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

which did not thrive when shaded by maize. By
June, the bean crop had covered the ground but further
growth was retarded by frost. Ploughing under
was in September with maize planted in December.
The presence of a bean crop at the !11aize grain filling
stage slightly reduced maize grain yield. Dolichos
bean reduced the yield of the relayed maize
crop and the subsequent maize crop, whilst white
jack bean, khaki jack bean and dhal significantly increased
maize grain yields (Arnold 1926-27).
Incorporation of immature vs. mature green manure
crops
Arnold (1926, 1927 and 1929) reported results from
a series of experiments that determined the effects
of leaving green manure crops to mature before incorporation
compared with ploughing under "the
green manure crops at first flowering. The other primary
objective of the trials was to determine
whether the ploughing in of two consecutive green
manure crops in the same season would have toxic
effects on the land or whether the additional organic
matter would be more beneficial than the ploughing
under of one mature crop. Sunnhemp, velvet bean
and dolichos bean were used in the experiment. Incorporation
of mature crops was 5-6 weeks later
than the incorporation of immature crops at flowering.
This work found out that the growing season was
too short to permit two velvet bean crops to mature
unless they were ploughed under before podding.
The biomass of matUre crops was double for velvet
bean and dolichos bean and four times for sunnhemp,
compared with two immature crops. Subsequent
maize yields obtained after immature green
manures were less than those from mature crops,
mostly rl.ue to a higher biomass in the mature crops.
One fully matured green manure crop was better
than two immature crops. These were also compareg
with the effect of a reaped mature velvet bean
on the subsequent maize.
These experiments also determined whether irrespective
of mass of green manure per unit land area,
immature crops ploughed under will ben~fit the
land as mU(;h as if the crops are fully grown. Mature
plants provide a higher percent of organic matter
than immature plants. Fully developed crops had a
more beneficial effect than a partially developed
crop. Mature green .manure crops of velvet bean
and sunnhemp more than trebled maize yield in the
first season after green manuring whilst immature
crops doubled maize yields compared with continu­
·ous unfertilized maize. The second season maize
after both ma'ture and immature green manure
crops did not benefit from green manuring.
Timson (1946) also conc1uded that ploughing under
of a green manure crop before the end of the rainy
season led to excessive leaching of the nitrogen.
Ploughing under at the end of April compared to
February and March resulted in higher grain yields
of the subsequent maize crop.
Green manure crops in rotations
Rotation experiments were carried out and green
manure legumes were recommended in the differ­
ent rotation systems; in pure crop production sys­
tems as well as for crop and livestock farming sys­
tems. An example of a recommended rotation, de­
signed to meet the needs of a grain farmer whose
income was solely dependent on maize and ground­
nut, is given below.
Year 1 = Maize + fertilizer
Year 2 =Green manure legume
Year 3 = Maize
Year 4 =Groundnut
A supply of humus in the soil' was maintained by
ploughing under the velvet bean and dolichos bean
green manures. The green manure crops followed
immediately after the maize crop that received mineral
fertilizer, hence the green manure benefited
from the resid ue of the fertilizer left in the soil. In
this way, an adequate supply of humus was retained
in the soil for the maize that followed the
green manure crop.
For dairy farmers, succulent legume crops were also
included in the rotation to provide ample feed for
livestock, for example.
Year 1 =Maize plus farmyard manure
Year 2 = Oats, velvet bean or dolichos bean mixtures
for hay
Year 3 = Maize plus mineral fertilizers
Year 4 = Sweet potatoes (succulent crop for winter
food for stockfeed)
The rotational experiments highlighted the drawbacks
of continuous (year-after-year) maize cropping
on the same land. Over 13 years, maize yields
were trebled in planned rotations compared with
unplanried rotations similar to those found in smallholder
communal areas, where continuous maize
cropping is very common. Leaving the fields fallow
was also · found to be less productive when compared
to inclusion ofgreen manures for fodder purposes
or exclusively as green manures. It was concluded
that when farm stocks and crops are judi­
, ciously combined, the permanent fertility of the soil
is increased and larger crops are secured. This was
calculated to be profitable and a form of insurance
against unfavorable seasons.
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Grain Legumes and Green Manures for Soil Fertility in Southern Africa
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