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

PERFORMANCE OF GREEN MANURES AND GRA1N LEGUMES ON
SEVERELY ACIDIC SOILS IN NORTHERN ZAMBIA t AND THEIR
EFFECT ON SOIL FERTILITY IMPROVEMENT
COST AH MALAMA and KENNETH KONDOWE
Misamfu Regional Research Centre, P. O. Box 410055, Kasama, Zambia
E-mail: misamfu@zamnet.zm
Abstract
Green manures have been used in various parts of Zambia, especially where soils are not acidic. TherethR green manures
have been reported to produce large amounts of biomass that leads to improved soil fertility once incorporated into
the soil. We assessed the production of above-ground biomass by two green manures and the grain production oftwo
grain legumes to see how they affect the fertility of an acidic Ultisol and an acidic Alfisol. Green manures were incorparated
at flowering while grain legume residues were incorporated after harvesting the grain. On the Ultisol, sunnhemp
produced the most above-ground biomass (2800 kg ha- l ) and velvet bean produced 2000 kg ha- l . Soyabean grain production
was 792 kg ha- l and cowpea grain yield was just 9.2 kg ha- 1 . However, on the Alfisol velvet bean produced the highest
above-ground biomass (2100 kg ha- 1 ) and sunnhemp produced 2000 kg ha- l • Grain yield was highest for soyabean
(1313 kg ha- l ) and lowest in velvet bean (83 kg ha- l ). Velvet bean constantly produced high above-ground biomass on
both soil types. Thus it can be used as a green manure on both soils. The results show that cowpea might be unsuitable
for grain production on the Ultisol while soyabean can be used for grain production. Cowpea seems inferior on both soil
types, while sunnhemp and velvet bean appear to be ideal for the production of biomass on both acid soils. Thus, these
two green manures can be promoted for soil fertility improvement on these acid soils in northern Zambia.
Key words: Green manures, soil acidity, Al saturation, P-fixation
Introduction
Soils in Northern Zambia are general acidic, infertile
and of low productivity. As in other parts of Southern
Africa, nitrogen is the nutrient most limiting
crop production on these soils. Use of the mineral
fertilizers needed to increase crop yields has become
an almost impossible option for smallholder farmers
due to the escalating prices resulting from the
removal of subsidies on fertilizers and other agricultural
inputs. This has seen many farmers resorting
to biological methods of soil fertility management.
Green manure use is one way to increase the basket
of options for small-scale farmers. Several green manure
legumes have been identified for use in Southern
African cropping systems. However, the boundary
conditions under which they perform best have
not been ascertained.
There is need to establish the soil and climatic conditions
for legume adaptation so they can be used to
improve soil fertility in specific environments. Increasing
human population densities and the resultant
pressure on land limits the growing of legumes
for green manure in some areas as farmers have to
grow crops that ensure they are food secure. The
increase in human population has seen intensification
of agriculture without replenishment of depleted
nutrients. Population pressure has led to expansion
of agricultural activities into marginal lands
resulting in crop production declines.
Research on practical options that are affordable to
farmers such as intercropping of green manures
with other crops to maximize area under cultivation
is necessary, as well as exploring the use of grain
legumes for home consumption and for soil fertility
improvement. Greert manuring was an integral part
of some local farming systems before inorganic fertilizers
became widely used. Most elderly smallscale
farmers recall arid appreciate the usefulness of
two green manures, Crotalaria spp. and MucurUJ.
spp., which have shown a high potential to fix atmospheric
nitrogen symbiotically in a wide range of
environments (Bowen et al. 1988; Kolar, et al. 1993;
MacColl, 1990; Yost et al. 1985).
Green manures have been reported to possess the
potential to accumulate up to 250 kg N ha- 1 yr'!.
(Giller and Wilson, 1991; Peoples and Herridge,
1990). This amount of N leads to an increase in yield
of cereals, reported to be between 600 and 4100 kg
ha- I (Peoples and Herridge, 1990). The use of organic
manures has been shown to improve soil organic
matter (Mwale et al. 2000) in non-acid soils of
southern Zambia. The improved organic matter
status in tum leads to improved soil structure and
better root aeration leading to improved water
Grain Legumes and Green Manures for Soil Fertility in Southern Africa 185