Grain Legumes and Green Manures for Soil Fertility in ... - cimmyt

LEGUMINOUS AGROFORESTRY OPTIONS FOR REPLENISHING
SOIL FERTILITY IN SOUTHERN AFRICA
PARAMU L. MAFONGOYA 1·, E. KUNTASHULAl, F. KWESIGA 2 , T. CHIRWAl,
R. CHINTU 1 , G. SILESHl 1 , and J. MATIBINll
'Zambia ICRAF/Agroforestry Project, P.O. Box 510046, Chipata, Zambia;
2SADC-ICRAF A grofores try Research Project, P. O. Box MP 163,
Mount Pleasant, Harare, Zimbabwe
(* Corresponding author, E-mail: mfongoya@zamnet.zm)
Abstract
Nitrogen is the major nutrient limiting maize production in Zambia and Southern Africa. Removal of subsidies on inorganic
fertilizers made them very expensive and most farmers cannot afford them. Short duration planted fallows using
a wide range of leguminous trees have been found to replenish soil fertility and increase subsequent maize yields.
Species such as Sesbania sesban, Tephrosia vogelii and Cajanus cajan have been found excellently suited for planted
fallow technology. These improved fallow crop rotations are being adopted by small-scale farmers in Eastern Zambia.
Since the seminal paper of Kwesiga and Coe (1994), research has been done to understand how the planted tree fallows
replenish soil fertility and improve maize yields.
A wide range of species has been screened as alternatives to sesbania fallows to overcome some of the limitations of sesbania.
Species such as Gliricidia sepium, Leucaena leucocephaJa have maintained maize yields of 3 t/ha over 8 years
of cropping when sesbania fallows yields declined to 1.1 t/ha after 3 years of cropping. The selection criteria for good
fallow species are high biomass production and litterfall. Maize yields after fallows were highly correlated to biomass
and litterfall yields. High quality biomass, which is low in lignin, polyphenol and high in N, is needed for higher maize
yields. Mixing of gliricidia and sesbania fallows resulted in higher maize yields compared to single species fallows (3.0
vs. 1.8 t/ha). Mechanisms on how mixed fallows work need further investigation.
Preseason inorganic N (N0 3 +NH4) was highly correlated with maize yield (r 2 = 0.62) and this could be used to select
fallow species and management practices. Nutrient budgets of N, P and K showed over 8 years that a positive balance of
Nand P was maintained for coppicing fallows while a negative balance of K started showing from the fourth year onwards
on fertilized maize, gliricidia, ieucaena and sesbania fallows. This points to the need to .use inorganic fertilizers
such P and K to supplement N supply from leguminous fallows. Improved fallows increased infiltration, reduced runoff,
increased water storage, and reduced soil loss. The order was sesbania = tephrosia > natural fallow =maize + fertilizer.
The biophysical limits of most fallow species and other emerging issues such as pests and diseases, the need to inoculate
with rhizobium, amount of N fixed by different species and provenances and soil acidification under improved
fallows are under further research.
Biomass transfer technology using biomass from leguminous trees was evaluated on maize and vegetable production in
the dambos (wetlands). Maize and vegetable yields were significantly increased by application of high quality biomass
from gliricidia and leuceana. However, financial analysis showed that it is not viable to apply biomass on a low value
crop like maize. Biomass transfer was economically viable on high value crops such a vegetables.
Key words: Eastern Zambia, improved fallows, soil fertility, nutrient budgets, nitrogen fixation and sustainability
Introduction
Soil infertility is now increasingly recognized as the
fundamental biophysical root cause for declining
food security is smallholder farmers of sub-Saharan
Africa (Sanchez et al. 1997). Maize is a staple food
crop in Southern Africa. Nitrogen is the major nutri-
ent that limits maize productivity, with phosphorus
and potassium in limited cases. Although inorganic
fertilizers are used in the region, the amounts applied
are normally insufficient to meet .crop de­
mands due to high costs and uncertain availability.
Mostcciuntries in southern Africa have developed
fertilizer recommendations for major crops, some-
Grain legumes and Green Manures for Soil Fertility in Southern Africa 141