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Soil Sampling. Automated probe for soil samples taking sample with soil probe.
Cristie Preston, Ph.D.

In short, yes, there are differences in methods. The phosphorus soil test is an index of phosphorus availability, which ultimately tells a farmer the likelihood of a response to fertilizer application. Universities have conducted many site years of research correlating plant available phosphorus in soil and yield.

It is well known that different test methods are better correlated with plant available phosphorus depending on soil pH. The most commonly used test methods are Mehlich-3, Bray P-1, and the Olsen P tests. These three methods use different extracting solutions that result in different soil test phosphorus results.

In general, the Mehlich-3 and Bray P-1 solutions are used when soil pH is below 7, and the Olsen P test is used when soil pH resides above 7.

The important thing to remember is that each of these solutions may not always be directly comparable or correlated with one another. Double-check which method your laboratory uses and make sure if you resample a field, that the same method is used if making comparisons.

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