch	Cassava (fresh roots)	Maize	Cowpea	Soybean
Bare soil (control)	16.4 def	3.0 e	0.6 a	0.6 de
Maize stover	16.4 def	3.3 cd	1.1 a	1.5 abc
Maize cobs	17.8; cdef	3.3 cd	1.1 a	1.4 abed
Oil palm leaves	17.1 def	3.2 cd	1.2 a	0.9 bcde
Rice straw	17.9 cdef	3.5 bed	1.0 a	1.5 abc
Rice husks	28.3 a	3.7 abc	1.1 a	0.8 de
Kikuyu grass straw	14.2 ef	3.3 cd	1.2 a	1.4 abed
Elephant/napier grass (Pennisetum)	16.6 def	3.3 ed	0.9 a	1.3 bed
Guinea grass	15.5 f	3.6 bed	2.1 b	1.5 ab
Andropogon straw	18.5 cdef	3.5 bed	1.0 a	1.2 bcde
Cattail straw (Typha)	16.7 def	3.1 cd	1.0 a	1.1 bcde
Cassava stem (chipped)	20.9 cd	3.8 abc	0.9 a	1.4 abcd
Pigeon pea tops	22.9 be	3.7 abc	1.1 a	0.9 cde
Pigeon pea stem (chipped)	19.9 café	3.5 bed	1.0 a	1.3 bcd
Legume husks	26.4 ab	4.4 a	1.0 a	1.5 abc
Soybean tops	22.9 be	4.2 ab	1.0 a	1.2 bcde
Hemp (Eupatroium)	18.8 cdef	3.6 abc	1.0 a	1.2 bcde
Mixed twigs (chipped)	18.5 cdef	3.4 bed	1.0 a	1.2 bcde
Sawdust	20.5 cde	3.7 abc	0.9 a	1.9 a
Black plastic	30.5 ab	3.0 cd	0.9a	1.1 bcde
Transluscent plastic	27.7 ab	2.7 d	1.0 a	1.1 bcde
Fine gravel	22.9 be	3.1 cd	1.0 a	1.0 bcde

Figures followed by similar Ietters are stastically similar within vertical roust (Okigbo and Lal. 1980)

Table 34 Effects of tillage methods and mulching on yield and yield components of yam tubers in eastern Nigeria

Treatment	Diameter (cm)	Length (cm)	Number	Mean tuber yield (Mg/ha)
Ridge, mulch	16.7 a	23.5 b	9936 b	15.4 a
Flat, mulch	17.1 a	21.7a	12916a	16.1 a
Ridge, no mulch	13.3 b	20.7 a	10385 b	12.8 b
Flat, no mulch	13.8 b	20.6 a	10128 b	13.4 b

Figures followed by similar letters are statistically similar within vertical rows.

(Maduakor et al., 1984}

Table 35 Yield response of maize and cowpea to different mulch materials and mineral fertilizer in an eastern Amazon Oxisol, Capitao Poco (CPATU) Para, Brazil, 1983

(kg/Mg)
Mulches used (10 Mg/ha of dm)	First crop (maize)*		Second crop (cowpea)**
	NPK (kg/ha) 20-80-60	NPK 0-0-0	NPK 30-80-60	NPK 0-0-0
Elephant grass	4646	2144	1227	80
Pueraria	5697	3342	1187	114
Weeds	4911	2215	1394	105
Sec. Forest (2 3 years)	4462	1560	1191	35
Sec. Forest (4 5 years)	4479	1807	1397	95
Rice husks	4398	1146	1487	163
Maize cobs + husks	4863	2101	1302	41
Bare soil	3539	78	1169	7
LSD. 5%	987		281
LSD. 1%	1321		376
LSD. 0.1%	1729		493

* Grain moisture content 14. 5%
** Grain moisture content 13, %

(Schoningh and Alkamper 1984)

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

(Mg/ha)
Treatment	Plantain	Banana
	Giant	Medium
No mulch, no fertilizer*
No mulch, fertilizer	18.0 a	16.7 a 7.5 a
Mulch, no fertilizer	17.2 a	15.8 a 9.5 a
Mulch and fertilizer	31.3 b	19.8 a 13.3 b

* 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

Parameter	Mulch	Fertilizer
Yield (Mg/ha)	22.8	4.8
Bunch weight (kg)	11.8	8.1
Plants harvested (% of planted)	116.0	36.0
Harvest duration (months)	10.0	6.0

(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

Species	Growth characteristics	Uses
Acioa bateri	Fast-growing shrub	Alley cropping, nitrogen fixation
Albizia falcate	Tree grows to 30 m	Erosion control, nitrogen fixation
Albizia lebbeck	Tree grows to 25 m	Erosion control, nitrogen fixation
Anthonotha	Fast-growing shrub	Alley cropping, nitrogen fixation macrophylla
Calliandra calothyrsus	Fast-growing shrub to 8 m, on acid soils	Alley cropping, nitrogen fixation
Cassia siamea	Shrub grows to 8 m. vigorous coppicing	Fuelwood, nitrogen fixation, lumber
Erythrina spp.	Tree grows to 20 m, often thorny, coppices shell	Live fences, nitrogen fixation, fuelwood, fodder
Flemingia macrophylla	Shrub grows to 3 m	Alley cropping, nitrogen fixation
Gliridia sepium	Fast-growing tree to 20 m. vigorous coppicing	Alley cropping, nitrogen fixation. forage, fodder. staking material
Inga spp.	Nitrogen-fixing shrub, acid- tolerant	Alley cropping. nitrogen fixation
Leucaena leucocephala	Tree grows to 20 m, fast-growing on non-acid soils. vigorous coppicinig	Fodder. fuelwood. erosion control, nitrogen fixation, alley cropping. staking material
Panagomia pinneta	Small tree. grows to 8m	Erosion control live hedges
Sesbania spp.	Fast-growing loss: tree	Erosion control, nitrogen fixations

(NRC, 1993a)

Table 39 Merits and limitations of agroforestry systems

Merits	Limitations
Reduction in fallow period and high cropping intensity over longer time period	High labor input
Erosion control and runoff management	Highly skilled management
Strengthening of nutrient cycling mechanisms leading to savings in fertilizer use	Low yields due to allelopathic effect and competition among trees and food crops for light, water, and nutrients
Alternate products (e.g., fodder, fuel, staking water mulch and food crops)	Limited application on soils of moderate to low soil fertility
Saving, land and decrease in need for clearing new land	Potentially high risks of pests and diseases
Improved traditional system, therefore, ecologically compatible	Difficulties in adoption due to traditional land tenure system

(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

Treatment	Runoff		Erosion (Mg/ha/yr)
	Total (mm)	Fraction of rainfall (%)
Plow-till	232	17.1	14.9
No-till	6	0.4	0.03
Leucaena 4 m	10	0.7	0.9
Leucaena 2 m	13	1.0	0.1
Gliricidia, 4 m	50	1.5	1.7
Gliricidia. 2 m	38	2.8	3.3

(Lal, 1989a)

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

Species	Net primary production of biomass (ka/ha/yr)
Acacia auriculiformis	3000-4000
Acacia mangium	2500 3500
Albizia falcata	4000-5000
Alchornea cordifolia	2000-3000
Calliandra calothyrsus	2500-3500
Cordia alliodora	2500-3500
Dalbergia latifolia	4000-5000
Erythrina poeppigiana	4000-6000
Gmelina arborea	1500-5000
Leucaena leucocephala	3000-5000

(NRC, 1993a)

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

(%)
Species	N	P	K
Cassia siamea (leaves)	1.91	0.18	1.03
Tephrosia sp.	3.73	0.28	1.78
gliricidia sp.	4 15	0.27	300
Leucaena leucocephala	3.85	0.17	1.46
Erythrina sp.	400	0.29	3.05

(FAO. 1190)

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

(kg/ha)
Year	Nutrients
	N	P	K	Ca	Mg	Total
1985	440	49	264	195	52	1489
1988	408	37	244	155	29	873
1989	275	15	133	74	17	514
1990	281	16	188	106	26	617

(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

(%)
Species	Biomass yield (Mg/ha/yr)	Nutrient yield
		N	P	K	Ca	Mg	Total
Acioa barteri	30	41	4	20	15	5	85
Alchornea cordifolia	4.0	85	6	48	42	8	189
Gliricidia sepium	5.5	169	11	149	104	18	451
Leucaena leucocephala	74	247	70	184	98	16	565

(Kang and Wilson. 1987)

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

(kg/ha)
Month	Leaf fall (Mg/ha)	Nutrient addition
		N	Ca	Mg	K
January	5.9	110.3	88.3	13.0	39.1
February	5.7	106.4	85.2	12.6	37.7
March	2.4	43.7	35.0	5.2	15.5
April	1.5	28.1	22.5	3.3	10.0
May	1.9	35.1	28.1	4.1	12.5
June	1.4	25.8	20.7	3.0	9.2
July	2.1	40.0	32.0	4.7	14.2
August	3.4	62.5	50.1	7.3	22.2
September	9.0	167.8	134.4	19.8	59.5
October	9.6	179.3	143.6	21.2	63.6
November	12.7	236.5	189.7	28.0	84.0
December	16.4	305.0	244.3	36.1	108.2
Total	172.0	1340.8	1073.9	158.4	475.7

(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

(kg ha)
Grain yield
	1985	1986	1988	1989	1990	Mean
Alley cropping	4890 a	3470 a	5657 a	4141	5376	4707
Control	3990 b	2240 a	5084 b	3353	4032	3740
Difference	900	1230	573	788	1344	967
Relative %	2.6	54.9	11.3	23.5	33.3	25.8

(Kang., 1993)

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

Treatment	Relative grain yield for different years *
	1982	1983	1984	1985	1986	1987	Mean
Plow-till	128	78	79	77	176	65	100
No-till	270	147	212	139	177	38	164
Leucuena, 4 m	177	91	103	73	51	39	89
Leucaena, 2 m	129	57	89	28	26	42	62
Gliricidia 4 m	168	106	119	105	80	37	103
Gliricidia, 2 m	124	95	120	72	41	40	82

* 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

Species	Row from the shrub	Grain yield (kg/ha)
Inga	1	723
	2	1942
	3	2162
	4	1986
	5	1975
	6	2052
	8	1923
	10	1875
Erythrina	1	1582
	2	2014
	3	1888
	4	2059
	5	2056
	6	2044
	8	2107
	10	1997
Leucaena	1	1718
	2	2084
	3	1948
	4	2128
	5	2253
	6	2321
	8	2106
	10	2419

(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.