Agriculture Reference
In-Depth Information
TABLE 15.1
Potential Benefits of Cover Crops
Interferences
Benefits to Crop Community
Impacts on soil structure
Enhanced root penetration in upper soil layers;
shielding of soil surface from sunlight, wind,
and the physical impact of raindrops; addition
of organic matter to soil; enhanced biological
activity in root zone
Improved water infiltration
Reduced soil crust formation
Decreased runoff
Less soil erosion
More stable soil aggregates
Increased percentage of macropores
Decreased soil compaction
Decreased bulk density
Impacts on soil fertility
Creation of cooler, moister surface and subsurface
habitat; fixation of nitrogen by Rhizobium
bacteria; carbon fixation (greater biomass);
capture of nutrients by roots
Increased organic matter content
Retention of nutrients in system
Prevention of leaching loss
Increased nitrogen content
Greater diversity of beneficial biota in soil
Impacts on pest
organisms
Addition of allelopathic compounds; removal of
resources (light and nutrients) needed by
weeds; creation of habitat for beneficial
predators, parasites, and parasitoids;
modification of microclimate
Inhibition of weeds by allelopathy
Competitive suppression of weeds
Control of soil pathogens by allelochemicals
Increased presence of beneficial organisms
Suppression of pest organisms
Source : Lal, R., E. Regnier, D. J. Exkert, W. M. Edwards, and R. Hammond. 1991. In W. L. Hargrove (ed.), Cover Crops for Clean Water .
pp. 1-14. Soil and Water Conservation Society: Iowa.; Altieri, M. A. 1995a. In M. A. Altieri, Agroecology: The Science of Sustainable Agriculture .
2nd ed., pp. 219-232. Westview Press: Boulder, CO.; Magdoff, F. and R. R. Weil. 2004. Soil Organic Matter in Sustainable Agriculture . Advances
in Agroecology Series. CRC Press: Boca Raton, Florida.
COVER CROPPING WITH RYE AND BELLBEANS
Multispecies cover crop systems often confer greater benefits to the agroecosystem than a cover crop of just one
of the component species. These benefits arise from interactions between the species in the mixture.
One such system has been studied at the Center for Agroecology's farm facility at the University of California,
Santa Cruz (UCSC). A legume (bellbean) is mixed with a grass (cereal rye) as a winter cover crop for vegetable
fields. This multispecies cover crop has been used by local farmers since the turn of the century. Farmers plant
the grass-legume mixture following the harvest of the summer crop, before winter rains begin. It is allowed to
grow through the cool, wet months of winter and is disked into the soil in March or early April. The summer
vegetable crop is then planted toward the end of May. The UCSC study used cabbage as the vegetable crop.
Rye produces significant amounts of biomass and limits weed growth in the plots, possibly through the release
of allelopathic chemicals (Putnam and DeFrank, 1983). Bellbeans bring nitrogen into the system through their
symbiotic relationship with nitrogen-fixing bacteria, but they produce limited biomass and have only a minor effect
on weed growth. When bellbeans and rye are planted together, the advantages of both are combined: the mixture
suppresses weed growth, is highly productive, and adds nitrogen to the system. But that is not all. The mixed
cover crop does a better job of increasing nitrogen levels in the soil than does a legume-only crop, even when the
legume-only crop has a higher legume biomass. It is possible that the increased organic matter being disked in
with the bellbeans slows decomposition, retaining more nitrogen in the soil.
The mixed cover crop also proves to be of benefit to the vegetable crop that follows it. Although cabbage
yield was highest in the bellbean-only treatment, it was not statistically different from the high cabbage yield of
the rye-bellbean treatment, and both yields were significantly higher than those for rye alone and the control.
Because of the greater bulk of organic matter it adds to the soil, the mixed cover crop would probably show the
greatest benefits over a period of many years.
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