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

picing and noncoppicing species reduces the level
of subsoil nitrate and controlled mesoplatys beetles.
However mixing gliricidia or tephrosia or sesbania
with herbaceous legumes such as mucuna or archer
dolichos reduced tree growth and hence maize yield.
These mixtures also lead to the build up of mesop:atys
beetle, which may have led to a larger attack
of sesbania by the beetles (Sileshi and Mafongoya,
2002).
Prediction of improved fallows performance
Many studies have shown a 3 to 4-fold increase in
maize grain yields after two year improved fallows.
Most of these studies were conducted under research
station conditions. However, when improved
fallows are tested in a wide range of environmental
conditions there is variability in maize
grain yields. The explanations advanced for this
variability is based on trial and error. There is need
for a predictive understanding of how fallows perform
in different agroecological conditions.
The work done for many years has shown how organic
decomposition and nutrient release is affected
by the levels of polyphenols, lignin and nitrogen
contents of the organic inputs (Mafongoya et al.
1998). Recently we have found also that maize
yields after fallows with various tree legumes were
negatively related to the L+P: N ratio (Figure 4).
Fallow species with high N, low lignin and low
polyphenols such as gliricidia and sesbania gave
higher maize yields compared to species such as
flemingia, calliandra and senna. This work has clearly
shown that it is not the quantity of polyphenols
which is critically important but also the quality of
the polyphenols as measured by their protein binding
capacity (Mafongoya et al. 2000). Legume species
for improved fallows can be screened for their
suitability based on the above characteristics.
Soil inorganic N before a cropping season is an accepted
test for soil N for soil productivity. Results
of our studies in Southern Africa show that preseason
inorganic N can also be an effective indicator
of plant available N after different improved fal­
48
lows. Our results indicate that preseason inorganic
N (N03 + NH4) can be more related to maize yield
than preseason N03 alone in a tropical soil with a
pronounced dry season (Figure 4). Large amounts
of NH4 can accumulate during a dry season, and it
may not be nitrified when the soil is sampled at the
beginning of the rainy. season. We concluded that
preseason inorganic N is a relatively rapid and simple
index that is related well to maize yield on N­
deficient soils and hence it can be used to screen fallow
species and management practices.
Improved fallows of S. sesban tested under a wide
range of conditions showed a strong linear relationship
between maize yield and standing biomass at
fallow clearance (r2=0.50). Preseason inorganic was
also related to standing biomass (r2= 0.60) and
standing biomass was related to clay content of the
sites (r2=0.SO). From these studies the impact of improved
fallows on maize yield were clearly related
(a)
35 y = -0_19x+ 3.24
1 R2 = 0_83
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