The recent expansion of urban agriculture presents an opportunity to increase food production in cities and to recycle nutrients from organics waste into new food through compost application. However, compost generally has a low nitrogen:phosphorus (N:P) ratio relative to crop nutrient demand, so that fertilizing exclusively with compost has the potential to lead to buildup of P in soil. We constructed N and P budgets for six urban farms in Minneapolis and St. Paul, and compared calculated N and P surplus with soil nitrate and phosphate concentrations. Farm managers provided vegetable harvest data, and information on inputs was collected by interviews with farm managers.
On average, 30 times more P was applied as compost than was removes as crops, and average soil phosphate levels exceeded 200 ppm, seven times higher than the threshold for "very high" soil P. Because only a small fraction (3%) of P added was removed, estimated P surplus increased linearly with P inputs (slope = 1.01, r2 = 0.999). In contrast, estimated N surplus was negative for five of the six farms, meaning that more N was removed as vegetables than was added as compost. Soil nitrate levels averaged 30 ppm, close to recommended amounts for vegetable production. These results support the hypothesis that compost application can lead to P buildup in urban soils and potential loss through leachate. In order to lessen this imbalance, other sources of N fertilizer may need to be used, without adding additional P to the soil. While composting coupled with urban agriculture presents an opportunity to increase nutrient recycling within urban ecosystems, this stoichiometric imbalance presents a potentially important constraint.