Fertile cities: Nutrient management practices in urban agriculture

Cities are increasingly targeted as centers for sustainable development and innovation of food systems. Urban agriculture (UA) is advocated by some as a multi-faceted approach to help achieve urban sustainability goals as it provides possible social, economic and environmental benefits. The role of UA in restoring resource cycles receives increasing attention, especially with regard to assimilating urban waste. However, there is little information on how nutrients are managed in UA in industrialized countries. To examine nutrient management in UA, data was collected from a total of 25 ground-based UA initiatives in the Netherlands on i) preferences for types of fertilizers, and ii) quantity and quality of fertilizers used including nutrient composition and organic matter content. The main inputs at urban farms were compost and manure, high in organic matter content. The total nutrient inputs were compared to nutrient demand, based on crop nutrient uptake, in order to determine nutrient balances. Results show that mean nutrient inputs exceeded mean crop demand by roughly 450% for total nitrogen, 600% for phosphorus and 250% for potassium. Mean inputs for plant-available nitrogen were comparable to crop uptake values. The surpluses, particularly for phosphorus, are higher than fertilizer application limits used for conventional farming in The Netherlands. While nutrient input calculations were subject to several uncertainties, e.g., due to lack of accuracy of the data supplied by the farmers, results show a salient indication of over-fertilization and thus a suboptimal nutrient use. If UA continues to expand across cities these observed nutrient surpluses may pose a risk for local surface waters and groundwater as well as soil quality. The need to improve nutrient management in UA is evident. Soil tests, harvest logging and book keeping of nutrient inputs would improve data quality and may help balance nutrient inputs with nutrient outputs.