Evaluating the spatial and temporal dynamics of farm and field phosphorus and potassium balances on a mixed crop and livestock farm

Methods to track P and K on farms over time at varying spatial scales can improve farm agronomic and environmental performance monitoring. An annual nutrient balance was used to determine P and K balances at varying spatial scales on a 128 ha mixed crop and dairy farm in a central Pennsylvania limestone valley for each of nine years. Alfalfa (Medicago sativa L.) and corn (Zea mays L.) occupied 60% and 40% of the cropland, respectively. Inputs of P and K to the farm exceeded outputs over the study period. Net increases of P and a decrease in K were determined in the aggregate of all fields over the 9-yr period. The balance of P and K varied with time within a field, and by field within a year because nutrient inputs and removals varied with crop selection, management tactics, and year in the rotation. Recognizing that increases in field P are in proportion to P entering the farm can help reduce P accumulation in fields by addressing surpluses at their source or balancing managed flows for the farm. Conversely, decreases in field K associated with forage crops may be the reason for additional supplemental K inputs to the farm. Monitoring nutrient stocks (soil testing) and flows (input/output balances) at different scales in conjunction with spatial patterns of nutrient balances can be the foundation for integrating farm activities and information technologies in a new generation of performance enhancing tools.