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Differentiating and Understanding the Mehlich 3, Bray, and Olsen Soil Phosphorus Tests

Soil testing is the key component for determining the need for phosphorus (P) fertilization. Also, if fertilization is required, test results guide the rate of application recommended to optimize production. Through extensive laboratory, greenhouse, and field research studies, soil tests are calibrated against the expectation of response to applied P. That is, the research determines both the relative index of P availability to the crop being grown and an indication of the probability and magnitude of yield increase one should expect when P is applied – thus providing the interpretation of test results. This research also determines which test is best at predicting available P and crop response to P application. Disciplines Agricultural Science | Agriculture | Agronomy and Crop Sciences This article is available at Iowa State University Digital Repository: http://lib.dr.iastate.edu/agron_conf/12 Differentiating and Understanding the Mehlich 3, Bray, and Olsen Soil Phosphorus Tests John E. Sawyer and Antonio P. Mallarino Department of Agronomy Iowa State University Soil testing is the key component for determining the need for phosphorus (P) fertilization. Also, if fertilization is required, test results guide the rate of application recommended to optimize production. Through extensive laboratory, greenhouse, and field research studies, soil tests are calibrated against the expectation of response to applied P. That is, the research determines both the relative index of P availability to the crop being grown and an indication of the probability and magnitude of yield increase one should expect when P is applied – thus providing the interpretation of test results. This research also determines which test is best at predicting available P and crop response to P application. Soil P Test Considerations Both the relative index of availability and interpretation of P need is specific to each soil test. Therefore it is imperative that one knows what soil test method was employed before you can adequately derive a P fertilization recommendation. This is an important consideration as there have been many methods developed to test soils for crop available P. Each can have a widely different interpretation index. For example, at a given soil test level the interpretation may be optimum for one test, but may be interpreted as low or high for another test. In Iowa for instance, at a test level of 15 ppm, the interpretation of the Bray-1 test is low, but for the Olsen test it is high (the Olsen test typically extracts less P than the Bray-1). Each test method has unique characteristics, and in the end, best application to specific soils and cropping systems. For P this is important and somewhat complicated in geographic areas where soils vary in chemical properties across the landscape – specifically the content of free lime (calcium or magnesium carbonates in calcareous soils). This variation has been highlighted by results from intense soil sampling. Some calcareous soils can cause certain P soil tests to behave differently than in noncalcareous soils, and thus to produce erroneous results. In the Midwest, the commonly used Bray-1 test has this difficulty. The Olsen test, on the other hand, does not have this problem (was developed and calibrated to determine available P in calcareous soils). Another important, but complicating issue is the efficiency of laboratory operations. Use of one extraction solution to provide multiple nutrient analyses can shave costs and increase through-put. Thus laboratories may be interested in employing tests that do not use traditionally recommended extractions for P. This efficiency underlies the interest in the Mehlich 3 test. Another, but sometimes non-evident issue is the analytical method utilized to measure the amount of P extracted from soil samples. Again, to increase laboratory efficiency by simultaneously determining multi-nutrient concentrations, laboratories have interest in shifting away from individual methods (in the case of P, colorimetric determination using the ascorbic acid method) to use of equipment that can make multiple element determinations on the same soil extract (specifically use of inductively coupled plasma or ICP). It is important to remember 1 Presented at the 19 Annual Crop Pest Management Short Course, University of Minnesota, November 22, 1999, St. Paul, MN.