Balancing phosphorus requirements for milk production and water quality

Phosphorus (P) is both an essential element for pasture and animal production and a limiting nutrient for nuisance weed and algal growth in many New Zealand water bodies. Research results, mainly measured under mowing, have shown that pasture production increased with fertiliser P application up to a target range of soil Olsen P values (20-30 mg/L for sedimentary and Allophanic soils; 35-45 mg/L for Pumice and Organic soils). A dairy farmlet trial in Taranaki on an Egmont Allophanic soil measured small increases in pasture production and larger economic responses in milksolids (MS) production as Olsen P levels increased from 30-50 in a management system with high stocking rates and a long lactation. The relative response in pasture production to Olsen P was similar under mowing and grazing, indicating that the MS response to high Olsen P measured on Allophanic soils would also apply to other soils. Only small amounts of dissolved P (< 1 kg/ha) are required to promote nuisance weed and algal growth in water bodies. P can be sourced from soil, fertiliser and dung and transported in dissolved and particulate forms from soil to water through overland and subsurface runoff flow. Data from a P runoff study on different soils is presented which shows that the potential for P to be lost to water bodies increases with Olsen P level. On a high anion storage capacity (ASC) Egmont Allophanic soil (83%), the concentration of dissolved reactive P (DRP) in overland flow was lower than the critical level of 0.03 mg/L at Olsen P levels of 20-110 mg/L. In contrast, on the more weakly weathered Waikoikoi Pallic soil with low ASC (15%), the critical DRP level was exceeded at an Olsen P of about 30 mg/L, and on the moderately weathered Waikiwi Brown soil (ASC 49%), at an Olsen P of about 50 mg/L. These results indicate that there is a greater risk of P enrichment of surface water bodies in areas with low ASC than in areas with high ASC soils.