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

Phosphorus (P) losses via surface runoff can be large. Soil water repellency (SWR) occurs in hotter months when P fertiliser is applied and could increase the risk of P loss by runoff. 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 runoff. A 3-yr trail was conducted on a grazed dairy pasture soil in the central North Island of New Zealand. The soil was classified as a 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. These fertiliser had water solubilises of 85, 34 and 12 g kg-1, respectively. Losses of P 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 runoff < 21 days of fertiliser application. In contract, P losses from the serpentine super and a new product were 2.2 and 1.6 kg P ha-1, respectively. The use of a low water soluble P fertiliser is recommended to prevent P losses in areas where SWR-induced runoff is likely.