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

GREEN MANURING IN ZIMBABWE FROM 1900 TO' 2002
LUCIA MUZA
Agronomy Research Institute, AREX (formerly Departmentof Research and Specialist
Services), Ministry of Lands qnd Rural Resettlement,
CY550, Causeway, Harare, Zimbabwe
Introduction
The ploughing under of crops for green manuring became popular with farmers in Zimbabwe, then Rhodesia,
during the 1900s. Green manuring was mainly practiced before planting a maize crop or potatoes, to help
supply N to that subsequent crop. Mineral fertilizers were not yet widely used and the ratio of maize to legume
green manure area reached 4:1 (Tattersfield, 1982). Several research questions were raised about greeri
manuring and a series of 'trials were conducted, mainly at Harare Research Station, to address the questions
that farmers had from their early experiences in the 1900s. This paper reviews some of those research areas
and findings on green manuring in the early 1900-1930s and traces the resurgence of recent work during the
1990s to 2000s in Zimbabwe.
Green Manuring from 1900 to the 19505
Screening of suitable green manure crops
A need to identify suitable green manure species
was addressed through the screening of nonleguminous
and leguminous crops. A series of experiments
was carried out for ten years, to screen
crops such as niger oil, sunflower, geotani bean, kaffir,
florida velvet bean, black velvet bean, sunnhemp
and mixtures of these crops. Arnold (1909­
1930) summarized the ten-vear results in the Annual
Reports of Salisbury (Harare) Agricultural Experiment
Station and in a series of journal articles.
Pure veivet bean, dolichos, sunnhemp and nigers
oil, resulted in 8; 8.5; 17 and 15 t of above-ground
biomass respectively at Harare. Sunnhemp was
found to have the highest N mobilization in the
above-ground biomass, whilst niger bean mobilized
the highest amounts of P and K. Arnold noted that
although sunnhemp produced the most green manure
biomass, the subsequent maize crop was less
vigourous compared with that after velvet bean,
which had lower above-ground biomass. This may
have been due to a high % N in the sunnhemp
which results in rapid mineralization of N, not in
synchrony with the N requirements of a subsequent
maize crop. The lower N % (slightly less than 2% N)
with velvet bean, resulted in a slower rate of N re·
lease, more likely in synchrony with the needs of
the subsequent maize crop. It was concluded that
no single green manure species was suit",ble for all
~oil types. Niger oil and sunflower generally gave
lower subsequent maize grain yields.
Legumes were found to be the best crops for green
manu:ing because of their ability to fix atmospheric
N for their own requirement and that of a subsequent
crop. Sunnhemp, velvet bean and dolichos
bean were identified as the best among a range of
potential green manure legumes and there was no
significant difference between these crops. Sunnhemp
became popular with equally beneficial results
chiefly because of its hardiness and suitability
to a wide range of soils, its ability to smother weeds
and ease of ploughing its residues under when used
for green manuring. Sunflower, a non-leguminous
crop, was found to be suitable for green manuring
purposes as well because of its ability to produce
high above-ground biomass (above 4 t/ha in most
cases), although its effect on the subsequent maize
was often lower than with the legumes (Arnold
1928).
Ploughing under green manure crops vs. harvesting
green manure crops for hay
Another research question of concern was whether
it was more ' economical to plough under a green
manure crop at flowering or leave the crop to mature
and harvest the haulms and seed for hay or silage.
A series of trials . was set up to answer this
question. Livestock owners found it more profitable
to use their leguminous crop for hay or silage. There
was a need to apply farmyard manure to maintain
soil fertility when legumes were harvested as hay.
Ploughing under of the whole crop increased maize
grain yield by 1370 kg/ha, compared to harvesting
the green manure legume as hay. Removing aboveground
biomass for other purposes like hay reduced
the subsequent maize grain yield by 6% with
sunnhemp, 16% with velvet bean, 11% with dolichos
bean and 13 % with niger oil green manures.
The difference between ploughing under and not
ploughing under was smaller with sunnhemp than
with the other crops. This was due to the great
Grain Legumes and Green Manure~ for Soil Fertility in Southern Africa 103