The results discussed above all clearly illustrate the importance of recently applied fertilizers and manures as sources of P in surface and subsurface runoff. In the context of overall annual losses from pastoral areas, the amounts of P transported in runoff derived from recently applied fertilizer in these examples are very significant. On average, the difference in TP loss from fertilized compared with unfertilized plots in rainfall-runoff in the papers reviewed here was roughly 4.5 times greater, while that in DRP was about 6 times greater. In many cases the differences were greater than an order of magnitude, and sometimes close to two orders of magnitude (Table 1). It has thus been suggested that when they do occur, event-specific P losses often make the dominant contribution (50–98%) to P loads in runoff from field plots (Withers et al., 2003). In one extreme case, approximately 7.75 kg P ha–1 was exported during an unexpected storm that occurred 2 d after the application of DAP, out of a total of 9.7 kg P ha–1 for the whole year (Nash et al., 2000b; Gourley et al., 2002; C.J.P. Gourley, personal communication, 2002), serving as a reminder of the importance of losses from recently applied P fertilizer, and their contribution to overall total annual P losses.
Surprisingly, despite the demonstrable significance of individual applications of fertilizer on P runoff from agricultural land, there have been very few studies comparing runoff from different fertilizer forms of P. We believe that this reflects a general lack of knowledge and/or interest in the wide range of solubilities, physical characteristics, and soil chemical reactions affecting different phosphate fertilizers. This topic is discussed in the following section.
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