Changes in green roof media properties from agricultural management and annual compost additions

The use of green roof technology to grow food on rooftops provides additional production space in urban areas. This comes with some challenges, as green roof media is typically fast draining and contains limited nutrients, requiring the use of irrigation and fertilizers to support crop plants. One nutrient management practice in use to supply the necessary crop nutrients is annual additions of compost. The long-term effect of this practice has not been studied but has the potential to increase organic matter content and therefore water holding capacity and weight of the green roof over time. Green roof platforms were constructed and treated with 0, 0.33, 0.66, and 1 kg/m² of compost annually. Media samples were taken at the start of the project and then annually after compost addition, after the growing season and in the following spring before compost addition and analyzed by Pennsylvania State University Agricultural Analytical Services Laboratory. While the compost treatments used in this study did have an effect on organic matter content and total phosphorus, they had no effect on soluble salts, any other macronutrient, or any micronutrient measured. No increase in media organic matter content over time was observed, suggesting that the compost application rates used in this study will not lead to soil building. An accumulation of macronutrients over the course of the growing season while fertilizers were being applied was observed, as was evidence of nutrient flushing, which has implications for the management of agricultural green roofs.