Application of potassium based on soil testing

Soil test correlation data are often used to identify a critical soil test value (CSTV), above which crop response to added fertilizer is not expected.

There are four forms of potassium in the soil: unavailable K (mineral K), available K (soluble K+1), non-exchangeable K (fixed or trapped K), and exchangeable K (ionic K+1). The total potassium in the soil exceeds 20,000 parts per million (ppm). 

The fixed K is a reserve source of potassium. At the same time, the exchangeable K (ionic K+1) is readily taken up by the plant’s root system and substitutes for potassium on the exchange sites. However, relatively small amounts of potassium (K+1) are available for plant uptake at any one time. 

However, the optimum potassium level in the soil test varies with the crop, projected yield, soil type, physical conditions, and other soil-related factors. Generally, higher levels of K+1 are needed in soils high in clay and organic matter than in sandy soils low in organic matter.

The common method to determine the available potassium in soil is to mix 1 g of air-dried soil with 10 mL of 1 normal ammonium acetate at pH= 7 and shake for 5 min. Available potassium is measured by analyzing the filtered extract on an atomic absorption spectrometer set on emission mode at 776 nm. The results are reported as parts per million (ppm) exchangeable potassium (K) in the soil.

Soils containing high levels of magnesium may also require high levels of potassium. Some crop advisors recommend potassium application based on optimum levels for light-colored, coarse-textured soils ranging from 150 to 175 ppm and dark-colored, heavy-textured soils ranging from 175 ppm to 250 ppm.

We recommend not increasing your soil potassium content above 150 ppm. Increasing the potassium content to 150 ppm means that you will be able to increase the soil K % base saturation from 1.4 to 2.23 (by dividing 150/67 = 2.23). It will be great to try other examples based on the soil testing results, and you will learn more. 

However, the objective of this education section is to Convert 150 ppm to Pounds per acre and calculate how much potassium you need to apply to increase the soil content to 150 ppm to achieve your projected yield. 

1. First, it is important to know that soil weight per acre = the soil volume × the soil bulk density. 

2. This is equal to (43560 ft2 /acre × 0.5 ft or 6") × 92 lb/ft3 ( the soil bulk density) ≈ 2,000,000 lb/acre ≈ 2 million pounds per acre.  So, let us consider that a factor of 2.

3. We can easily use a factor of 2 to convert the amount of ppm to pounds per acre of available potassium provided in the soil testing report. 

4. We know that the amount of available potassium in the chemical composition of potassium oxide (K2O) in commercial fertilizers needs to be calculated by an additional conversion factor of 1.2, i.e., the conversion of K to K2O, as follows:

5. For K2O = [2x 39.1 (39.1, is K atomic mass)] + [16 (16, is oxygen atomic mass)] = 78.2 + 16 = 94.2. 

6. 1.2 is the result of dividing 94.2/ 78.2 = 1.2., or the conversion of K to K2O. 

6. The soil testing of available potassium = 67 ppm K when multiplied by 2 x 1.2 , i.e., (67 x 2x 1.2) = 161 pounds of K2O which currently available in one-acre soil.

7. To increase the soil content of K from 67 ppm to 150 ppm, as found in the graph, we need to add 83 ppm of K [(150- 67) = 83)]. 

8. Or you need to add (83 x 2 x 1.2 = 199) 199 pounds of K2O. 

9. The K content in the Muriate of Potash (potassium chloride, KCl) fertilizer is  62% (0-0-62) (every 100 pounds of Muriate of Potash contains 62% of K2O)

10. Therefore, it is necessary to apply 321 pounds of potash per acre [( 199 ÷ 0.62) = 321.3] to increase the soil content of potassium (K) up to 150 ppm to meet the crop demands and achieve your projected yield. 

Finally, note that:

• 1 ppm of soil testing K matches an estimated rate of 4.0 pounds of potash per acre. Therefore, using the graph information, multiply how much you need to add K by 4.0 lbs./Potash/ ac to your crop. 

• However, such an estimated rate varies per crop, soil, and other climatic conditions. We will certainly advise you on the best rate of application.     

• We strongly recommend that the percentage saturation of K should not exceed 5%, i.e., the maximum % saturation base of K in soil analysis. To understand better the % saturation base, please read the following chapter titled "The Soil Cation Exchange Capacity (CEC) and % Soil Base Saturation", under the Education chapter.