The Right Source of Potassium

Potassium, or K, is unique in a way that soils have a relatively large amount in the Midwest region.  Some soils can range from 15,000-20,000 ppm total K.  Even though there are large amounts of this macronutrient, it must be in an exchangeable form.  It is estimated to be only 1-5% is available and most soil contains approximately 1-3 ppm in the soil solution. 

Why is potassium so important?  Afterall, when reviewing commercial fertilizers, it is listed as the third number in the grade or analysis.  The functions of K in the plant are numerous.  It plays a key role in cell growth, photosynthesis, stomata function, enzyme activation, nutrient absorption and water retention just to name a few.  Using corn as an example, K deficient plants can experience marginal leaf necrosis, reduced plant height, poor reproduction and stalk lodging. 

There are multiple variables to consider when evaluating soil K levels.  Weather, residue, soil type and sampling habits can all play a large role in test levels.  Weather will affect the uptake of accessible soil potassium simply by not having enough moisture for uptake.  It is important to understand just because the crop is showing signs and experiencing symptoms of K deficiency, that environment dictates uptake of this macronutrient. 

When choosing the right source of potassium, or potash, do not over think what type of K is correct.  The K is nutritionally the same in all fertilizers.  However, many commercial fertilizers are not only K.  Some crops are sensitive to individual nutrient salts.  There are many soils with insufficient amounts of individual nutrients, and this is why it is important to select the correct type of fertilizer for individual cropping systems, soils and maintenance programs.

Here are some examples of potassium fertilizers and their composition.  Potassium chloride (KCI; 0-0-60) This is the most common for bulk blending and row crop agriculture.  This would not be a good choice for crops such as almonds since they are sensitive to Cl^-.  Potassium nitrate (KNO₃; 13-0-44) is an additional source of nitrogen and a good choice for chloride sensitive crops.  Potassium magnesium sulfate (K₂SO₄•2MgSO₄; 0-0-22) used to supplement magnesium and sulfur with K.  Potassium sulfate (K₂SO₄; 0-0-50-18) has a low salt index and includes sulfur.  Potassium thiosulfate (K₂O₃S₂; 0-0-25-17) is a liquid fertilizer and can be used in sidedress applications in dry conditions.  Potassium hydroxide (KOH; 0-0-75 has a high pH.  Often used as a liquid fertilizer that is absent of chloride and best used in acidic soils.   

Sample timing is critical when accessing K levels.  It is advised to pull samples after the same crop each time.  Soybeans, for example, will release potassium back into the soil much faster than corn.  As soon as the cell ruptures K is released and available.  It does not take a microbial process for availability.  This is why soil test levels vary greatly depending on sampling time and previous crop. 

Extreme, dry conditions can also impact soil test K levels as they become tightly bound to clay particles.  When making management decisions for potassium, always sample at the same time of year, after the same crop type, at the same depth/location and be mindful of current weather/environmental conditions when analyzing data.

Source: International Plant Nutrition Institute. (2012). 4R Plant Nutrition: A manual for improving the management of plant nutrition (pp. 40-41). Norcross, GA, USA: International Plant Nutrition Institute