Potassium (K) is a crucial nutrient for plant growth. It is one of the three primary nutrients (along with nitrogen and phosphorus) because natural levels of potassium in soil often limit plant growth.

Potassium is attracted to the negatives charges on soil particles that make up the cation exchange complex. Thus loss from agricultural fields is rarely an economic problem, with the possible exception of sandy soils with very low cation exchange capacity (CEC) values. Unused potassium fertilizer from one year can be used the next.

Potassium loss from agricultural fields is not an environmental concern.

Yield response to potassium

A network of experiments across Missouri has shown that there is little or no yield response to K fertilizer when previous fertilizer management has maintained soil test levels at the target levels recommended by the University of Missouri.

A network of experiments measuring soybean, corn, and wheat response to potassium was conducted all across Missouri between 2000 and 2002. These were conducted in cooperation with the University of Missouri variety trials, mainly on farmer fields. Cooperating farmers had maintained soil test K at levels consistent with University of Missouri recommendations. Most experimental fields had soil test levels between 200 and 360 lb ammonium acetate K/acre; the University’s target level is 280 lb/acre for a soil with typical silt loam CEC.

Average yield response to K in these experiments was very nearly zero (see table). This is expected and intended in the University of Missouri’s recommendation system. At the target soil test level, the soil should be able to supply the crop’s entire potassium need. Fertilization is then to replenish the soil supply as it is used up. Timing of K fertilization is not critical if soil test values have been maintained at or near the target level.

Crop	Average yield response to K	Number of experiments
Soybean	-0.5	40
Corn	-1.0	21
Wheat	1.5	8

See the complete reports: Soybean (PDF), Corn (PDF), Wheat (PDF).

Field-scale response to potassium

Measuring field-scale response to P, K, or both is fairly easy to accomplish using yield monitor data. Yields can be compared from side-by-side strips with and without P/K to see how large the response is, where it is occurring in the field, and to monitor and understand patterns of P and K response on a given landscape or farm.

Soil tests are designed to predict the probability of a yield response to P and K. On-farm measurement tells you much more accurately whether you got a response and how large it was. Soil tests are interpreted based on experiments done under different conditions and on different soils than are likely to exist on your farm.

Yield responses to K may depend on how much K is being supplied by your subsoil.

Resources:

• Field-scale P/K response (PPTX)
• Yield response to P/K fertilizers over landscapes (PDF)

Potassium in Missouri subsoils

Soil samples to a 3-foot depth show that the subsoil is a greater reservoir of K than the topsoil over most of Missouri.

Current University of Missouri fertilizer recommendations have a target level of 280 lb K/acre (140 ppm K) for a typical silt loam soil with CEC = 12. Soil K is measured with ammonium acetate extraction and atomic absorption photometry. Measurements of soil K in 308 deep soil samples across Missouri show that the subsoil usually contains more than three times this much K in most of Missouri. See the subsoil potassium map (PDF).

Variable-rate potassium

Variable-rate P and K increase costs to producers, both in soil sampling and in application. Where is this cost recouped? It has to be either by increasing crop yield, or by reducing P and K use enough to more than cover the cost of using this type of management.

A statewide network of on-farm small-plot experiments showed that under prevailing P and K management conditions, response to the current year’s P and K applications is near zero. This suggests that producers are managing P and K in such a way that the soil can supply all that the crop needs, and that it is not possible to increase yield by managing P and K differently.

Although variable-rate P and K management may sometimes reduce the total amount of P and K applied, I think there is potential to enhance these savings using the variable-rate approach.

In the network of experiments mentioned above, there was evidence of a small yield response to K in corn, soybean, and wheat when soil test levels were below 200 lb K/acre. So why is the target level for soil test K set at 280 lb K/acre for a typical silt loam by the University of Missouri? I don’t know the history, but it makes sense to me that there should be a ‘cushion’ between the known ‘critical value’ and the recommended ‘target value’. This is because of the spatial variability of K in the field. See an example for K variability (PDF).

Having a ‘cushion’ built into the target value for soil test K ensures that even the lowest-testing parts of the field are at full yield potential. Variable-rate management may offer a cheaper way to ensure that the whole field is at full yield potential.

By taking spatially intensive soil samples, the low-testing areas of the field can be located and fertilized to achieve their full yield potential. The ‘cushion’ is no longer needed. This means that a lower target level for soil test K can be used, and savings on fertilizer will be increased to where they should easily exceed the cost of variable-rate management.

Potassium placement

P and K placement is a hot (and hopeful) topic. Many producers hope that they can save big money by going with placement.

What can placement actually do for you? Multiple studies in Missouri have failed to find that placement of P and/or K can increase yields. That leaves the possibility that placement will allow you to lower your rates. This is where the typical producer considering a placement machine is pinning his or her hopes.

Placement can improve the efficiency with which this year’s fertilizer is delivered to the crop. Remember that for producers who follow soil test recommendations, this year’s fertilizer is not actually necessary to attain full yield. Fertilization only serves to replenish the soil’s ability to supply all of the crop’s needs. This means that increasing the efficiency of this year’s fertilizer is a moot point.

The only way to take advantage of greater efficiency is to lower rates and allow soil test levels to drop. Then your repayment on the time and money that you spend on placement is the interest on the money that you saved on P and K. The kicker that many producers have overlooked is that you can’t put on a half-rate of P and K and let soil test drop forever. If you’re 100% efficient with delivering half of what the crop needs, where is the other half coming from? The soil. And the soil’s supply is being depleted. Eventually you will get to a point where you are cutting into your yields unless you go back to applying full removal rates of P and K with your placement machine. At that point, your savings have ended but your obligation to band P and K every year is permanent.

Grain potassium removal

Grain nutrient removal per bushel of yield appears to vary from year to year and region to region based on samples collected at a range of sites around Missouri from 2006 to 2008 for corn (PDF), soybean (PDF), and wheat (PDF).

The variability seen means that calculated grain removal values for N, P, and K will not always be accurate. Most fertilizer recommendations use these removal values as part of the basis for rate decisions. Current National Research Council feed grain values for K as K₂0₅, which are currently the basis for University of Missouri nutrient removal calculations, are shown in golden brown.

Applying nutrients at rates to replace those removed by the crop is a sound approach, but putting too much faith in the ‘book values’ for removal could lead to problems. Soil testing should be used as another source of information about the nutrient status of soils.