Summer 2005

Soil Phosphorus Testing for the Iowa P Index and Manure Management Plans in Calcareous Soils


By Jeremy Klatt, Iowa Department of Natural Resources

There are four soil testing analytical methods calibrated for use with the Iowa phosphorus (P) index. These four tests are the Bray P 1, Olsen and Mehlich-3 tests determined colorometrically; and the Mehlich-3 test determined using an inductively coupled plasma spectrometer (ICP).

While the Olsen and Mehlich-3 are suitable across virtually all Iowa soils, the Bray P 1 test is not suitable for use in calcareous soils. This is because Bray P 1 extract, which is a weak acid, is largely neutralized by calcareous soils and therefore loses its extracting power and underestimates available P. Because of this, many soil testing labs may run Bray P 1 on samples with a pH of less than 7.4 and use the Olsen test on samples where the pH is greater than 7.4.

Having two different soil test methods used in the same field creates a problem when running the P index because a field average soil P value is needed for each field in the manure management plan. Because each soil test is interpreted differently, different soil tests can not be averaged together.

SamplingThe easiest way to avoid this problem is to specify to the soil lab that you'd prefer the Olsen or Mehlich-3 (either colorimetric or ICP) method used on all samples.

If the Olsen or Mehlich-3 test is not specified and the soil lab results have both Bray P 1 and Olsen values within the same field, Olsen values should be adjusted so they can be included in the average with the Bray P 1 samples. Iowa research by Antonio Mallarino (Department of Agronomy, ISU) has shown a strong relationship between the Olsen and Bray P 1 soil tests, and therefore the Olsen value can provide an accurate estimation of the Bray P 1 value. This research has shown that on average the Olsen P test extracts about 60 percent of the P extracted by the Bray P 1 and Mehlich-3 colorimetric P tests, but the actual range for non calcareous soils varies from 50 to 70 percent depending on various soil chemical properties. This is the reason why planning ahead for either Olsen or Mehlich-3 for all samples is the best option.

To adjust the Olsen value, divide it by 0.6. For example, if an Olsen test has a value of 30 ppm, dividing this number by 0.6 would give an approximated Bray P 1 value of 50 ppm. The value of 50 ppm then could be used in an average that included Bray P 1 soil test results.

This process is acceptable when developing a manure management plan for DNR.

For more information on soil testing for P, refer to the ISU Extension publication: General guide for crop nutrient recommendations in Iowa (PM 1688). It is available on-line at: or can be ordered through the ISU Extension Distribution Center by calling (515) 294-5247.


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