Effects of Repeated Application of Four Rates of Two Composts Varying in Carbon: Nitrogen Ratio on Soil Nitrogen and Leaf Lettuce Yield

Abstract

The effects of repeated application of two composts differing in carbon: nitrogen (C: N) ratio on soil NO3-N, soil NH4-N, and leaf lettuce yield was studied over three sequential crop cycles from 1995 to 1996. One compost type (HiCN) was prepared primarily from yard wastes and had a C: N ratio of 29 to 32:1 The other compost (LoCN) was a compost composed of a mixture of crude materials including yard wastes, feedlot manures, and vegetable trimmings and had a C: N ratio of 10 to 12:1. Before transplanting leaf lettuce, both composts were applied and incorporated in the same plots repeatedly over three crop cycles at rates of 9, 18, 36, and 54 Mg·ha–1 (dry mass) in each application. In the first crop cycle, no differences were observed for weekly soil NO3-N, NH4-N, or leaf lettuce yield among compost types or rates. In the second and third crop cycles, weekly soil NO3-N and soil NH4-N were directly related to LoCN compost application rates. First harvest lettuce yield was also directly related to LoCN rate in the second and third cycles, but total yield was not related to LoCN rate. In the second and third cycles, soil NO3-N and early and total lettuce yield were inversely related to rate of application of the HiCN material. Weekly soil NH4–N was not consistently related to application rates of HiCN or LoCN material.