Opportunities and Challenges for the Inclusion of Soil-improving Crops in Vegetable Production Systems

Abstract

The ability of certain crops to improve soil fertility or physical conditions has long been recognized. As early as the Chow dynasty (1134-247 B.C.) in China, there were reports of crops whose value for soil improvement was “greater than silkworm excrement” (Pieters, 1927). Romans around the time of Christ likewise waxed eloquently on the value of green manures, as in this line from Virgil: “Sow your wheat on land where grew the bean, the slender vetch or the fragile stalks of the bitter lupine.” The potential benefits of soil-improving crops (SIC-s) are many and varied. In addition to the continued use of legumes as a source of biologically fixed N, in more recent years, SICs have been commonly included in cropping systems to retard soil erosion and build up organic matter. In addition to these objectives, a 1988 survey of New England farmers using cover crops revealed that nearly half expected to derive some weed control from the SIC (Schonbeck, 1988). Fewer used SIC to conserve nutrients or make certain elements (such as P) more available, or to control insects and pests. Growers with soil compaction problems may grow a deeprooted SIC to break up dense soil layers (Foulds, 1989). No single SIC can impart all the above benefits to a particular cropping system. In addition, there are some potential negative effects of SICs, including: depletion of soil moisture, lowering of spring soil temperatures, disruption of field operations, allelopathy, and the creation of habitat for harmful organisms. Even nitrate pollution of groundwater is a potential problem with the improper management of legumes in cropping systems. The realized value of the SIC to a particular cropping system will, of course, depend on the selection of species, management of those species, and interaction with the rest of the cropping system, including crops, weather, weed