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

Questions and Answers
Improving the Productivity of Grain Legumes and Green Manures
To Obed I. Lungu and Kalaluka Munyinda
Q: Was the reduction in yields after adding MAP
not due to N2 fixation reduction by the presence of
N in the MAP?
A: No, all treatments received optimal N, and N2
fixation was not being evaluated in this study.
PAPR has no N, but these treatments received N
from either urea or ammonium nitrate.
Q:
1) Were these trials conducted on station or on
farm?
2) If on farm, do farmers still get the benefits of
PAPR if they plant late or weed less effectively?
3) What is the cost of getting P20S per unit of
nutrient by applying PAPR compared to DAP,
considering the bulk of PAPR is inert material?
A:
1) The trials were researcher-designed and farmer
managed. They were both on farm and on station.
2) PAPR is just a substitute or alternative to MAP,
TSP, etc. Farmers would therefore manage their
crops with PAPR in the conventional way.
3) At only 10% P20S in PAPR, the material is bulky.
The product that will be promoted will be
beneficiated by a physical process to at least 15%
P20S as already demonstrated.
Additionally, the utilization of PAPR is being
promoted for areas close to the deposits of PR.
Q: Should RP work still be considered a priority
given that so much work has been done, but these
materials are most often not available nor
sufficiency reactive?
A: Yes, PR work is still needed, but there should be
a shift from basic research on-station to promotion
and demonstration on-farm. Soils are acutely
deficient in P, and adequate P application is in
excess of 80 kg P20S ha·1, which is beyond what
small farmers can afford especially since imported
fertilizers in the region cost at least four times their
cost outside Africa. The local product would be
developed and made available to farmers if there
was the political will and the policy and
institutional support.
To Atusaye Mwalwanda, et al.
Q: I do not think the effect of S increase was
studied, when looking at your treatments?
A: The fertilizer source used was unfortunately a
Grain Legumes and Green Manures for Soil Fertility in Southern Africa
compound fertilizer 23:21:0:4S, hence it is difficult to
isolate the individual effects of nutrients. However,
the response to the inorganic fertilizer is an
indication of the deficiency of the elements in the
soils at the study sites.
Q: In your first conclusion your attribute an increase
in yield to P and S, but what is the economic return
for the extra yield versus the cost of nutrients?
A: Economic analysis was not done in this study.
There is need to do that analysis to get the benefit
from using inorganic fertilizer compared with not
using.
To Lackson K Phiri, et al.
Q: Ca and Mg are usually leached into the sub-soil.
In your assessment of the residual effect of the
liming materials tested on soil Ca and Mg levels,
what was the justification for tracing the two bases
down to 20 cm soil depth only?
A: I agree that there is the possibility of Ca and Mg
leaching in a high rainfall envirorunent such as
where the trial was located. But the trial was only in
the second year so our interestwas to see what was
happening in the rooting depth (0 - 20 cm) and later
assess the leaching of these cations down the soil
profile. Unfortunately funding ended prematurely.
Q: Your results do not mention the effect of
increasing lime material on available phosphorous,
but only mention pH and exchangeable AP+ effects.
Why did you not mention P availability since it is
very much influenced by AP+?
A: Certainly P is critical. Indeed in the middle of
the trial in the 1995/96 season, P deficiency
symptoms were observed such that an additional 20
kg P20s/ha had to be added to correct P deficiency.
General Discussion
C: How much yield benefit after a fallow is
necessary for the system to yield more than with
two years of cropping? The naive answer is "twice
as much". But maize next year is not worth as much
as maize in your hand - an effect economists call
"discounting". This means the yield improvement
must be more than two times for the fallow to be
viable. On the other hand, the fallow may take less
labour and may benefit long term-fertility or reduce
weed populations, which implies that less than
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