This review has highlighted the complex nature of P runoff dynamics, and shown that some assumptions that are generally made regarding the forms and nature of P loss from agricultural land do not necessarily hold true. The potential influence that event-specific losses of P from recently applied fertilizer may have on P exports from agricultural land has been overlooked in many previous studies. The studies reviewed here establish that event-specific P losses may have much greater and wider significance than has been realized. The pattern of loss of P from recently applied soluble fertilizer appears to be consistent and predictable, within the confines of local climatic conditions. No single management measure will control all sources of P, but there are a number of options available to land users that could be put into practice without compromising agricultural production to an unacceptable level, or possibly at all. Best management practices have been shown to be most effective when a variety of approaches and/or practices are applied in concert. However, highlighting the risk associated with event-specific P transfers and targeting short-term decision-making appears to be the most readily applicable method for mitigating P loss.
It is our view that the appropriate management of fertilizer applications, in particular in regard to fertilizer forms and P solubility, and especially where fertilizer is applied via broadcasting (e.g., to permanent pasture and other perennial crops) should be considered a major tool to mitigate potential losses of P to the environment. Limited studies indicate that the use of slow-release DAPR could significantly decrease the potential for event-specific P losses from recently applied fertilizer. The DAPRs or DAPR–soluble P blends are potentially as effective as fully soluble P fertilizers, under maintenance conditions, on very significant proportions of agricultural land in many countries. The substitution of one type of fertilizer (slow-release) for another (fully soluble) in areas where P is required for maintenance situations is a very simple and inexpensive mechanism that farmers could use to minimize the risk of loss of P from their land, compared with many of the other strategies discussed above. Alternatively, soluble P fertilizers such as DAP can be modified to substantially reduce short-term susceptibility to runoff. Using this type of approach to fertilizer management along with a tool identifying critical source areas, such as the P index or PLRI model, would give farmers a targeted strategy enabling the potential environmental effects of their activities to be managed in an efficient and cost-effective way, and may significantly reduce total amounts of P loss from pastoral and other agricultural land.
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