There’s Good News About the Potassium in Your Soil

As you look over your fields this winter, be thankful for the tons of potassium (K) present in every acre. It is the most abundant mineral macro-nutrient on earth, which is good because most crops utilize large quantities of it. Among other functions in plants, K is involved in the regulation of water uptake and utilization. The bad news is that not all K present in each acre are available to growing crops, and the ability to determine how much is available can be imprecise.

Soil testing is the time-honored method of assessing K availability; the process encompasses several components that must work together to deliver meaningful recommendations at the end. It is not simply measuring how much of an element is present in soil. In fact, elemental analysis of soil is useless for fertility management. Recently such a report provided by a chemical analytical laboratory was sent for recommendations: the results indicated a little over 1.5 tons of K present. However, there was no way to extrapolate or interpret what that number meant as there 1) there was no documentation of the method used to determine it; 2) consequently there was no indication as to whether this was low, medium, or high in the overall scheme of things, and 3) therefore no justification of observed crop conditions or prediction of whether the crop would respond to fertilization could be made.

Recently the validity of soil testing has come under fire in some quarters due to perceived variability issues. One part of the argument is that soil test K values of a field change through the year. This is not a new phenomenon, nor is it unmanageable to the point of abandoning soil testing.

First, we need to define what we are measuring with the soil test. As noted above, there can be tons of K in each acre of soil, but only a fraction of the total is useable by growing plants. The portion that is most readily available is present in the soil water (called soil solution). This amount is rather small at any point in time, and inadequate to provide enough K for agronomic crops. Soil solution K is replenished by, and in equilibrium with K held on clay edges and organic matter in the soil matrix. The larger quantity of K in soils is part of mica, feldspars, and other K-bearing minerals which reflect the soil parent material and its weathering over time.

Soil tests for agronomic crops measure soil solution K and the K held on clays and organic matter. The number reported is called exchangeable K, and can vary among different laboratories due to the chemistry of the extracting solutions or reaction time.
Extraction and measurement are only part of the back ground needed for meaningful soil testing. Calibration of various soil test levels with correlation to plant responses is necessary for recommendations. Without the calibration/correlation process, soil test extraction numbers are no more useful than a total elemental analysis.

What can cause variability in exchangeable K measurements? A pessimistic view is “what can go wrong”?

First, this is a complex system where we are seeking a snapshot at one moment of what the growing plant will utilize in the course of a growing season. So plants are part culprit – they use up the easily available soil solution K. Moreover, as vegetative growth rates increase, they use more each day until reproductive growth initiates, therefore soil solution K must be replenished from the other K pools in the soil. Replenishment from mineral weathering however continues at a slower, geological rate likely contributing to lower soil test K values in the fall.

The effect of soil moisture is less clear, but obviously important. Many factors are involved such as mineralogy, initial soil test K levels, depth measured, and fertilizer programs. Historically, soil samples have been dried by testing laboratories; recently a few Mid-western laboratories began offering moist soil test options based on their local research.

This discussion is simply an introduction to some issues of soil testing for K. What do we do about it?

•   Sample the same time of year to remove the seasonal variation component.
•   Take good, consistent, representative samples.
•   Use a good laboratory.
•   Development nutrient budgets of additions and removals.
•   Keep an eye on problem areas.

Through the support of the Mississippi Soybean Promotion Board, Mississippi State University is finishing the third year of a statewide study on timeliness of soil testing, and beginning the third year of modern soil test calibration and correlation for soybeans in the Delta region.