THE BASICS OF SOIL POTASSIUM

Of the three major nutrients, potassium (K) probably receives the least amount of attention in research and education programs in this part of the Corn Belt.  This does not necissarily mean that it is less important than nitrogen (N) or phosphorus(P).  When we respond to questions about fertilizer use, there are probably fewer about the management of this nutrient.  The chemistry of potassium in soils is certainly not as complicated as the chemistry on N and P.  However, because of the high price of this nutrient, it’s appropriate to take a look at the basics and review what we know.

In our soils, K is described as exisiting in three forms. These are generally agreed to be :1) unavailable, 2) slowly available, and 3) readily available.  All forms coexist at any point in time in soils and there is a constant change from one form to another.  That is, these three forms are said to be in equiulibrium with each other.

Most soils contain in the neighborhood of 40,000 lb.K/acre in the top 7 inches (plow layer).  Most of this large reservoir of K is present in minerals such as feldspars and micas.  Therefore, the very large majority of total K in the soil (about 98%) is not readily available to crop production in any given year.  Over many years, these minerals break down releasing K that was previously not available for crop production. this conversion of “unavailable”  K to “available” K is called weathering.  This process, however, is too slow to supply all of the K needs of field crops in any given year. 

“Slowly available” K is associated with the clay minerals.  Briefly, clay minerals consist of layers separated by water.  The layers of some clay minerals collapse and trap or “fix” the K.  In other types of clay minerals, the layers can collapse when dry and separate when water is added to the soil.  Potassium is released as these mineral expand when wet  and is not “fixed”.

“Readily available” K is associated with but is not trapped by the clay minerals.  This K is found in the thin film of water around every soil particle regardless of size.  This is the form that is easily absorbed by plants.  This is also the form that is measured in the routine analysis of soil samples to generate a recommendation for potash fertilizer use.

In soils, potassium can move from one form to another. If a crop is removing large amounts of K, some of the “slowly availabe” K is changed to “readily available” K .  Likewise, some (very small amounts) of the “unavailable” K  can be released from soil minerals to the “readily available ” K.  This transformation, however, is a very slow process.  The transformation from “slowly available” to “readily available”  K is more rapid.

The challenge in crop production is to measure the concentration of “readily available” K and to relate this measurement to to the amount of potash fertilizer that must be supplied to achieve optimum crop yield.  A laboratory procedure to do this was developed many years ago and it works.  It’s a routine procedure that’s used in most soil testing laboratories in the North Central states.  A soil sample is extracted with ammonium acetate solution having a specific or standard concentration.  The K measured after extraction is classified as being very low, low, medium, high, or very high and the rate of fertilizer potash needed for optimum yield is adjusted accordingly. 

The yields listed in Table 1 are taken from research plots in farmers’ fields.  At the Fillmore County site, the optimum rate of applied potash was somewhere between 150 and 200 lb. K2O per acre.  On the other hand, potash use did not increase corn yield at the Yellow Medicine County site.  What’s the difference in the two sites when the yields were about the same?  The soil test K value for the Fillmore County site was 75 ppm which is classified as low.  In Yellow Medicine County, the soil test for K was 204 ppm which is classified as being very high.

Table 1. Corn yield as affected by rate of applied potash fertilizer.

 

Potash Applied                                  Fillmore County                 Yellow Medicine County

     lb. K2O/acre                                ———————-bu./acre——————————–

           0                                                         124                                            208

         50                                                         178                                            205

       100                                                         191                                            206

       150                                                         196                                            203

       200                                                         201                                            206

These results show that the soil at the Yellow Medicine County site supplied adequate amounts of “readily available K” for optimum yield whereas the soil at the Fillmore County research site did not.  These yield data provide a good documentation for the use of analysis of soil samples as a guide to fertilizer use.

There is also the theory that the amount of K or potash used is a fertilizer program should be equal to the amount expected to be removed by the intended crop.  While this “crop removal” approach to fertilization may seem logical, it can lead to mistakes in fertilizer use—mistakes that are not needed when prices are high.  Let’s use the information in Table 1 as an example.

Potassium removal in corn grain is about 0.25 lb. K2O per bushel.  A 200 bu. per acre crop, therefore, will remove about 50 lb. K2O per acre. For the research site in Fillmore County, the application of 50 lb. K2O per acre would not have been adequate for optimum yield. It would have been a mistake to use the “crop removal” approach to fertilization at this site.  Yield would have suffered with subsequent reduced profit.  In the Yellow Medicine County field, potash fertilizer was not needed for optimum yield.  Money spent on potash fertilizer for this field would have been wasted because there was no increase in yield when the potash fertilizer was applied.  The yield information summarized in Table 1 shows that the use of the “crop removal” approach can be expensive and it’s easy to make a mistake.

The “crop removal” approach to fertilization ignores the fact that the soil can supply some or part of the nutrient demand of a crop.  That’s the purpose of soil testing.  With soil testing, the first step is to determine the relative level of the nutrient in question.  Knowing this, we can then determine the amount to apply in a fertilizer program. If we understand some of the basics of K in soils, we can understand why the “crop removal” approach to fertilization does not work.