Summer Annual Green Manures: Plant and Soil Nutrient Returns for Fresh Market Lettuce Production

Lindsey B. Esterly (M.S.--Horticulture, August 2000), Department of Horticulture, Clemson University, Clemson, SC 29634-0375.

Winter annual legume cover crops are commonly used to provide N to subsequent crops.  In the southeastern United States, cool season crop production extends beyond the window of opportunity for the establishment of most winter annual legumes.  This experiment was conducted to evaluate the contributions of two summer annual green manures, velvet bean (Mucuna deeringiana) and Ladino white clover (Trifolium repens) for fresh market lettuce production.  Conducted in the Piedmont region of South Carolina, this study determined the N, P, and K concentrations contained in cover crop dry matter and soil N, P, and K additions following cover crop incorporation.  Lettuce cultivars ‘Buttercrunch’, ‘Nevada’ and ‘Red Sails’ were grown to measure yield responses within three rates of NH4 NO3 fertilizer application following cover crop and fallow treatments. The experiment was conducted as a randomized complete block, split-split plot design with three replications.  Legume cover crop additions did not increase soil NO3 levels three weeks following incorporation.  Plant available P and K additions were significantly greater following velvet bean incorporation and were determined to be 59 and 260 kg ha-1 respectively.  Following incorporation, plant available soil P and K values exceeded both the nutrients contained in cover crop tissues and nutrient additions due to environmental factors.  Increases in P and K transactions are attributed to increased biological activities involved in the breakdown of organic plant materials.  Lettuce fresh weights following velvet bean were sixty-seven percent greater than the average yielded in fallow plots.  NH4 NO3 additions did not affect lettuce yields.  A slight increase in labor and machinery costs are not expected to outweigh the long-term soil improving benefits derived from legume cover crop rotations.