Effects of Residual Soil Nitrogen and Applied Nitrogen on Yields of Head Lettuce

Abstract

Field studies previously conducted in the Matanuska Valley have determined that head lettuce production can be optimized by applying approximately 100 lbs per acre of nitrogen (N) as a fertilizer supplement when residual soil N levels are low (Carling et al., 1987 and 1988). However, conditions in grower's fields often are such that significant quantities of residual N fertilizer may remain in the soil from one growing season to the next. Maximizing the utilization of residual N makes sense both economically as this N has substantial value as a plant nutrient, and ecologically as N may contribute to groundwater contamination if permitted to leach from the soil profile. A field study was conducted during the 1988 growing season to examine the effects of residual soil N in combination with various levels of spring-applied N fertilizer on head lettuce yields. Residual soil N is defined as N present in the soil and detected by a soil test prior to the application of fertilizer in the spring. This study had two primary objectives: to promote maximum utilization of N through accurate interpretation of soil test results and to evaluate interactions between residual and spring-applied N. The results of the first year of this study were reported by Michaelson etal. (1989). The experiment was repeated during the 1989 growing season and the results of that study are contained in this report.