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

een renewed interest in green manure technologies
cis the price of mmeralfertiliser has increased.
The Risk Management Project (RMPJ is a project under
the CIMMYT Natural Resources Management
Group with a broad objective of improving farm
incomes and food self-reliance for poor smallholder
farmers in Zimbabwe and Malawi l;>y addressing
problems of low soil fertility, climatic variability,
low and unstable agro ecosystem productivity
through the use of simulation modelling and farmer
participatory research. Risk Management Project
staff have been working in very close collaboration
with a group of 14 farmers in the Zimuto smallholder
area of Masvingo since 1999. The work has
focussed on farmer-led on-farm experimentation
with several legume-based soil fertility technologies
coming out of the Soil Fertility Network trials. The
approach used aims to enable farmers, together
with researchers, to analyse and understand farmer
strategies and practices of soil fertility management
and to identify technologies that both meet farmers'
needs and are sustainable. Preliminary results from
the fieldwork and from focussed group discussions
with the farmers indicate the robustness of mucuna
under smallholder conditions and great interest in
the mucuna technology amongst the farmers.
However, selection of an appropriate technology
and management options is complicated by climatic
variability. This means that management options
must be assessed on a probabilistic basis. Mo.reover,
the development of appropriate technologies,
and the testing of the components, is complicated
by season-to-season variability. Experiments to test
different technologies must be run over many seasons
to obtain reliable results. This is expensive
and, in many cases, impracticaL Simulation models,
which integrate the major physical and biological
processes, provide a solution to this problem.
Simulation Study
Simulations were carried out using the APSIM
(Agricultural Production Systems SIMuJator) model
in conjunction with a 47-year long-term weather
dataset for Masvingo, which is around 30 km south
of the study area and about 50 mm/year drier. The
APSIM software system allows a wide range of configurations
of crops, sequences, mixtures and management
practices to be simulated. It provides a
flexible structure for the simulation of climatic and
soil management effects on growth of crops in farming
systems and changes in the resource base. A
detaileQ>~scription pf APSIM, including its capabilities,_d~~Jgn
features, structure, user interface and
the derivation of its main biological and environmental
modules is provided by McCown, Hammer,
Hargreaves, Holzworth and Freebaim (1995).
The simulation set-up consisted of growing either:
1. A maize crop, receiving various levels of N,
year after year. The N levels ranged from 0 kg
N /ha to 100 kg N /ha.
2. A crop of mucuna from the opening rains of the
season. The crop of mucuna was managed in
two ways:
a) either the crop was grown to maturity
and harvested on the 1 st of July with
60% of the residues incorporated on the
1 st of November just before maize planting
(Management 1).
b) or, the mucuna is harvested at the beginning
of grain fill with 90% of the
mucuna material incorporated at that
time (Management 2).
3. The muctina crop was grown in rotation with
an unfertilised maize crop (cv. SC501). Two
cropping systems were simulated for both residue
management systems with either one maize
crop after every mucuna crop (mucuna-maize
rotation) or two maize crops after every mucuna
crop (mucuna-maize-maize rotatioll).
Results and Discussions
Simuiation runs on maize response to different
amounts of mineral fertiliser and on maize following
a mucuna crop were done using the long-term
climatic data from Masvingo. On moderate fertility
soils typical of most of the topland fields found in
Zimuto, maize grain yields in the absence of mineral
fertilisers were simulated to be, on average, 494
kg/ha. The yields from such unfertilised crops
range from total crop failure to about 1600 kg/ha.­
These values are similar to values quoted elsewhere
from on-farm and on-station results (Shamudzarira
and Robertson, 2002) and are similar to measured
yields for unfertilised maize in the area. Figure 1
shows the simulated responses to a range of different
amounts of mineral fertiliser additions over
seven seasons on a typica'lly low fertility soil. There
is enormous variation in maize response to any
given rate of fertiliser applied, with -the range being
greater at higher rates of N applied. Smallholder
farmers in this area normally cite the "risk" associated
with the wide variations in yield with N application
(Figure 1) as one of the reasons they use
small amounts of mineral fertilisers. The simulations
also show that in 20% of the seasons there is
no benefit in use of mineral fertilisers in these environments.
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Grain legumes and Green Manures for Soil Fertility in Southern Africa