Soil - Agroecology: The Ecology of Sustainable Food Systems - page 108

noncrop species, whose primary role is biomass produc-
tion and the return of organic matter to the soil. But
present-day agriculture, with its focus on the market, has
reduced plant diversity so greatly that very little organic
matter is returned to the soil.

Cover Crops

Cover cropping, where a plant cover is grown specifically
to produce plant matter for incorporation, as a “green
manure” into the soil, is another important source of organic
matter. Cover crop plants are usually grown in rotation with
a crop or during a time of the year that the crop can't be
grown. When legumes are used as cover crops, either alone
or in combination with nonlegume species, the quality of
the biomass can be greatly improved. The resultant biomass
can be incorporated into the soil, or left on the surface as
a protective mulch until it decomposes.
In research done at UC Santa Cruz (Gliessman, 1987),
a local variety of fava bean called bellbean ( Vicia faba )
was grown as a cover crop in combination with either
cerealrye or barley during the winter wet season fallow
period. It was shown that the total dry matter produced in
the grass/legume mixtures was almost double that of the
legume alone. After 3 years of cover crop use, organic
matter levels in soils under mixed covers improved as
much as 8.8%. Interestingly, soils under the legume-only
cover actually dropped slightly in organic matter content

Crop Residue

An important source of organic matter is crop residue.
Many farmers are experimenting with better ways of
returning to the soil the parts of the crop that are not
destined for human or animal use. A major concern has
been how to deal with potential pest or disease organisms
that residue may harbor and pass on to a subsequent crop.
Proper timing of incorporation of the residue into the soil,
rotating crops, and composting the residue away from the
field and then returning the finished compost are possible
ways of overcoming this problem. Research on these and
other management strategies are helping transform crop
residue from a problematic by-product into a valuable part
of soil organic matter management (Franzluebbers, 2004)
(Figure 8.5).

FIGURE 8.5 Burning of crop residue in Taiwan. Burning is a common method of removing crop residue. Although it returns
some nutrients to the soil and helps control pests and diseases, burning can cause significant air pollution and prevents crop residue
from being incorporated into the soil as organic matter. When crop residue is seen as a valuable and useful resource for maintaining
soil organic matter, techniques for incorporating it into the soil can be developed as alternatives to burning.