Potassium is involved in many vital plant processes, including standability and the regulation of water loss. But picking the best timing, rate and method of application all depend on using your soil test correctly.
Know how your fertilizer recommendation was calculated to tailor potassium applications to soil type
The purpose of a soil test is to let your soil do the talking. Understanding what you’re hearing and seeing, then dialing in the correct rate of potassium (K) fertilizer based on testing method, leads to a healthy soil–crop–farmer relationship.
"Problems arise when advisers use the same recommendation procedure across different soil test extraction methods," explains Farm Journal Field Agronomist Ken Ferrie. "There are numerous ways to analyze soil potassium levels. You, or your adviser, must understand which testing technique was used in order to interpret the results and formulate an accurate fertilizer recommendation."
K recommendations based on soil test findings can be detailed in the following ways:
- parts per million (ppm)
- pounds (you can convert ppm to pound per acre by multiplying the ppm number by 2)
- ppm or pounds, but adjusted based on the cation exchange capacity (CEC) of the soil
- base saturation of cations
- ppm, base saturation and CEC
Method 1. The simplest way to make a potassium recommendation is to base it on ppm or pounds of K shown on the soil test. "Typically, this type of recommendation is geared to crop removal, along with building up or pulling back soil test levels," Ferrie says. "The goal is to keep the soil potassium level in the optimum range.
"You can tell if this is the source of your adviser’s recommendation by reading your soil test; it will only report pounds or ppm of K, along with the soil’s phosphorus and pH levels. If you use computer software to formulate this kind of recommendation, the only thing you’ll need to enter will be the K reading from the soil test."
The ppm or pound method has some shortcomings. "If there’s a lot of variability in your soils the recommended rate of fertilizer might fall short," Ferrie says. "Most laboratories would say you want somewhere around 350 lb. to 400 lb., or 175 ppm to 200 ppm, of K per acre. But, actually, whether a certain level of K is high or low depends on the soil’s cation exchange capacity (a measure of the soil’s ability to hold nutrients) or texture class.
"Soils with higher cation exchange capacities need higher potassium levels to ensure sufficient K in the soil solution. For example, 190 ppm K might be excessive in a soil with a CEC of 6; you could get luxury feeding of potassium by the crop.
Some soils might benefit from potassium applications throughout the season, such as at sidedressing.
"On the other hand, 190 ppm K may be too low for optimum plant growth in a soil with a CEC of 25. We and others have documented this by tissue testing and scouting for deficiencies.
"So a recommendation based only on pounds or ppm only works on a narrow range of soils with limited variability. It is fine if you farm uniform soil, but most farmers don’t."
Method 2. The next simplest way to make a recommendation is to look at ppm of K and then adjust the recommendation based on the soil’s texture or CEC. "This fixes one of the weaknesses of the first method." Ferrie says. "To do this you have to know the CEC of the soils you farm. You can get the CEC from your soil test or by looking at soil survey information online or in a soil survey publication.
"If you want your soil test laboratory to analyze the CEC of your soils, pull samples by soil type rather than by a standard grid system. You can use the smart-grid system, in which you pull the same number of samples, but adjust the location of the grids to stay within soil textures."
Handbooks published by land-grant universities often contain this type of recommendation, based on ppm K and soil texture or CEC, Ferrie notes. (The more sand, the lower the CEC; the more clay, the higher the CEC.) But some recommend different K levels for various CECs or texture classes, so follow your state recommendations.
Method 3. A third method for formulating K recommendations is to adjust the fertilizer rate based on the soil’s base saturation level of potassium. "This involves comparing the base saturation of potassium to the base saturation of magnesium and calcium," Ferrie says. "Most advisers who use this method try to hold base saturation of K between 3% and 5%."
This method also has a weakness. A recommendation made only on base saturation might be inaccurate on low- or high-CEC soils. "If a soil test reads only 85 ppm K, but the CEC of the soil is only 4, the base saturation percentage would be high enough that a K application would not be recommended," Ferrie says. "But with only 85 ppm K, you won’t have enough K to meet the demand of the growing crop.
"On the other end of the spectrum, a soil with a CEC of 30 could have 200 ppm K and still not reach the desired base saturation level. In fact, with a CEC of 30, it might not be economically feasible to reach the desired base saturation level. In soils like these, you would need to think about annual K applications, banding and making multiple applications throughout the season, to keep K available as the plants need it."
- Late Spring 2013