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

Table 3. Above·ground biomass (t/ha) and N, Pand K
contents (kg/ha) in biomass of green manures grown at
- Makoholi in 1989/90
Dry biomass N f' K
(t/ha)
Dolichos 1.9 46.7 6.1 39.2
Cowpea 1.7 41.9 5.2 34.9
Sunflower 3.3 41.2 8.2 94.1
Sunnhemp 1.5 40.7 5.1 25.5
Soyabean 1.3 26.7 4.7 18.4
Table 4. Average %Nitrogen and %Phosphorus in velvet bean,
sunnhemp and cowpea above·ground biomass and root!; at time of
incorporating in April 1996 .
Velvet bean Sunnhemp Cowpea
Above ground Roots Above ground Roots Above ground Roots
biomass biomass biomass
Nitrogen 1.9 1.38 3.00 0.84 2.16 1.52
Phosphorus 0.13 0.17 0.12 0.04 0.18 0.14
MUla et al 2000
maize grain yields in recent years were less than 500
kg/ha. Soil pH ranged from 4.1 to 4.8 and there
was no correction for pH. Velvet bean, surmhemp
and fish bean were planted with 100 kg/ha P20S or
without phosphorus.
Biomass production by the three legumes is shown
in Table 6 and the grain yield of the maize test crop
after green manuring forChihota and Zvimba
(where a maize crop was harvested) are in Table 7.
Velvet bean performed the best, with six fields gen­
erating an above-ground biomass of over 4 t/ha
when phosphorus was applied whilst four plots
wi~h no phosphorus also produced a biomass above
4 t/ha. Surmhemp performance on the degraded
soil was very variable. Dieback of the plants after
crop emergence was common at most sites.
Velvet .bean gave reasonable biomass on extremely
nutrient depleted and somewhat acidic soils and
has the potential to rehabilitate degraded fields
when coupled with lime and phosphorus. Low pH
and P levels in the soil inhibit legume growth; hence
P and lime should be added.
There is still a need to screen more potential green
manures, to expand the legume base.
Green manuring extension work in Chihota
In the 1999/2000 season, four technologies were se­
lected to help smallholder farmers in Chihota com­
munal area to sustainability improve the crop pro­
ductivity of their farms through improverl soil fer­
tility management practices. This pilot project was
led by the extension sen-ice in Marondera District.
Green manuring was one of the technologies se­
1able 5. Total nitrogen and phosphorus (kg ha· 1) in ab.ove·
ground biomass during the 1995/96 season
Site Velvet bean Sunnhemp Cowpea
N PzOs N PZOI N PZOI
Chiwundura 207 14 68 3 23 2
Chihota 87 6 74 3 22 2
Mlezu 62 4 287 12 102 8
MUla et al 2000
Table 6. Dry biomass production (kg/ha) by three green manure
legumes, on exhausted sandy soils in nor\hern Zimbabwe, 1996/97
season
Communal Area Velvet bean Sunnhemp Fish bean
+ P . P + P . P + P . P
Gokwe South (1) 2368 1916 1688 858 0 0
Gokwe South (2) 1826 1964 809 1000 0 0
Nyazura (1) 8020 7240 0 0 0 0
Nyazura (2) 6490 6610 0 0 0 0
Chiduku (1) 1257 1865 grazed grazed 70 34
Chiduku (2) 4538 2703 116 13 64 66
Mangwende (1 ) 318 317 311 290 145 145
Mangwende (2) 5351 5250 5000 5040 3127 3125
Zvimha (1) 2410 1260 0 0 0 0
Zvimba (2) 85U 1620 0 0 0 0
Chihota (1) 10665 5290 8460 2315 0 0
Chihota (2) 4275 3405 505 550 0 0
AIter Hikwa et al 1998
lected and 411 farmers participated, in farmer
groups, in demonstrations of green manuring on
their farms. Generally, it was found that green manuring
was a new technology to most of the farmers.
Few had tried it or seen it. Forty percent of the
experimenting farmers tried green manuring on
their own fields whilst 83 farmers outside the
groups also used it (Mwenye and Kuwaza, 2001).
There is still a great need to expose far more farmers
to green manuring through working with other extension
districts in Zimbabwe.
Current Work and the Future
Green manuring work in Zimbabwe is still going
on, with the Agronomy Research Institute looking
at the possibilities of · combining mulching using
green manure legumes and minimum tillage. The
University of Zimbabwe and the Agronomy Institute
are also experimenting with different green manures
to control Striga. Agronomy Institute, Crop
Breeding Institute and ICRAF are researching the
possibilities of intercropping Sesbania sesban with
velvet bean and bushy and trailing cowpea. Researchers
on livestock feeds at Research stations and
the University of Zimbabwe are also looking at the
suitability of velvet bean for use in stock feeds. The
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Grain legumes and Green Manures for Soil Fertility in Southern Africa