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

few studies characterized the household assets
of the host farmers. Consequently it is difficult
to assess the reasons why some farmers may
favour one technology and others another.
• Few papers presented actually showed clear
hypotheses for the experiments.
• It is evident from the papers presented that host
farmers are beginning to develop their own local
taxonomies. These need to be catalogued to
enable wider dissemination.
Suggestions arising:
• There is a need to collate information from different
trials in the region into GIS databases to
look at soil, climate and social interactions.
• Ex ante market studies on legumes are required.
This will meet a growing need to assess market
demands for legumes before they are promoted
in an area.
• Need a synthesis study on results ~e have to
date concerning the relative merits and benefits
of intercropping and rotations.
• Combinations of inorganics and organics need
further attention. More and detailed studies are
required on the synergistic effects of organiC
and inorganic fertility amendments. At the
same time, work is required to develop simple
and transferable messages.
• Detailed economic analyses of many of the interventions
bring into question their appropriateness
for smallholder farming systems. If research
intends that the smallholder farmer is to
benefit from their work, it is essential that research
take on a greater participatory emphasis
in problem identification, development and
evaluation of interventions.
legume residues and concluded t fertilizer N
applications were necessary for sustained production
(based on one abnormal year).
2. N availability/dynamics in soil:
• Mineral-N in soil does not correlate well with N
recovery by maize from preceding legumes nor
with maize yield response.
• Mineral-N dynamics suggest that mineralized
N is flushed through the soil profile before
maize roots are present to extract it, leading to
poor synchrony of N availability and N uptake
by maize.
3. N recovery from legumes (and fertilizer) by
subsequent maize crops:
• Measured using 15N techniques by Chikowo et
al.
• Net N inputs from legumes were < 10 kg/ha for
soybean, pigeonpea and erotolaria but> 80 kg/
ha for mucuna.
• N recovery was always < 36%; being least for
mucuna (12%) and greater for legumes with
small N inputs. Their high percent recovery
possibly being due to their low total N input.
• N recovery from fertilizer was 2x N recovery
. from mucuna, which had similar inputs (95 and
84 kg-N fha, respectively).
Issues from the questions and discussion:
• Economics of green manures: What marginal
increment/ yield gain is necessary for farmers
to take up the technology?
• The multiple uses of green manures need 'to be
considered in maize/green manure-grain legume
systems; e.g., animals that graze residues.
Legume Benefits on Maize Productivity
and Soil Properties
Main issues from the three presentations in this
session were:
1. Maize response to legumes in rotations and
intercrops.
2. N availability / dyna.mics in soil as affected by
green manures and grain legumes in rotations
and intercrops.
3. N recovery from legumes (and fertilizer) by
subsequent maize crops.
1. Maize response to legumes in rotations and
intercrops:
• In two studies, legumes gave very large maize
yield increases; by 2-3x the yields without fertilizer.
• BUT one study found only weak responses to
Improving the Productivity of Grain Legumes
and Green Manures
Highlight points from the papers:
1. Agronomic effectiveness of phosphate rock
products, mono-ammonium phosphate and
lime on grain legume productivity in some
Zambian soils (abed I. Lungu and Kalaluka
Munyinda)
• Partially acidulated phosphate rock (PAPR)
maintained a high level of soil P than mono ammonium
phosphate (MAP)
• Lime increased P effectiveness and legume biomass
productivity
• Optimal P application rate for legumes was 80
kg P20S per ha
• Simply processed PAPR (acidulated with sulphuric
acid) was agronomically as effective as
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Grain Legumes and Green Manures for Soil Fertility in Southern Africa