Chemistry Reference
In-Depth Information
to the development of the DSS, which makes practical recommendations for
appropriate use of organic materials based on their N, polyphenol and lignin
contents, resulting in four categories of materials (Classes I to IV). The DSS
recognises the need for certain organic resource to be applied together with
mineral inputs, consistent with the Second Paradigm.
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3.2
Availability and Production of Organic Resources in African
Farming Systems
In areas with absence of relatively long fallow periods, available organic
resources usually consist of (a combination of) crop residues, animal manure,
composted household waste, and prunings from trees or shrubs within plots or
along their borders. Due to the low current productivity of farming systems,
any of the above is usually available in limited quantities. Manure availability
is largely determined by livestock densities, grazing and feeding systems, and
storage measures.
15
In Western Kenya, for instance, it was observed that the
use of organic inputs decreased strongly with the distance from the homestead
and/or grazing sites, and differed between the crops.
7
Vegetable crops grown in
home gardens received most of the organic resources, followed by the cash and
grain crops grown in the close and mid-distance fields. Virtually no organic
resources were applied to the remote fields, due to the extra effort required to
transport coarse materials to distant parts of the farm. Moreover, in many
areas, crop residues have competing uses and are often removed from the land
to serve as livestock feed or fencing material or to be sold to other farmers. In
the West-African Sahel, herdsmen are often allowed to graze on crop residues
through specific arrangements between sedentary farmers and pastoralists.
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Organic resource availability is also different for different farm typologies with
poorer farmers having less access to organic inputs and often still selling part
of whatever resources they have to better-off farming families.
5
The lack of organic inputs within farms and the need to use such inputs for
restoring soil fertility has been recognised for a long time and several attempts
have been made to increase the availability of organic resources within
smallholder farms. Several windows in time and space can be identified to
produce organic resources (see Table 2) and all of these have been the focus of
specific measures, often referred to as 'improved systems', over the past three
decades. Simultaneous in situ production of organic matter has been most
widely studied in agroforestry systems. In the West-African savannah, for
instance, hedgerow trees in alley cropping systems were observed to produce
940-6027 kg dry matter ha
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season
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After many years of adaptation and
promotion of agroforestry systems, one could conclude: (i) that several
agroforestry systems are technically sound, but only for specific soil fertility,
rainfall and crop conditions, and (ii) that several constraints to adoption of
agroforestry systems exist, related to land tenure, the time for such systems to
deliver the expected goods and services, the need for extra labour, and the lack
of appropriate seed systems, amongst others.
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Long-duration herbaceous
