Agriculture Reference
In-Depth Information
Keywords:
soil acidity, soybean, nodulation, liming, N and P uptake
1.
I
NTRODUCTION
Soybean (
Glycine max
(L) Merr) accounts for 40% of the national edible vegetable oil
and seed cake output in Zimbabwe (Whingwiri, 1996). On average a 100 g of soybean
contains 1700 KJ energy, 40 g protein, 20 g fat and 10 g water. The inclusion of soybean in
cropping systems significantly improves soil N status as the crop can fix up to half of its N
requirements when in symbiosis with Rhizobia thus reducing the amount of N farmers have
to apply as fertilizer (Wynch and Rains, 1978). Successful production of the crop in the
smallholder sector can be a source of protein source for poor households most of which have
been affected by the HIV/AIDS pandemic and therefore require a good diet to improve their
health. Soybean can significantly contribute to sustainable soil fertility management if the
stover in incorporated into the soil after harvesting grain.
Soil acidity is a major constraint to crop production in tropical and subtropical areas as
well as cultivated temperate areas. About 27% of soils in tropical Africa are classified as
acidic (Pandey
et al.,
1994) and low soil pH is the number one growth-limiting factor (Jones,
1975; Copeland, 1976). Reduced plant growth in acid soils is mainly due to Al and Mn
toxicities as well as deficiencies of P, Ca and Mg (Mugwira and Haque, 1993). In Zimbabwe
the majority of smallholder farming areas are located on sandy soils which are low in organic
matter (<1%) and weakly buffered, and in high rainfall zones the soils are also acidic and
often deficient in P, Ca and Mg (Mugwira and Nyamangara, 1998). Like any other legume,
soybean is sensitive to soil acidity (Al and Mn toxicity) and poor growth has been reported on
acid sandy soils in Zimbabwe (Mpepereki, 1994).
Despite the documented positive effects of lime, its adoption by smallholder farmers has
been poor mainly because of inaccessibility and high transportation costs although the lime
itself is relatively cheap. The financially constrained farmers opt to buy seed and mineral
fertilizers before they consider lime (Dhliwayo et al., 1998).
The breeding or selection of crop cultivars tolerant to soil acidity offers an opportunity to
sustain crop production in high rainfall areas of Zimbabwe, and related farming systems in
sub Saharan Africa.The growth of legumes under acid conditions depends on the tolerance by
both the host plant and the bacteria species it forms a symbiosis relationship with.
Reduced
performance by a legume and/or rhizobia under acidic conditions can be a result of the
negative environmental conditions induced by acidity such as toxicities of Al, Fe, Mn and
deficiencies in Ca, Mg, P and Mo both of which hinder growth, survival and development of
the rhizobia and the plant (Giller, 2001). Deficiencies of the major cations Ca and Mg are
common due to the small amounts of these cations in acid soil and to the secondary effect of
high concentrations of H
+
inhibiting uptake of these cations by plants (Andrew, 1978). Al
decreases the initiation of nodules at concentrations that have no discernible effect on growth
of the legume plant or functioning of nodules (Robson, 1983). In acid soil rhizobia fails
because it cannot grow and colonise the host rhizosphere (Keyser et al., 1979). Few rhizobia
strains can grow under conditions more acidic than pH 4.5 (Graham, 1992).
A corresponding problem, with acidity, particularly when exchangeable Al
3+
is high is
high P fixation (Graham, 1992) and soybean has relatively high P requirements. Phosphorus
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