Agriculture Reference
In-Depth Information
Soil fertility
The impact of fertilizers on intercrop-weed mixtures is a function of
inherent species-specific responses to fertility conditions and changes in
interspecific competitive relationships created by those responses (Bantilan,
Palada & Harwood,1974).When barley/pea intercrops were grown in mixture
with Brassica hirta , fertilization with ammonium sulfate reduced the growth
of pea relative to barley (Liebman, 1989), diminished the intercrop's ability to
compete for light and N, and increased B. hirta 's photosynthetic rate, leaf area,
total biomass production, and seed output (Liebman & Robichaux, 1990).
Comparison of B. hirta biomass in pure stand and in mixture with barley and
pea indicated that competition from the intercrop reduced the weed'sbiomass
by 49% when fertilizer was used, but by 87% when it was not (Liebman &
Robichaux, 1990). In contrast, Olasantan, Lucas & Ezumah (1994) found that
application of calcium ammonium nitrate to a maize/cassava mixture
increased the yield of both maize and cassava, but maintained the relative
balance between the two crop species. Fertilizer application reduced weed
biomass (a mixture of many grass, sedge, and broadleaf species) by increasing
the intercrop's leaf area index and light interception ability.
Because many intercrops are composed of legumes, which use atmospheric
N 2 , and non-legumes, which use inorganic forms of N from the soil pool and
fertilizer, a key research question is how to manage the N nutrition of differ-
ent intercrop components so that total yield is optimized and weed growth is
minimized. Can the location, timing, and form of fertilizer N application be
adjusted to match the N requirements of non-legume crops, while withhold-
ing inorganic N from legumes and weeds? Research is also needed to under-
stand how different types of N management affect crop-crop interactions in
mixtures containing only non-leguminous species and how these relation-
ships affect weed performance. Additional research is needed to determine
whether management of non-N nutrients should differ between intercrops
and sole crops.
Living mulches and smother crops
The preceding discussion has focused on how intercropping may
affect weeds when each component crop is intended to be a source of food,
feed, or fiber. Here we consider the impacts of intercropping with species that
are intended mostly for other purposes, such as soil conservation, nitrogen
fixation, and weed control, though they may produce food and other harvest-
able products incidentally. Used in this way, such species are “minor” compo-
nents in mixture with “main crops,” and are commonly called living mulches
and smother crops (Shetty & Rao, 1981; Paine & Harrison, 1993; De Haan,
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