II.2.1.3. Monoculture
Monoculture describes the practice of planting crops with the
same patterns of growth resulting from genetic similarity. Examples include
Wheat fields or Cassava or Potatoes. These cultivars have uniform growing
requirements and habit resulting in greater yields on less land because
planting, maintenance (including pest control) and harvesting can be
standardized. This standardization results in less waste and loss from
inefficient harvesting and planting. It is also beneficial because a crop can
be tailor planted for a location that has special problems - like soil salt or
drought or a short growing season. Monoculture can lead
to large scale crop failure as this single genetic variant or cultivar becomes
susceptible to a disease. The Irish potato famine in the UK in 1846 was caused
by susceptibility to Phytophthora infestans. Each crop then had to be
replaced by a new cultivar imported from another country that had used a
different genetic variant that was not susceptible to the pathogen (Richard,
1979)
II.2.1.4. Fallows
Traditionally, fallowing was a common practice in most SSA
rural areas. Fields were cropped and then left to rest for one or two seasons
before returning to the same plot. In most cases natural fallow of natural
grasses are allowed to invade the growing fields and then burned and residues
incorporated in soil before sowing again. Benefits of short-duration fallows to
crop yields are sometimes related to the amount of biomass accumulated during
fallow. The practice has disappeared with time due to more demand of land for
producing more food. In order to cut down longer period of fallow on field,
short term fallow using shrubs and legume known to fix nitrogen in soil have
been sought after. The system is very common in agroforestry system and is
known as improved fallow practice. Thus, an accelerated fallow is where
specific fast growing leguminous trees, shrubs, legumes, and other plants are
used to improve soil fertility faster than would occur otherwise, while an
enriched fallow is where trees or shrubs of economic value are planted into the
fallow so that farmer can drive some income from them while the land is
regenerating (Garrity,1999). Improved fallows of perennials and herbaceous
cover crops can suppress weeds, particularly over a number of years and might
be an important component of integrated weed management strategies. Tree
fallows, have distinct advantages over herbaceous fallows, particularly in
seasonally dry climates; because they may take up nutrients from deep soil
layers, and accumulate a large quantity of biomass through which nutrients can
be recycled, and nitrogen fixing trees may add nitrogen to the system through
Biological Nitrogen Fixation (BNF) (Louise et al., 1998).
However, despite proved advantages of improved fallow practice
across most areas in SSA, the practice has not been widely adopted due to land
scarcity that does not allow the farmer to leave his land without producing
even for a short term. Smallholder farmer always seeks maximizing production
through strategic use of limited resources on farm. At higher population
densities, however, scarcity of land means that there is a higher opportunity
cost in putting land in fallow, and intensive continuous cultivation systems
may dominate (Drechsel et al., 1996).
Enriched fallows address this problem to some extent, in that
species that are able to provide some economic benefits, such as fruit or nuts
are planted in preference to species that only improve soil fertility (Cairns
and Garrity, 1999; Sanches, 1999). Other practical benefits to farmer may
include production of fodder, honey, firewood, or bean poles or light timber
for construction. Improved fallow practice seems to be irrelevant for Rwandan
farmers due to shortage of land and need for continuous food production for
household consumption (Nshimyumukiza and Benda, 2005).
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