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

4500
4000 .
3500
to
'" 3000 - , "-··1992
. • . 1993
0>
0<
-0 2 500 - .... - . 1994
1\j --1995
2000
--_·01(·····_·1996
>=
c
'm
i!l
1500
1000
500
0
0 20 40 60 80 100
N rate kg/h.
- - 1997
···--+-­ 1996
Figure 1. Maize grain yield responses on a low fertility soil to
varying amounts of applied N(kg/hal simulated using tong-term
weather data from Masvingo
Agronomic nitrogen use efficiencies (NUEs) were
calculated from the simulated maize yields as extra
kg grain produced divided by extra kg of N applied.
Averaged over all years, agronomic NUEs
declined from around 45-56 kg grain per kg of applied
N at low application rates to about 33-38 at
high N rates. In Figure 2, the agronomic NUE for a
crop receiving 10 kg N/ha is plotted against the
yield for an unfertilised maize crop. The data suggests
that responses to low rates of N (commonly
used by smallholder farmers) are generally larger
on low than with high fertility soils.
Simulations were also conducted for the same climatic
record for maize yields following a mucuna
crop on a moderate fertility sandy soil. The mucuna
was managed in two ways: - either harvested at maturity
(1 July) and then incorporated just before
maize planting (Management 1), or harvested and
incorporated at flowering (Management 2). For
each management system, two cycles of simulations
were done, one in which a single crop of maize was
grown following mucuna and the other in which
two maize crops were grown in succession after a
mucuna crop.
Despite having fewer seasons in which maize is
grown in the mucuna-maize and mucuna-maizemaize
rotations when compared to continuous sole
maize, maize grain yields averaged over the 47-year
record are predicted to be 3-5 times higher in rotations
that include mucuna (Figure 3). With continuous
sole maize cropping, just over 20 tonnes of
maize grain is realised over 47 years compared to
totals of between 80 and 120 tonnes in rotations that
include mucuna (Figure 4).
At a 50% probability level, unfertilised maize grain
yields are 3.5-4.0 t/ha for the mucuna-maize rotations
and 3.5-4.5 t/ha for the mucuna-maize-maize
rotations (Figure 5). These yields are far greater
than the 200-300 kg/ha obtained for continuous sole
maize cropping at the same probability level.
Grain Legumes and Green Manures for Soil Fertility in Southern Africa
. .
~ ..e ".
. .' / . '.
YIeld response 10 10 kg Nnw
Figure 2. Simulated maize yield response to a 10 kg N/ha
application for a crop grown on soils of different fertility status
300• . 0,-______________---,
025 00.0+_-------------~~
i 2(100 .0
.
>. I ~OO.O+_------~~
.,
•
i 1 00 0 0 +_------~
Figure 3. Mean maize grain yields for the 47·year record for
different cropping systems
OCOOOr-----------------------------------~
~~------------------~----------~~.~
70000+_----------------~----~~~=---~
~~+-------------------~~~----~~--~
.~
~50000~--------------~~_,~~--------~
jf~+_-----------~L-~~-------------~
i~r-------~#7~------------------~
u~+_---~~-------~--~~----------~
l0000+_~~~~~------------------------_i
SoIematze 'MxlJna-rreize1 - Mucuna-maize2
- 'Muruna-maize-rT'9ize1 .... 'Muruna-rrnlze-malZ.e2
Figure 4. Cumulative maize grain production over 47 years for the
different production systems
The loss in maize production from the piece of land
where mucuna is grown in the first season, coupled
with labour constraints, has been given by many
workers as one of the limitations in the uptake of
green manure technologies by farmers (Kumwenda,
Waddington, Snapp, Jones and Blackie, 1997).
However, in Northern Malawi on fairly good sandy
soils, a one season sole crop green manure can increase
maize yields from 200-300 kg/ha to up to 4
000 kg/ha. The data presented here also suggests at
the 50% probability level, a yield surplus from an
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89