How are soil tests correlated to plant uptake?

Cristie Preston, Ph.D.

Cristie Preston

Cristie Preston, Ph.D.


Senior Agronomist

Agriculture has always been an integral part of Dr. Cristie Preston’s life. She grew up in southwest Virginia and had interest in crop and animal agriculture since an early age. Once she began college, she initially chose to study animal science but switched to soil science. Dr. Preston attributes her decision partly to an influential professor who told her, “You can’t understand animals until you understand what they eat.” She received a Bachelor of Science degree in animal science and a Master of Science degree in crop and soil environmental science from Virginia Tech. Dr. Preston holds a Ph.D. in agronomy, focusing on soil fertility from Kansas State University. While completing her advanced degrees, Dr. Preston conducted more than six years of field and lab research. Dr. Preston has experience in laboratory research measuring volatility loss from urea-based fertilizers. Her field research has focused on phosphorus availability and the interactions with tillage and placement. She also has extensive experience in working with large data sets and analysis. Her main priority is helping growers to identify yield-limiting factors and fix those issues as cost efficiently as possible.

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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).

Relative yield formula

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.

relative soybeen yield response

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).

Cristie Preston, Ph.D.
Nutrien | Senior Agronomist