Field assessment of soil quality as affected by compost and fertilizer application in a broccoli ®eld (San Benito County, California)

Selected in-®eld physical, chemical and biological indicators were measured for the rapid assessment of soil quality changes in a Sorrento silty clay loam as a result of compost and ammonium nitrate application to a broccoli ®eld (San Benito County, CA). Plots were laid out in a randomized complete block design with four replications of 0, 22 and 44 Mg haÿ1 compost treatments which were split to include fertilizer (165 kg N haÿ1) and no-fertilizer subplots. Soil samples were taken on 11 and 24 October 1995 during the active growth phase of the crop, and soil quality evaluation was compared to crop nutrient content and yield which were determined at harvest in November. Surface application of ammonium nitrate initially stimulated soil nitri®cation and acidi®cation processes in the top 7.6 cm as evidenced by an 80-fold increase in nitrate-N and accumulation of nitrite, a 1.5-unit increase in electrical conductivity (EC) and a 1.4-unit decrease in pH. Following irrigation, this pattern was reversed by nitrate leaching and root uptake, although nitri®cation and acidi®cation effects remained detectable at both sampling depths (0±7.6 and 0±20 cm). Nitri®cation was positively correlated to soil respiration and negatively correlated to soil water content. The estimated nitrate-N levels of fertilizer-containing plots in the top 20 cm were two times higher than those reported in the literature as minimal levels for optimal corn growth and indicated a high risk for groundwater contamination in this irrigated ®eld by taking into account the rapid water in®ltration and low soil buffering capacity. The detected short-term bene®cial effects of compost application were the stabilization of pH and the decrease of water in®ltration rate. Stabilization of pH prevented acidi®cation effects due to fertilizer application at both sampling depths. The high soil EC of plots receiving 44 Mg haÿ1 of compost at the 0±20 cm depth probably resulted from a high compost salt content, other than nitrates, and warns against repeated use of high EC composts that may result in N depletion, reduced nutrient cycling and impaired crop growth. The relatively small differences among treatments in crop yield, head number and weight, leaf and petiole nutrients indicated that there was suf®cient residual fertility to grow a crop in the absence of any amendments. However, the relevance of selected soil quality indicators to plant productivity and health was evidenced by the strong correlation of soil nitrate-N with leaf-N and head weight, despite the adequate-to-excessive amount of soil nitrate-N in most treatments. # 1999 Elsevier Science B.V. All rights reserved.