PRINCIPLES OF INTERPRETING SOIL TEST RESULTS – Why do Different Labs Give Different Results and Interpretation?

Different chemicals and testing methods extract different quantities of nutrient from soil.

Therefore, different testing methods are used for different soil properties and conditions. The success or failure of any chemical soil test greatly depends on the mode of action of the extracting chemical, or the extraction method used. The last two must simulate or correlate with plant uptake for a particular soil.

Therefore, different laboratories may give different results for the same sample.

In most cases, even when the numerical results are different, due to the use of different analytical methods, the interpretation is mostly the same in terms of sufficiency levels of the nutrients, i.e. deficient, adequate, high etc.

In case one lab uses testing methods which are not adequate for the specific soil and conditions, the interpretation itself might differ.

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. This may be very confusing.

Differences in laboratory procedures, such as different dilution, shaking rates, extraction time, filter paper etc. may also affect the results and lead to differences in the soil test results, even if the laboratories use the same testing methods. 

 Before sending soil samples to the lab

Before sending soil samples to the lab, make sure that the lab you choose uses the appropriate  testing methods for your soil.

For example, two common testing methods for phosphorus are Bray-1 (uses 0.025M HCl and 0.03M NH4F) and Olsen (uses 0.5M sodium bicarbonate). The Bray method extracts more phosphorus than Olsen and is more adequate for soils with pH<7.0, while Olsen is more adequate for calcareous soils.