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.”
Potassium 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
Three Ways to Analyze Soil Potassium Levels in Soil
Base potassium levels on ppm or pounds of K shown on the soil test
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. 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. 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.
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.
Look potassium ppm and adjust the recommendation based on the soil’s texture or CEC
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 cation exchange capacity (CEC). This fixes one of the weaknesses of the first method. 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. (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.
Adjust the fertilizer rate based on the soil’s base saturation level of potassium.
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. 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. 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.
Another consideration when making fertilizer recommendations from base saturation levels is you must know how the soil test for calcium was conducted.
While labs can use different procedures to extract K, they all tend to obtain similar results in pounds or ppm. But when they use different procedures to extract calcium, they can come up with different results.
Calcium extraction methods are not a concern if your recommendations are based only on ppm K, or on ppm and CEC figures or soil texture from a soil survey, rather than a lab analysis. But they become a factor if you base K recommendations on base saturation.
Calcium is the biggest contributor in the calculation of CEC. If a lab reports a higher calcium load, both the CEC and the base saturation percentage of calcium will increase. A higher base saturation of calcium causes the base saturation percentage of other nutrients, including K, to go down, because the percentages must total 100.
Consequently, one lab’s results may call for an application of K, while another lab’s results indicate optimum soil K levels.
4 Factors to Consider When Analyzing Potassium Rates in Soil
The most precise K recommendations for variable soils consider all four factors:
- The CEC (cation exchange capacity)
- The soil type
- The nutrient level in the soil
- The base saturation
Considering all of these factors together lets you make recommendations based not just on the total amount of potassium to apply, but on the timing and method of application.
On soils with a low CEC and adequate base saturation levels of K, but low ppm, we must apply enough potassium to at least cover crop removal, and adjust the timing and method of application. These soils typically are sandy and subject to leaching. So apply K fertilizer in the spring, close to the time of plant uptake. Consider broadcasting part of your potash and applying the rest in starter fertilizer, at sidedressing and through irrigation systems. On very heavy soil, with a very high CEC, low base saturation and high ppm, you may have to band some K fertilizer because it may not be economically feasible to raise the ppm high enough.
You can avoid many problems in soil test analysis by sticking with one lab, so the same procedures are used every time. Most labs have their own standards for high, medium and low ratings, based on which extraction procedure they use. Then, based on your various soil types, understand the strengths and weaknesses of whichever method you use to calculate fertilizer rates.
How Base Saturation is Calculated
To use base saturation to make potassium (K) recommendations, your soil test must report both CEC and base saturation of cations (potassium, magnesium, calcium and hydrogen). Base saturation is the percentage of CEC composed of each cation. The following steps detail how CEC and base saturation are determined:
Step 1: Using the ppm reading for calcium, magnesium, potassium and hydrogen, and the weight, in mil-equivalents (meq), of each element, calculate how much each element contributes to CEC.
Let’s say your soil test reads 2,156 ppm calcium. The meq of calcium is 200.
2,156 ÷ 200 = 10.78The magnesium reading is 342 ppm. The meq of magnesium is 120.
342 ÷ 120 = 2.85The K test reading is 280 ppm. The meq of potassium is 390.
280 ÷ 390 = 0.72The hydrogen reading is 13.6 ppm. The meq of hydrogen is 10.
13.6 ÷ 10 = 1.36If your soil test includes a reading for sodium (Na on your soil test report), include sodium in your calculation. The meq of sodium is 230.
Step 2: Add the contributions of each element to get the CEC:
10.78 2.85 0.72 1.36 = 15.71 That is the CEC for this soil.(Note: In this example and in the adjacent story, the term cation exchange capacity, or CEC, refers to a “calculated” CEC, which is used by most soil testing laboratories and in soil surveys. A calculated CEC is somewhat different from a “true” CEC, which usually is used to make environmental assessments.)
- Step 3: To determine base saturation, calculate what percentage each element contributes to CEC, by dividing the element’s contribution to CEC by the total CEC and multiplying by 100.
For calcium, 10.78 ÷15.7 x 100 = 68.7% base saturation
For magnesium, 2.85 ÷ 15.7 x 100 = 18.2% base saturation
For potassium, 72 ÷ 15.7 x 100 = 4.6% base saturation
For hydrogen, 1.36 ÷ 15.7 x 100 = 8.7% base saturation
Why Calcium Readings Differ in Soil Testing
The reason soil testing labs might get different readings for calcium is that there are different ways to treat calcium that is bonded so tightly to other elements or soil exchange sites that it probably is unavailable to plants, explains Farm Journal Field Agronomist Ken Ferrie. Although it sounds like a contradiction in terms, this tightly bonded calcium is often called “free calcium.” Some soils contain more free calcium than others.
Some labs use extraction methods that are strong enough to break the bonds of free calcium. It then is counted as exchangeable calcium (which is available to plants) on the soil test. This results in a higher calcium reading than another lab that uses a weaker extraction technique. Some labs use a regression equation to account for the free calcium. However, there is no standard equation, so labs can still get different results.
Here’s an example of how problems can arise: Say you farm in Tennessee or Georgia, in an area that has mostly light soils with little free calcium. You send your soil sample to a local lab that handles primarily handles soil from the area. That lab does not run a regression equation because there is little need for it. But if an Ohio farmer, with soils with a lot of free calcium, sends his sample to that same lab, he will get a higher calcium reading than if he sends it to a lab that runs a regression equation or uses a different method of extracting calcium.
Because the calcium reading has the greatest effect on CEC and base saturation percentage, different labs can get different CEC and base saturation results from the same soil sample. The use of different extraction methods and regression equations also cause labs to have different standards for high, low and medium soil potassium (K) levels. If you use the same methodology to make a K recommendation for soils that are high and low in free calcium, but you do not use a regression equation, you will have problems.
For example, Ferrie sent a sample of the same soil to two labs. Both used the same calcium extraction method (Mehlich III), but one applied a regression equation and the other didn’t. That led to different readings for CEC and base saturation, as seen below. Lab A found Sample 2 to be in the low K range. But Lab B, which ran a regression equation, reports K in the optimum range.
(Remember, a regression equation is required only if a lab uses an extraction method that measures salts such as free calcium and the end user is making recommendations based on base saturation levels.)
To formulate consistent recommendations, a few consultants and advisers build their own regression equations, for use with soil tests from different labs and for various soil types.
