Urban agriculture, which relies heavily on compost applications for soil fertility, has the potential to recycle phosphorus (P) from organic waste back into the human food system. However, excessive application rates of high-P compost result in characteristically low P-use efficiency by garden crops, and potentially high rates of P leachate from garden soils. Here, we report the results of the first two years of a five-year experiment measuring the recovery and loss of phosphorus from compost applied to urban garden plots, in which carrots, green peppers, and bush beans were grown. Compost from municipal organics waste or from cow manure was applied at low rates, targeted at either crop nitrogen (N) demand (P was applied in excess), or crop P demand (crops received supplemental N). We measured soil P, crop yields, and P leachate, to evaluate whether targeted compost application can result in more efficient recycling of P while maintaining yields.
Results/Conclusions
Over the first 18 months of the study, total soil P increased 20%, and plant-available soil P increased 37%, from initial levels in the highest-P treatment (municipal compost applied to meet crop N demand). Control plots receiving no compost decreased 3% in total soil P and 13% in plan-available P during this time. There was no effect of soil amendment treatment on crop yields during the first two growing seasons. Manure compost applied to meet crop N demand had significantly higher P leachate rates than other treatments (0.27 g P m-2 y-1), more than double the rates measured in the no-compost treatment. The ratio of P lost as leachate relative to P recovered as crops increased significantly between year 1 and year 2 for the manure compost (crop N demand) treatment, and decreased significantly between years for the no-compost treatment. These results indicate that even carefully targeting compost application for crop requirements can result in a buildup and loss of P from garden soil, potentially constraining the ability of urban agriculture to recycle phosphorus in urban ecosystems.