How will phosphorus be targeted and enforced by state agencies?
Environmental concern has forced many states to consider the development of recommendations for manure and watershed management based on threshold soil P levels. In most cases, agencies proposing these levels plan to adopt a single threshold value for all areas under their jurisdiction. However, threshold soil P levels are too limited to be the sole criterion to guide manure management and P applications. For example, adjacent fields having similar soil test P levels but differing susceptibilities to surface runoff and erosion, due to contrasting topography and management, should not have similar soil P thresholds or management recommendations. Therefore, threshold soil P levels will have little value unless they are used in conjunction with an estimate of a site-specific potential for surface runoff and erosion.
A sounder approach advocated by researchers and an increasing number of farm advisors personnel is to identify critical source-areas where high soil P levels coincide with high surface runoff and erosion potentials. This approach addresses P management at multi-field or watershed scales. Further, a comprehensive P management strategy must address down-gradient water quality impacts such as the proximity of P-sensitive waters. Conventionally applied remediations may not produce the desired results and may prove to be an inefficient and costly approach to the problem if this source-area perspective for targeting application of P fertility, surface runoff and erosion control technology is not used. The issue of nutrient balance with the crop utilization potential must still be addressed. Since the resources invested in balancing the nutrients generally will not promote milk production nor reduce associated production costs, nutrient utilization should be accounted for before a new facility is constructed. Otherwise, accommodating increased costs in the operating cash flow might be difficult after the facility is financed and in operation.
A simple P index has been developed by USDA-NRCS in cooperation with several research scientists as a screening tool for use by field staffs, watershed planners, and farmers to rank the vulnerability of fields as sources of P loss in surface runoff. The index accounts for and ranks transport and source factors controlling P loss in surface runoff and sites where the risk of P movement is expected to be highest. The index is being considered for implementation by NRCS as part of their national nutrient management planning strategy to help identify agricultural areas or practices that have the lowest P loss potential. It will identify management options available to land users that will allow them flexibility in developing remedial strategies.
