Influence of aggregate size on phosphorus loss and ryegrass yield in a soil cultivated intermittently

Abstract

AbstractIntermittent cultivation for forage cropping and subsequent grazing by dairy cattle during winter is practiced in southern New Zealand to protect remaining pastures from poaching damage by cattle. However, this practice may degrade soil aggregate structure affecting pasture yield and nutrient loss. A study was conducted to determine the ability of different aggregate sizes to supply phosphorus (P) to ryegrass in a soil used for winter forage cropping and grazed by dairy cattle 2 out of 7 years. Over 35 weeks ryegrass was grown in pots containing one of six wet‐sieved soil aggregate size classes and leached every 2 weeks. Soil analyses included water soluble P, inorganic and organic P fractions, and P in leachate. Results showed that organic P was preferentially lost in leachate or mineralized and used by ryegrass with decreasing aggregate size, although this did not affect yield. Ryegrass yield was best correlated with bicarbonate extractable P in aggregates, and P loss with water soluble inorganic P after minimum disturbance. These findings suggest that although intermittent cultivation did not affect yield, if this practice decreases aggregate size in the future, it may increase the risk of P loss.