Plant Nutrient Removal by Crops

In a Thread (that has gone somewhat off course) titled Fertilizer Choices, Coastal bermuda... there were various amounts of potassium reported as being removed by Coastal bermudagrass.

 

The discussion was on the amount of potassium that is taken up by Coastal bermudagrass on a per ton basis. Apparently, there is some confusion when crossing state lines, on the recommended amounts of potassium that need to be applied to compensate for this plant uptake based on statements in that Thread. Here is an attempt to clear up this confusion.

 

All plants take up potassium as K⁺, not as K₂O represented by the third number on the fertilizer bag or in the fertilizer blend. When a plant analysis is done, the result for potassium is reported and percent K, not K₂O. When a soil test is done, potassium considered as plant available by extraction is reported as parts per million K or even as pounds per acre and the fertilizer recommendation for potassium is given as pounds of K₂O to apply per acre. The potash in the fertilizer (0-0-60) is not even K₂O but is KCl, or potassium chloride, referred to as muriate of potash. Sometimes the potassium is supplied as potassium sulfate (K₂SO₄), or even as potassium nitrate (KNO₃) but rarely.

 

According to Kelling and Matocha, (1990, in Soil Testing and Plant Analysis, 3rd Edition, edited by R. L. Westerman, a Soil Science Society of America Publication), 1.8% K in Coastal bermudagrass is the upper end of adequate and 2.2% K is listed as high. Using an average between these two values, or 2.0% K in a ton of Coastal bermudagrass on a dry matter basis amounts to plant uptake of 40 lb of K. If we want to replace this amount of potassium by fertilizing the soil, we need to apply 48 lb of K₂O per ton of Coastal bermudagrass dry matter removed as hay, because there is only 83% K in K₂O. This fits very closely with the 48 pounds of K₂O per ton indicated by the old Potash/Phosphate Institute (Now IPNI, or International Plant Nutrition Institute) as taken up by bermudagrass.

 

So if two tons of 15% moisture Coastal bermudagrass hay is removed per acre, about 85 pounds of K₂O per acre (rounded up from 83.4 lbs) would need to be applied as fertilizer to replace the potassium removed by the Coastal bermudagrass hay. On a dry matter basis, two tons of Coastal bermudagrass removes 96 pounds of K₂O. This is very close to the current amount of K₂O reported by IPNI (50 lb/ton) as being removed from the soil by Coastal bermudagrass, or 41.5 lb of K.

 

(If you are interested in the chemistry, the formula weight of K is 39.1 grams per mole. Two moles of K in K₂O become 78.2 plus the molecular weight of Oxygen is 16, or a total molecular weight of 94.2 for K₂O. The weight of K in K₂O, 78.2 divided by the molecular weight of K₂O is 0.83, or 83% K in K₂O.)

 

(Why is the letter K used to represent potassium when there is no letter 'K' in potassium? The origin of the symbol K used for potassium comes from kalium, the Latin name for potassium. And word potassium is derived from potash.)

 

For your information, IPNI provides plant nutrient removal data for many crops on their web site at:

 

http://ift.tt/1SfQxNI

Plant Nutrient Removal by Crops