A gronomy J our n al • Volume 10 0 , I s sue 6 • 2 0 0 8 1587 C over cropping is an essential component of many production systems. Cover crops are any ground covers grown to add N and organic matter into the soil, suppress pest populations, and reduce soil erosion and nutrient leaching (Creamer and Baldwin, 2000;Aguiar et al., 2001;Hartwig and Ammon, 2002;Hutchinson and McGiff en, 2000). Th ese species and management practices need to be carefully chosen to optimize the benefi ts and yield of cash crops (Al-Khatib et al., 1997;Scott and Knudsen, 1999).Cover crops may aff ect nutrient availability in subsequent cash crops (Cookson et al., 1998). Residue decomposition modeling has been useful for establishing patterns of nutrient release and plant demand (Myers et al., 1997). Carefully scheduled cover crops can enhance N uptake and increase the yield of cash crops (Th orup-Kristensen, 2006). If a cover crop produces large amounts of biomass and is not given suffi cient time to decompose, nutrients immobilized in the cover crop residue may not be available at the early growth stages of the following cash crops. Th e following cash crop yield could also be reduced due to an allelopathic eff ect of cover crop residues (Kessavalou and Walters, 1997;Weston et al., 1989).Th erefore, careful cover crop management is required for optimal yield of cash crops.Cover crops may also aff ect weed density in the following cash crops. Appropriate use of cover crops can reduce weed populations and the amount of inputs required for weed control (Teasdale, 1996;Caamal-Maldonado et al., 2001). In addition, using cover crops as surface mulches in reduced tillage systems may alter weed population dynamics (Burgos and Talbert, 1996;Hutchinson and McGiff en, 2000;Buhler, 1995). Cover crop and cash crop management can be especially critical during transition from conventional to organic farming system as changes in plant species diversity and species composition may occur during organic transition (Hald, 1999;Ngouajio et al., 2003).A typical crop rotation in the low desert of California and Arizona is a cool-season vegetable crop in the fall and a warmseason crop in the spring. Many vegetable growers fallow their fi eld during the hot summer months, providing an excellent window to grow warm-season cover crops that are adapted to the intense desert heat. Sudangrass and cowpea are adapted to tropical conditions, tolerate high temperatures, and can produce high biomass in a short period (Creamer and Baldwin, 2000). Additionally, both cowpea and sudangrass can reduce weed populations for the rotation crops (Ngouajio et al., 2003). Th e combination of pest suppression and N fi xation from cowpea is especially useful in organic production systems where no synthetic pesticides and N fertilizers can be used.We investigate the potential of cowpea and sudangrass cover crops for improvement of a lettuce-cantaloupe rotation system in the low desert of southern California. Cover crops were evaluated for their eff ects on soil temperature, N availabi...