Determining Compost Carryover for Optimal Use in an Organic Corn Squash Rotation

Organically certified farms using compost to improve or maintain fertility rarely consider compost carryover and its impact on the determination of economically optimal application rates. Compost carryover is comprised of nutrient and non-nutrient elements. Both affect crop growth, yet carryover is typically described primarily in terms of nitrogen (N)-carryover only. This study tested a new method for estimating compost carryover on organically certified land and expressed carryover in units that capture both the nutrient and non-nutrient components. Compost carryover for five treatment rates was estimated over four years in an organically certified field trial in a corn and squash rotation. Nitrate (NO3-), phosphorus (P), soil organic matter (SOM) were investigated to determine the residual effect of a one-time compost application. Implications for fertility management and farm profitability were considered. The new method successfully modeled carryover, determining that compost had a persistent and positive effect on crop yields, evident even three years after an initial one-time application. No NO3- carryover was observed in any year, suggesting that yield responses were due primarily to non-N carryover. Compositional changes in SOM corresponding to compost input three years earlier suggested that compost was able to influence non-nutritive soil properties many years after incorporation. High value cash-crops are necessary in organic rotations to offset the high input cost of compost use. In organic fertility management, compost is an important and economical source of non-N fertility, which benefits crop yield many years after incorporation. When used with a dedicated N-fixing cover crop in a rotation that includes a high value cash-crop, complete fertility goals could be met in a sustainable manner. (153 pages