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United States Department of Agriculture

Agricultural Research Service

Title: Setting and Justifying Upper Critical Limits for Phosphorus in Soils

Authors
item Sibbesen, E. - RESEARCH CENTRE FOULUM
item Sharpley, Andrew

Submitted to: Centre for Agriculture and Biosciences International
Publication Type: Book / Chapter
Publication Acceptance Date: October 30, 1996
Publication Date: N/A

Interpretive Summary: The mobility of phosphorus (P) in soil is low compared to most other plant nutrients because of the generally low solubility of P compounds and strong P-binding capacity of soil material. A plant root gets most of its P from within 2 mm from the root surface during its period of active P uptake. Soils have to have a critical pool of plant-available soil P to ensure P nutrition of crops. This implies that available P pool must be considerably larger than the P uptake of a single crop. The question is how large relative to optimum P nutrition of crops. Numerous field experiments on types and rates of P fertilizers have been conducted and related to soil-P testing for the guidance of P fertilization. Even so, the widespread use of P fertilizers and manures in many regions of Europe and the USA has increased soil-P status levels above what is necessary for the crop. 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: There are two main issues regarding the level of soil phosphorus (P): (i) a certain critical level is necessary for economic crop production, and (ii) part of the soil P is lost from agricultural land to the aquatic environment by wind erosion, surface runoff, and leaching. The paper deals with attempts to set levels of soil P relative to crop production and to the potential for P loss by surface runoff. Calculations were made of ratios between amounts of soil P extracted with different P-test methods used in Europe, the USA, and Canada. The critical levels of soil-test P calibrated in Europe were generally greater than the ones calibrated in the USA and Canada, but large differences in estimated levels were also observed within countries, and the levels also varied with type of crop and yield potential. Soil-test P level was related to concentration of dissolved orthophosphate (DP) in surface runoff, but the relation was found dto differ between soil types, crops, and runoff episodes. The variability in runoff volume and erosion as a result of climatic, topographic, and agronomic factors, however, plays a larger role than soil-P in determining the losses of P in plot experiments. Critical soil-test P values, suggested by some USA states relative to P loss potential, were four to eight times above the critical levels of soil-test P relative to crop production. Thus, a comprehensive approach is needed for reliable, yet flexible recommendations of fertilizer and manure P management integrating soil-test P with estimates of potential runoff and erosion losses.

Last Modified: 10/20/2014
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