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Increasing animal production in the state or its concentration in certain areas is increasing the amounts of manure being applied to the land. Often, the manure is applied at rates or at a frequency that exceed the phosphorus (P) needs of crops and even the amount removed in harvested products. Animal manure can supply the nitrogen (N) and P needed by crops as well as other nutrients. Due to its relative N and P content and potential N losses, however, continued use of rates that supply the N removed in corn grain (and in soybean grain too) results in P accumulation in soils. Accumulation in excess of crops needs may increase the potential for eutrophication of surface waters. Eutrophication means that nutrient levels in water, especially P, are high and excessive algae growth occurs, which could create imbalances in the water ecosystem and the esthetic value or water bodies such as lakes or streams. This problem is compounded because soils of many grain crop production areas already have soil-test P levels that are at or above levels that optimize grain yields. The upper limit for amounts of manure that could be applied with minimum environmental pollution could be ultimately determined by the P level in the topsoil and the potential for soil erosion, water runoff, or P leaching through the soil profile that can reach surface (through tile flow) or groundwater. This issue is calling for better estimates of P in soils, especially in manured soils, not only for crop production purposes but also for environmental resources conservation. Thus, questions have arisen concerning soil P testing for agronomic purposes or environmental purposes. A major question is the relative importance of a soil test value in terms of potential losses of P with erosion and runoff and if upper soil P limits can be reasonably determined to minimize P pollution of surface water supplies. A discussion of these alternatives necessitates a preliminary discussion of some basic aspects of P in soils, of the various types of tests, and their potential use for environmental purposes in comparison with routine soil tests for crop production.

Soil Phosphorus Testing for Crop Production and Environmental Purposes