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Soils - Part 6: Phosphorus and Potassium in the Soil

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Soil Testing for Available Phosphorus

The standard soil tests for available phosphorus are generally weak acid extractants which dissolve portions of the calcium, iron and aluminum phosphates in the soil. In the sodium bicarbonate test, also known as Olsen P, for high pH soils, the bicarbonate ion removes calcium from the system, solubilizing the calcium phosphates, which are measured as an “index” of available phosphorus. All presently used soil tests for phosphorus are of no value unless correlated and calibrated with crop response to applied phosphorus. Correlation tests relate the chemical testing to plant response, and calibration studies measure field response to phosphorus applications based on soil test levels. All phosphorus soil tests are an “index” of availability. The fact that the results are reported in terms of parts-per-million or pounds-per-acre has no interpretative value by itself. The phosphorus “index” is not a “quantitative” value, i.e., it does not indicate a quantity of phosphorus in the soil. Its value is based on the correlated and calibrated predictability of economic crop response.

It is not known precisely what phosphorus compounds are dissolved or measured by the soil test extractants. The extractants probably measure all of the labile phosphorus and, perhaps, some of the non-labile forms. The tests used at the University of Nebraska Soil Test Lab (Bray and Kurtz No. 1 and the sodium bicarbonate extractants) relate well to crop response to applied phosphorus, but are not infallible. The probability of phosphorus response decreases rapidly as the soil test increases from nearly 100 percent when the test is very low, to less than 5 percent when the test is high. However, the long term economics make following UNL recommendations the most profitable decision.

Accurately predicting the precise amount of phosphorus that a given soil can supply to a given plant is essentially impossible. Phosphorus reactions in the soil are complex. The present extracting agents do not measure the actual rate of phosphorus release from the relatively unavailable form to the soil solution where it is available to the plants. The present research data base uses the soil test “index” value from various extracting agents to provide “relative level” for the soil’s ability to supply phosphorus, i.e., very low, low, medium, high, and very high. On a soil testing very low, the amount of phosphorus to apply for corn would be different than with wheat because all crops either don’t have the same ability to extract phosphorus from the soil and/or they take up phosphorus in larger amounts. How well the phosphorus recommendation meets the need of the crop from a particular field depends on several factors, e.g., how variable the field is and how well the soil sample sent to the laboratory represents the field. All chemical tests have some degree of error associated with them; but, in the case of soil phosphorus determinations, the error is very small. Of more concern is the lack of field research on the influence of subsoil phosphorus on crop yield. Considerable field research has been done to arrive at the present ability to make phosphorus recommendations, but considerably more field and laboratory research are needed to improve soil test procedures and to access the role of subsoil phosphorus. At the present time, there is inadequate support for this type of research.

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