CORN Newsletter: 2004-27, August 16, 2004

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C.O.R.N Newsletter 2004-27
August 16, 2004 - August 24, 2004

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Soil Tests for Plant Available Phosphorus
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Over the last few years questions have arisen about the tests for soil phosphorus (P), especially the relationship between the Bray-Kurtz P1 and the Mehlich 3 soil phosphorus tests. Both tests have been used for Ohio soils and measure plant available P, not the total P concentration in the soil. The Bray-Kurtz P1 test has often been called just the Bray P1 test. The following is a discussion of these tests and includes basic information about the development of successful soil tests.

Purposes of a Soil Test:
A useful test should be able to separate soils that are deficient in P from those that are sufficient in P in regard to optimum plant growth. In addition, information about plant response to the application of fertilizer P should be part of the test. Several tests have been developed for measuring plant available P in the soil. The kind of test that should be used depends on the soil. Consequently, appropriate P soil tests are based on the kind of soils that occur in various regions of the United States. Many years of research are required to develop a useful soil test for the soils of a region.

Development of a Soil Test:
The development of a soil test involves two primary aspects of study. The first aspect involves a study to establish the chemical extracting solution that shows the highest correlation (strongest relationship) between the concentration of P extracted from the soil to the amount of P taken up by the plant. In this way, the extracting solution relates what is in the soil to what is used by the plant. This research is usually done under greenhouse conditions where many different extracting solutions are evaluated on different soils, and usually for many different plants. This has been the case for both the Bray-Kurtz P1 and Mehlich 3 extracting solutions for the soil P tests.

The second aspect involves calibrating the soil test to determine the relationship between the concentration of P in the soil to a response of the crop under study, usually bushels/acre or tons/acre of yield. This provides information about the optimum crop response (yield) at a given concentration or range of concentrations of P in the soil. The determination of an estimate of optimum crop yield at a given soil P concentration will allow for the separation of soil P concentrations into deficient and sufficient categories. In addition, if the soil is in the P deficient category, the calibration research will provide information on how much increase in crop yield will likely be obtained with increases in the concentrations of soil P. From the calibration research, a recommendation for applying specific amounts of fertilizer P to the soil can then be developed. The calibration work is done under field conditions and involves replicated field research on different soils.

Bray-Kurtz P1 Test:
The Bray-Kurtz P1 test was developed in 1945 at the University of Illinois by Dr. Bray and Dr. Kurtz. The correlation with plant uptake of P ranges from 0.70 to 0.94 for the soils of the North Central Region of the USA. The detection limit of the test is 1 part per million (ppm) P and the reproducibility is plus or minus ( + ) 10%. It is recommended by the USDA North Central Regional Soil Testing and Plant Analysis research committee (NCR-13) for use on soils with pH 7.0 or less. The Ohio State University fertilizer recommendations for agronomic crops grown in Ohio soils are based on this soil P test.

Mehlich 3 Test:
The Mehlich 3 test for plant available P in the soil was developed by Dr. Mehlich at North Carolina State University in 1984. It was a modification of his earlier tests, Mehlich 1 and Mehlich 2. The test has been referred to by some as the “Universal Extractant” method since both P and K can be determined in the same solution at the same time by appropriate laboratory instrumentation. Limited crop response work on soils of the North Central region has shown the correlation with P uptake by the plant to range from 0.83 to 0.99. After many years of study, the NCR-13 Committee approved the Mehlich 3 test for North Central soils that have a pH of 7.0 or less. The detection limit of the test is 1 ppm with a reproducibility of +10%.

How do the two test compare?
It has been found across most of the soils of the North Central Region that generally the Mehlich 3 tests extracts more P from the soil than the Bray-Kurtz P1 test. Limited research in Ohio has shown that on samples of Crosby soils the Mehlich 3 test extracted, on the average, 30 ppm P, compared to 19 ppm with the Bray-Kurtz P1 test on the same Crosby soil samples. On Ohio Hoytville soil samples, Mehlich 3 extracted 30 ppm P, compared to 16 ppm with the Bray-Kurtz P1 test. Although the data is somewhat limited, a linear relationship can be determined across a range of soil P concentrations from results of the two tests.

What about making P fertilizer recommendations from the Mehlich 3 test?
Since there is limited field calibration of the Mehlich 3 to crop response for Ohio soils, and since the fertilizer recommendations for agronomic crops are based on the Bray-Kurtz P1 test, soil testing laboratories that use the Mehlich 3 test should make an estimation of what the P concentration would be if the Bray-Kurtz P1 test were used. This can be done by doing both tests on hundreds of soil samples and from this information develop a linear equation describing the relationship of the P concentrations. This is the best approach until field research can be done that evaluates crop response to the soil P concentration as determined with the Mehlich 3 test. However, it has been emphasized by the NCR-13 committee that a laboratory should not run the Mehlich 3 test and then refer to the results as being from a Bray-Kurtz P1 test. It is important in the future that fertilizer recommendations based on for the Mehlich 3 test be developed from actual field crop response calibration work on as many soils as possible.

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C.O.R.N. is a summary of crop observations, related information, and appropriate recommendations for Ohio Crop Producers and Industry. C.O.R.N. is produced by the Ohio State University Extension Agronomy Team, State Specialists at The Ohio State University and Ohio Agricultural Research and Development Center. C.O.R.N. Questions are directed to State Specialists, Extension Associates, and Agents associated with Ohio State University Extension and the Ohio Agricultural Research and Development Center at The Ohio State University.

Information presented above and where trade names are used, they are supplied with the understanding that no discrimination is intended and no endorsement by Ohio State University Extension is implied. Although every attempt is made to produce information that is complete, timely, and accurate, the pesticide user bears responsibility of consulting the pesticide label and adhering to those directions.

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Issued in furtherance of Cooperative Extension work, Acts of May 8 and June 30, 1914, in cooperation with the U.S. Department of Agriculture, Keith L. Smith, Director, Ohio State University Extension.

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