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United States Department of Agriculture

Agricultural Research Service

Title: Development of a Phosphorus Index for Pastures

Author
item MOORE, PHILIP

Submitted to: Meeting Abstract
Publication Type: Proceedings
Publication Acceptance Date: September 1, 1999
Publication Date: N/A

Interpretive Summary: Currently, there is a trend among state and federal agencies to restrict animal manure and phosphorus fertilizer applications to land when the amount of phosphorus in the soil is considered high. They are doing this because they (mistakenly) believe that high levels of soil test phosphorus automatically result in high amounts phosphorus pollution in run-off water. As a consequence, animal producers are not being allowed to fertilize with manure on some or all of their fields. This causes the manure (which is normally a valuable renewable resource) to become a waste product, with associated costs. These new restrictions on manure applications on high soil test P soils may be unwarranted. Recent studies conducted at several locations around the country strongly suggest that soil test P has little, if any, effect on P concentrations in runoff water from pastures fertilized with manure. Other factors, such as manure application rate, soluble P in the manure, amount of P in the diet, and timing of fertilizer addition appear to be much more important in determining P levels in runoff water. The objectives of this paper are to describe the Phosphorus Index (PI) that is currently being used (in the place of upper cutoff limits for soil test P) and to describe the methods that are being used to modify the PI for use with pasture systems. We strongly believe that this PI for pastures will help keep farmers in business, by increasing their options for land application of manure, and reduce P runoff.

Technical Abstract: At present, many government agencies are placing upper cutoff limits on soil test phosphorus (P), with the assumption that high levels of P in the soil result in high levels of P in runoff. As a consequence, animal producers are not being allowed to fertilize with manure on some or all of their fields. This causes the manure, a valuable renewable resource, to become a waste product, with associated waste management costs. If growers have to pay for transporting the manure to areas with low soil test P soils, then it could become a large economic burden. These new restrictions on manure applications on high soil test P soils may be unwarranted. Recent studies conducted at several locations around the country strongly suggest that soil test P has little, if any, effect on P concentrations in runoff water from pastures fertilized with manure. Other factors, such as manure application rate, soluble P in the manure, amount of P in the diet, and timing of fertilizer addition appear to be much more important in determining P levels in runoff water. The objectives of this research are to: (1) evaluate the effects of various management practices and environmental conditions (such as soil test P) on P runoff, (2) use that data to modify the Phosphorus Index (PI) for pasture systems, (3) validate the modified PI in the Eucha/Spavinaw watershed on three different soil types, (4) evaluate the management options and risk of P runoff associated with using current NRCS guidelines versus the modified PI, and (5) use the modified PI for writing nutrient management plans. We believe that the PI should help keep animal producers in business by providing more management options, while greatly reducing the amount of P runoff from agricultural lands.

Last Modified: 9/29/2014
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