Direct Exports of Phosphorus from Fertilizers Applied to Grazed Pastures

Since its discovery in 1669, phosphorus (P) in the form of fertilizer has become an essential input for many agroecosystems. By introducing a concentrated P source, fertilizers increase short-term P export potential soon after their application and longer-term export potential by increasing soil fertility (legacy P). The 4R concept was developed to help mitigate P exports from the fertilizers that sustain agricultural productivity. This review investigates the factors affecting P exports soon after the application of mineral fertilizers to pasture-based grazing systems and studies quantifying its potential impact in different systems, with an emphasis on Australasia. Initially, P fertilizers and reactions that might affect their short-term P export potential are reviewed, along with P transport pathways, the forms of P exported from grazing systems, factors affecting P mobilization into water, and studies demonstrating the possible short-term effects of fertilizer application on P exports. Using that foundation, we review studies quantifying the short-term impact of fertilizer application in different regions; they show that under poor management, recently applied fertilizer can contribute a considerable proportion (30-80%) of total farm P exports in drainage, but when fertilizer is well-managed, that figure is expected to be <10%. We then use three model systems of varying hydrology that are common to Australasia to demonstrate the principles for selecting fertilizers that are likely to minimize P exports soon after their application.