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

Questions and Answers
Screening of Annual Legumes for Adaptation and Use
To Paul Mapfumo, et al.
Q:If the indigenous legumes dominate natural
fallows, why is maize productivity so low after 1-2
years of natural fallows? .
A: The current weed management approach in most
farming areas is aimed atldepleting the weed
(including these legumes) seed bank through clean
weeding. Therefore the current populations are
probably too low to make an impact in the short
term.
Q: If these 'indifallows' are left for two or more
years, will they contribute more to productivity?
A: The duration of the fallow does playa big role in
indigenous legume species diversity and
abundance. In Chikwaka, legume biomass from
one-year fallows was far less than that from twoyear
fallows for both species richness and
abundance.
To Bongani Ncube, et al.
Q: Would it be useful to separate the woody tissue
from leaves and their separate response to soil
fertility? Will the woody tissue immobilize N?
A: Our research shows that it is beneficial to
inco~porate both leaves and stems soon after
harvesting the grain. Stems decompose
substantially within the year of incorporation
(based on a five-year rainfall period). There seems
to be better synchrony for maize under this system.
Q: Pigeon pea has been researched as long ago as
the early 80's in Zimbabwe. Did you have an
unsprayed crop and was spraying economic?
A: This was a nursery trial aimed at evaluating
varieties, so we did not put up any control plots.
Our aim was to assess what would grow in the
semi-arid region of Matabeleland.
C: There is a lot of information about pigeonpea
adaptability in Zimbabwe from Matopos, Makaholi,
Panmure and Mlezu. See Agronomy Institute
Annual Reports from 1987, 1988 and 1989.
A: It is very difficult to get access to this type of
grey literature from the 1980's. None of the current
literature we have reviewed makes reference to this
early work.
Q: Did the research look at "weed suppression" on
the experiments? In Mozambique it was found that
just two weedings were needed when pigeonpea
was intercropped with maize.
A: Our aim was to keep the crop as clean ·as
possible; so weeding was done every time weed
regeneration occurred.
To Richard Foti, et al.
Q:
1. The 18 kg N ha- 1 for maize seems low for this
heavy feeder crop. Is this practice not leading to
nutrient mining?
2. Does the evaluation of the returns to a crop
include the quantification of some of the more
indirect crop values such as barter and exchange for
labour?
A:
1. Farmers are already mining soils by not using any
inorganic fertilizers at all and are therefore
foregoing extra income that they could generate
from using low rates of fertilizers, earn more
income, buy more fertilizers and move upwards.
Insisting tha t farmers use rates of fertilizer that they
cannot afford and that fail to generate a competitive
rate of return on their investment is retrogressive.
2. Yes the analysis uses the opportunity cost of these
resources and products. This will vary for and are
different for different people and areas, but we
cannot do the analysis for each and every farmer.
So we need to compromise and do the analysis for
one set of prices. We then do sensitivity analysis to
alternative prices.
C: In response to the first question, low rates of N
fertilizer is not the problem contributing to soil
mining. It is lack of investment in soil fertility in
general by smallholder farmers, be it inorganics,
legumes or manure. In dry regions, that is the cause
of low productivity and low soil fertility. Low rates
of N better suit the investment profile of semi arid
farmers and therefore ,r~ "lore likely to be adopted
than the higher "optimal" rates that arestill
recommended.
C: Work from southern Zimbabwe by ICRISAT +
SDARMP have shown that 18 kg N ha- 1 gives the
most economic response. Above 18 kg N ha- 1 often
gives no more yield.
Q : "Results from the @R.isk analysis appear very
close to what farmers in semi arid areas are actually
doing anyway, except for cattle manure in the drier
Grain legumes and Green Manures. for Soil Fertility in Southern Africa
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