One approach for using ecological knowledge to increase the range of tactics available for weed management involves managing soil conditions to suppress weed emergence, growth, and competitive ability (Gallandt et al.,1999; Liebman and Davis, 2000). As a consequence of the fixed root habit, both weeds and crops are affected by soil conditions. Weeds and crops may differ in their responses to those conditions, however. Just as certain herbicides are toxic to weeds but do little or no harm to crops, certain edaphic factors may suppress weeds but have neutral or positive effects on crops.

Legume residues, composts, and manures are widely used in organic and low-external-input farming systems to maintain soil productivity. Experiments conducted with several crop–weed combinations indicate that use of these soil amendments in place of synthetic fertilizer can also reduce weed density, biomass production, and competitive ability, at the same time maintaining or improving crop performance. This has been true for sweet corn and field corn grown in competition with Chenopodium album (Dyck and Liebman, 1995; Dyck et al.,1995); dry bean in competition with Brassica. kaber (Liebman and Ohno, 1998); and potato in competition with Chenopodium album and other weed species (Gallandt et al.,1998). Several factors appear to be responsible for the differential responses of crop and weed species to soil amendments.

First, the substantial differences in seed size that exist between many weeds and crops (Mohler, 1996) may convey differential susceptibility to early-season stresses that limit photosynthesis and nutrient uptake. Because of greater seed reserves, seedlings of large-seeded crops, such as corn and soybean, are likely to be more tolerant of low-nutrient conditions and soilborne phytotoxins, whereas seedlings of small-seeded annual weeds are likely to be less tolerant of these and other stresses (Westoby et al., 1996).

Second, many annual weed species are better adapted than crops for rapid nutrient uptake and biomass production early in the growing season (Alkämper, 1976; DiTomaso, 1995). Consequently, under conditions of high nutrient availability, many small-seeded weeds are highly competitive with large-seeded crops, despite their lack of seed reserves. Changes in the timing of nutrient availability have been investigated for their potential to alter weed growth and competitive ability. By delaying the application of synthetic fertilizer until several weeks after crop and weed germination, the growth of weeds such as B. kaber, Chenopodium album, and Veronica hederifolia can be reduced and the yield of crops such as corn and wheat can be increased (Alkämper et al., 1979; Angonin et al., 1996). Certain crop residues, composts, and manures may serve as slow-release nutrient sources that are better synchronized with the nutrient demands of crops rather than weeds (Dyck et al., 1995; Liebman and Davis, 2000). Specific rates of nutrient release from organic soil amendments will depend on substrate quality, soil temperature and moisture conditions, and other factors, but could be regulated advantageously.

Third, crop residues, composts, and manures serve as sources of biochemicals that can affect crop and weed growth. Some of these compounds are growth inhibiting, whereas others are growth promoting. In addition to containing nitrogen, which generally stimulates plant growth, legume green manures can release a range of phytotoxic compounds (Liebman and Ohno,

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