MASS BALANCE MODELING OF POTASSIUM LOSSES FROM GRAZED DAIRY PASTURE

A simple mass balance model was used to estimate the annual losses of potassium (K) from three dairy farms. The results showed that while the amount lost from each farm varied, 74-92% of the losses were directly caused by the grazing dairy cow through product removal from the farm, excreta transfer to unproductive areas, and preferential flow of urine K through the soil to beyond the effective plant rooting depth. A net loss of K leads to reductions in soil K, and for one of the farms (on an organic soil with a low total soil K content) it was possible to show that the predictions of K losses made by the model were comparable with actual changes in soil K status. The K losses per stock unit (su) estimated by this model for the three farms ranged from 3.4 to 4.7 kg K/su and contrasted with a loss of 4.0 kg K/su in the model used by the Ministry of Agriculture and Fisheries (MAF) for predicting K fertiliser requirements. Refinements to the MAF model are suggested for calculating the animal loss factor.