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United States Department of Agriculture

Agricultural Research Service

Title: INTERPRETING SOIL TEST PHOSPHORUS FOR ENVIRONMENTAL PURPOSES

Authors
item Beegle, Douglas - PENN STATE UNIVERSITY
item Sharpley, Andrew
item Graetz, Don - UNIVERSITY OF FLORIDA

Submitted to: SERA-IEG 17 Bulletin
Publication Type: Other
Publication Acceptance Date: May 1, 1998
Publication Date: N/A

Interpretive Summary: The amounts of phosphorus (P) extracted by recommended soil test methods have been found to correlate with the availability of P to a crop. The strength of this correlation is the basis for selecting a particular soil test extractant for a given combination of soil, crop, and growing conditions. To interpret a soil test, we must know the relationship between the amount of P by a given soil test and expected crop response fo each crop. The process of determining the degree of limitation to crop growth or the probability of getting growth response to applied P at a given soil test level is known as soil test calibration and must be determined experimentally in the field. A common procedure for calibrating a soil test is to grow the crop on soils representative of those where the test will be used and covering the range of extractable P likely to be encountered. Even so, many soils of the northeast U.S. have P levels above ewhat is necessary for crops. Part of the reason may be that it is difficult to define the optimum soil-P status level or that some farmers have practiced 'insurance fertilization' or have been wrongly advised. Environmentally sensitive or threshold soil-P levels are those above which the potential for P loss in runoff exceeds any crop production concerns. Quantification of these levels is critical for development of P management guidelines for water quality as well as crop production goals. In order to estimate defensible, upper, critical soil-P limits that are environmentally sound, we must first develop analytical methods that measure soil-P availability relevant to the release of soil P to runoff, quantify the relationship between soil and runoff P, and identify transport potential for a site.

Technical Abstract: Recent experiences of several states to interpret soil test phosphorus for environmental purposes have proved to be very controversial and difficult to define in a practical way. First we need to ask, do routine soil test extractants, designed to assess plant availability of a nutrient, measure the forms of the nutrient most important to eutrophication or other negative environmental impacts? If the soil test is appropriate, what should be the basis for interpreting the results for environmental purposes? Some would simply extend the levels used for interpretation for crop response and say a soil test that is above the level where a crop response is expected is in excess of crop needs and therefore is potentially polluting. However, it cannot be assumed that there is a direct relationship between the soil test calibration for crop response to nutrients and nutrient pollution potential. If soil tests are to be properly interpreted for predicting the probability of nutrient pollution, calibrations that specifically relate the soil test to some measure of environmental response, such as P in runoff, will be necessary. The calibration of soil test P to reflect the potential for environmental impact is more difficult than for crop yield response for several reasons. These reasons include a variation in what is an acceptable or unacceptable impact, an inadequate data base defining a relationship between soil and runoff P, and establishing what site characteristics play a critical role in controlling P loss in runoff.

Last Modified: 5/25/2015
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