How are soil tests correlated to plant uptake?

Several containers with soil samples for testing in the laboratory.

On Farmer Q&A, we previously answered a question about the difference between phosphorus soil test methods, but how exactly are soil test methods correlated to plant nutrient availability?

Soil test correlation is the process of determining the relationship between plant nutrient uptake (or yield) and the amount of nutrient extracted by a particular soil test method. Universities employ field experiments to determine relative yield (defined in the equation below) responses across a wide range of soil nutrient levels to establish this relationship (see figure below).

Overall, what does a correlation graph tell us? A correlation graph helps determine the critical values or optimum range for soil test levels — the soil test level at which an economical relative yield is achieved and the probability of yield response to fertilizer decreases.

Check out this related story on testing for potassium: comparing the dry soil test to the moist soil test.

Relationship between relative soybean yield response to potassium and soil test potassium measured using the dry soil test and moist soil test (adapted from Mallarino, 2012).