Soil and water environmental effects of fertilizer-, manure-, and compost-based fertility practices in an organic vegetable cropping system

Degraded soil quality, which decreases agricultural productivity and increases nonpoint source pollution of surface water, may be ameliorated by employing soil organic matter enhancing management, such as practiced by compost use in organic farming. The value of compost applied at rates lower than those required to supply crop nutrient needs requires investigation because applying compost at agronomic nitrogen rates may not be economically feasible for organic vegetable producers. We conducted field research during 2000-2002 on a Luvisol to compare the nutrient and non-nutrient effects of various rates and timings of mixed poultry litter-yard waste compost with a traditional organic fertilizer (poultry litter) and inorganic fertilizer on environmental soil attributes and water quality in an organic vegetable crop rotation. Soil organic C, total N, and available P increased 60%, 68%, and 225%, respectively, above the control with the application of 144 Mg ha^-1 compost (dry wt.) during the 3-year study, but the low rate of compost (31 Mg ha^-1) did not affect soil C or N. Compost N mineralization was not synchronous with sweet corn N assimilation, resulting in excess root zone nitrate that would have posed a leaching risk without the use of a winter rye N-scavenging cover crop. The concentrations of nitrate N that leached below the tillage zone occasionally exceeded the 10 mg L^-1 health standard but were not different among the agronomic rates of compost, poultry litter, fertilizer, and control treatments for nearly every sampling event. Despite increasing runoff water concentrations of N and P, the high compost rate reduced the amounts of N and P that were transported from the soil surface by five-fold and four-fold, respectively, compared to the inorganic fertilizer due to a four-fold reduction in runoff volume. Crop yields did not benefit from low compost rates during the 3-year duration of the study; however, improvements in some bulk density and porosity indicated that benefits of longer term, low compost rate additions may accrue over time.