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

implied that most fanners prefer to invest their labour
inputs where returns are already favourable,
thus apparently following a nutrient concentration
strategy - the poorer or less productive the field, the
less applied. The ameliorative role of manure to soil
was not a new concept to the smallholder farming
community where cattle manure was viewed as a
. common and significant soil fertility input in maize
systems (Tanner and Mugwira, 1984; Mugwira and
Murwira, 1997). Livestock manure applications to
soil have been known to increase soil pH, infiltration
rates, water holding capacities and decrease
bulk densities (Grant, 1967; Murwira, 1993).
Preliminary results from this study have revealed
that cultivation of grain legumes for food is at a low
scale in all the three Natural Regions, be it as rotations
or intercrops while exploitation of legumes for
soil fertility management is virtually non-existent.
While information on the role of leguminous plants
in replenishing soil fertility is available (Sanchez,
1995; Giller, 1998; Mapfumo, 2000), these results
suggest that the strategies to translate this valuable
information to the smallholder farming community
need diversification. Currently there is a general belief
among smallholder communities that legumes
are a women's crop (Mapfumo et al., 200tb) and this
belief coupled with poor extension methods and
over emphasis on cereal production, have led to reduced
legume cultivation. Application of leguminous
residues in arable farming systems provides a
ready supply of N to growing crops. While very few
farmers appreciate the role of grain legume residues
in soil amelioration, some results have shown that
in cereal cultivation, N contributions from legumes
can be as high as 250 kg N ha- 1 yrl (Giller, 2001).
However, in poor sandy soils, reported values have
mostly been less than 30 kg N ha- 1 (Mapfumo, 2000).
It is imperative to note that the impact of legumes
on soil productivity may not only be restricted to N
contributions, which has been the major focus of
previous work on organic input research. The quantity
and quality of C supplied by many of these organic
materials may also playa significant role in
soil productivity. Information of the role of decomposable
C on nutrient release and soil amelioration
from high quality organics including legume residues
is not well documented (Kirchmann and Bergquist,
1989). This information is essential in guiding
farmers and land-managers to optimally use
their organic resources, both in the short and in the
long term. Legume resid ues are most beneficial in
providing nutrients in the short-term, an option
more likely to be appealing to most smallholder
farmers (Palm et al., 2001). In the wake of diminishing
resources, the groyving of legumes and utilizing
their residues may be a realistic way of increasing
soil available C in sandy soils. This study aims to
address the practicalities of these issues. Are we as
'researchers doing enough to promote soil organic
matter build-up in our inherently poor soils? Participatory
experiments with fanners in Murewa (NR
II) suggested that Cajanus cajan (pigeonpea) could
be successfully grown by farmers yielding very
high biomass of up to 23 t ha- 1 in 2 years (Mapfumo
et al., 2001).
While it is difficult to make conclusive statements
based on these preliminary results, a few lessons
can be drawn. In Zimuto, use of organic fertilizers
in arable farming showed that soil C reserves could
be improved judging from the relatively high contents
of soil organic C, compared to the soils in
Chinyika where mineral fertilizer usage takes precedence.
Although cultivation in Chinyika is barely 20
years old, soil C and N are already depressed
probably stemming from the heavy dependency on
mineral fertilizer with little or no organic inputs. We
therefore conclude that there is merit to develop
strategies for the use of organic inputs, to not only
improve the soil organic C status, but also crop
yields through efficient nutrient uptake. The term
organic fertilizer should be given a new meaning
for the smallholder environment to not only mean
manure qut crop residues as well. For the nonowners
of cattle, the window of opportunity rests
with the growing of legumes with the N-rich stover
being retained in the field.
It is imperative to investigate how nutrient availability
is related to the quantity and quality of C
supplied in organic resources used by farmers in the
medium- to long-term if combined use of organics
and mineral fertilizer is to be optimized.
References
Anderson, }.M. and Ingram, }.S.1. 1993. Tropical Soil
Biology and Fertility: A Handbook of Methods. Second
Edition. CAB International. Wallingford,
UK. 221 pp.
Ashworth, V.A. 1990. Agricultural Technology and the
Communal Farm Sector. Main Report. Background
paper prepared for the Zimbabwe Agricultural
Sector Memorandum. The World. Bank, Agriculture
Division, Southern Africa Department,
Washington DC, USA. 159 pp.
Giller, K.E. 1998. Tropical legumes: Providers and
plunderers of nitrogen. In: Carbon and Nutrient
Dynamics in Natural and Agricultural Tropical Ecosystems.
L. Bergstrom and H. Kirchmann (Eds.).
CAB International, Wallingford, UK, pp. 33-46.
Giller, K.E. 2001. Nitrogen Fixation in Tropical Crop-
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Grain legumes and Green Manures for Soil Fertility in Southern Africa