COS 116-4 - The effects of annual compost additions to an agricultural green roof on growing media organic matter and nutrient content over time.

Abstract: 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 that rooftop farmers are using to supply the necessary crop nutrients is annual additions of compost. The long-term effect of this practice has not been well studied, but could lead to one of two outcomes. Increases in organic matter content and therefore water holding capacity and weight of the green roof could result in structural issues, or relatively quick breakdown of the compost would lead to increased nutrient leaching in runoff. Green roof platforms were set up at Kentucky State University’s Harold R. Benson Research and Demonstration Farm to determine which outcome is likely and what effect the amount of compost added may have. Compost treatments included 0.33, 0.66 and 1 kg/m² of compost, and a no compost control, based on the actual 0.64 kg/m² compost additions of a rooftop farm. Organic fertilizers meals were used to supply additional nutrients to meet crop needs. 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. These samples were analyzed using a modified (DTPA) saturated media extract method, for pH, total soluble salts, nitrate-nitrogen, ammonia-nitrogen, phosphorus, potassium, calcium, magnesium, boron, copper, iron, manganese, sodium, zinc, and organic matter content. Compost additions had no effect on media pH, soluble salts, or nutrient content. Organic matter content was higher by 0.5-0.9% in the compost treatments than the no compost control but did not exhibit a dose response or an increase with successive compost additions. Media pH decreased slightly over the course of the study with dips at fall samplings. Phosphorus, copper, and iron did show slight increases over the course of the study whereas the other measured nutrients decreased. Measured values of soluble salts, nitrate-nitrogen, phosphorus, potassium, calcium, magnesium, boron, sodium, and zinc exceeded the optimal range for green roofs, in some cases exceeding levels recommended for plant health and prevention of nutrient leaching by up to two orders of magnitude. The significance of these findings for crop production and the use of compost for nutrient management on agricultural green roofs will be discussed.