Sheryl,
Organic crop yields are sometimes lower than conventional yields due to
inadequate (N) nitrogen availability or excessive weed competition,
particularly during the transition, but this really depends on alot of
interacting and site-specific factors. Stanhill (1990) reviewed about 200
studies comparing organic and conventional cropping systems in Europe and N.
America and found that organic yields averaged about 9-10% less. This is an
interesting and important finding but may not tell you much about a specific
region or crop due to geographic differences in soils, climate, pest
pressures, farmer experience and support, market demands, etc. The results
of studies coming out since then have been variable.
At the Sustainable Agriculture Farming Systems (SAFS) project at the
University of California at Davis, we've had lower organic corn and tomato
yields (compared to conventional years) in some years due to N
immobilization by soil biota and weed pressure. However, bean yields have
often been higher under organic management.
Nitrogen mineralization/immobilization is sometimes less predictable in
organic systems and may present problems for crops with high N demands, such
as corn. However, after 10 years of farming organically these soils have
been pretty consistent in supplying adequate N. Last year we had a weed
problem in the organic corn plots. Crow damage at germination resulted in
poor stands in the organic and conventional treatments. Consequently, weed
pressure was high due to the gaps. Herbicides were used in the conventional
system to prevent yield loss but no economically feasible control options
were available in the organic system once the corn was too high to cultivate.
Organic tomato yields at the SAFS site have equalled conventional yields
over the last few years. Weeds can be hand hoed out of this high-value crop
and other pest groups (insects, nematodes, and diseases) are not a problem.
This is not the case in the eastern U.S., however, where diseases can be a
real problem in the humid climate.
Overall, the findings of the SAFS project indicate that equal crop yields
can eventually be achieved in this area with organic methods but that
economic risk may be greater, particularly during the transition, and
production costs may be higher, depending upon the crop.
Stanhill 1990. The comparitive productivity of organic agriculture.
Agriculture Ecosystems & Environment 30: 1-26.
M. Sean Clark
Research Manager
Sustainable Agriculture Farming Systems Project
Department of Agronomy and Range Science
University of California
Davis, CA 95616
msclark@ucdavis.edu
TEL:(530) 752-2023
FAX:(530) 752-4361
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