Potassium Test


Purpose:  Potassium (K) enhances disease resistance in plants by strengthening stalks and stems, contributes to a thicker cuticle (leaf surface layer) which guards against disease and water loss, controls the turgor pressure within plants to prevent wilting, and enhances fruit size, flavor, texture and development.  Soil potassium is found in three forms; trapped between clay layers (relatively unavailable), adsorbed on the surface of soil colloids (exchangeable), and in the soil solution (available).  Available potassium supply for maximum crop production depends on the type of clay mineral in the soil parent material (some minerals have more potassium than others) and its resistance to weathering actions.


Potassium is generally not considered a pollutant in Kansas surface water, and is naturally high in most Kansas soils.  There may be some conditions where low K levels limit crop growth, and extremely excessive levels of K in soil can limit plant uptake of other nutrients like magnesium and calcium.  One should monitor the K levels on their farm, to make sure that levels are adequate for crop growth, but not excessive.


Tools:  the following procedure used the LaMotte garden soil test kit model EM, Code 5934.  Other methods of measuring soil potassium are also possible.  Some test kits are more accurate than others.  Before using a particular “quick test “method, compare it to laboratory results.  The tests recommended in this handbook have all been compared to KSU soil test laboratory results, and have been deemed satisfactory for initial screening purposes.   Note:  the LaMotte soil test kit correctly identified very low and very high potassium  level in soils, but did not distinguish between medium and high (intermediate test) soils.




1)    Fill test tube to line 7 with Potassium Extracting Solution.

2)    Use 0.5 g spoon to add four measures of soil sample to test tube.

3)    Cap and shake vigorously for one minute.

4)    Remove cap and allow soil to settle.

5)    Use a clean pipet (dropper with the red bulb) to transfer the clear liquid to another clean test tube.  Be careful not to pull up any soil into the dropper tube.  Fill a second test tube to line 5 with the liquid.  Note that if additional extract is needed to fill the tube to line 5, start with another tube and repeat steps 1-4.

6)    Add one potassium indicator tablet to the soil extract in the second tube.

7)    Cap and shake to dissolve the tablet.  A purplish color will appear.

8)    Add potassium test solution, two drops at a time.  Keep a running count of the drops used.  Swirl the test tube after each addition to mix the contents.  Stop adding drops when the color changes from purplish to blue.  Record the total number of drops added.

9)    Use the potassium end point color chart as a guide in reading this color change.  Read the test result from this table:


Number of Drops Added

Potassium Level in Soil




Very High




Medium High




Medium Low



20 or more

Very Low





Interpretation - LaMotte Nutrient  Tests (N, P and K):  The interpretive guides provided with the kits give results as high, medium or low.  Other values are possible, such as zero, trace, medium high, etc.  Most commercial soil test labs provide results in units such as lb/a or ppm.  Use the following table to convert the LaMotte colormetric values of low, medium, and high to approximate ranges in ppm (parts per million).  Then use ppm values to determine if additional nutrition is required for the crop you are growing.






LaMotte Level:

Nutrient level range in ppm


























Generally speaking, if a soil falls in the low range for a nutrient, crops will be deficient, and can be improved either in quantity or quality by the supplementation of the deficient nutrient.  The medium range is usually an adequate level for most crops.  The high range is also adequate for crop growth and yield, and may be necessary for heavy feeding crops such as corn.  However, for other crops this could be excessive and could lead to nutrient pollution in surface water run-off.


KSU Extension guidelines vary slightly from the LaMotte table above, with the Horticultural recommendations slanted slightly higher than the table values listed above (need a higher level of each to be in the “medium” and “high” categories).  The Agronomy, or field crops recommendations on the other hand, are consistently lower than the LaMotte guidelines.  Thus, this table may be used with the caveat to consult KSU publications and expertise for the specific crops you are growing.  However, this table combined with the LaMotte tests can be used to determine if the nutrient levels are about right, or are too high or too low.


Use the following table to score your phosphorus, nitrogen, and potassium test results.  A score of 4 is the best rating, 3 is good, 2 is fair, and 1 is poor.






Nutrient levels are in the “medium” range.

Nutrient levels slightly above or below “medium.”

Nutrient in question is “low” or “medium low,” and may be deficient.

Nutrient level is “medium high” or higher, and may be contributing to water pollution, even though crop growth is adequate.