The mitigation of phosphorus losses from a water-repellent soil used for grazed dairy farming

Abstract

Phosphorus (P) losses via surface runoff can be large. Soil water repellency (SWR) occurs in warm and dry conditions. If SWR coincides with P fertiliser applications, the risk of P loss by surface runoff may increase. Losses during hotter months can be particularly detrimental to surface water quality because warmer waters help algal growth. We hypothesized that, compared to highly water-soluble superphosphate, the application of P as a low water-soluble fertiliser would decrease P losses caused by SWR-induced surface runoff. A three-year trial was conducted on a grazed dairy pasture soil in the central North Island of New Zealand. The soil was classified as moderate to severely SWR by water droplet penetration time and molarity of ethanol tests. Phosphorus (40 kg P ha⁻¹) was applied in autumn as superphosphate, serpentine super and as a new fertiliser product similar to di-calcic phosphate. These fertilisers had water solubilities of 85, 34 and 12 g P kg⁻¹ fertiliser, respectively. Yields of P lost were <1 kg P ha⁻¹ for the first year, but in the third year >8 kg P ha⁻¹ was lost from the superphosphate treatment due to SWR-induced surface runoff within 21 days after fertiliser application. In contrast, P losses in the third year from lower water-soluble P fertilisers, serpentine super and the new product, were 2.2 and 1.6 kg P ha⁻¹, respectively. The use of a low water-soluble P fertiliser is therefore recommended to prevent P losses in areas where SWR-induced surface runoff is likely and unavoidable.