The Enid News and Eagle, Enid, OK

December 15, 2012

Bermuda needs sufficient potassium to grow

By Roger Don Gribble
Enid News and Eagle

ENID, Okla. — Bermuda grass removes a large amount of potassium from the soil.

The removal of potassium is nearly equal to the amount of nitrogen taken up by the crop. We normally think one ton of Bermuda grass hay removes 50 pounds of nitrogen; well, it also removes about 45 pounds of potassium per ton. A concentration that high indicates just how important this nutrient can be for Bermuda grass growth and development.

Although forage grasses are not generally responsive to potassium as they are to nitrogen, it still is critically important to maintain adequate levels in your soils. This is especially true in a hay production system where little potassium is returned to the soils in the form of crop residues or manure.

Plants obtain potassium from the soil in the form of a positively charged cation (K+). K+ is attracted to negatively charged clay particles. This attraction causes K+ to be immobile in the soil. The K+ attracted to clay particles is called exchangeable K and can be readily released into the soil solution as plant uptake removes K+. Therefore, the K+ in the soil solution and the exchangeable K are said to be readily available soil potassium.

This readily available K+ only represents about 1 to 2 percent of the total potassium in most soils. Some of the remainder is referred to as non-exchangeable or relatively unavailable. This constitutes about 2 to 10 percent of the total potassium in the soil.

Non-exchangeable K is found in certain clay minerals that contain pore spaces large enough for K+ to become trapped or “fixed.” This fixed, unavailable potassium represents 90 to 98 percent of the total potassium in the soil. This “fixation” can occur after fertilizer applications and render fertilizer K less available. Best of all, this fixation can be reversed. This is because there is an equilibrium that exists among non-exchangeable and soil solution potassium. Therefore, as Bermuda grass removes K+ from the soil solution, more K+ is released from the exchangeable components. As these two components are reduced, non-exchangeable “fixed” potassium is released.

During periods of rapid uptake by the growing Bermuda grass, the release of non-exchangeable potassium can be insufficient to meet the Bermuda grass needs and limits growth.

If soil test potassium is below sufficient levels, potassium fertilizers should be applied. Fertilizer potassium usually is supplied as potassium chloride, but you might utilize other potassium fertilizers like, potassium sulfate or Sul-Po-Mag or K-Mag.

Watch your soil test fertilizer needs closely. Northwest and north central Oklahoma has not been receiving normal rainfall in recent years, and as a result, soil test values have been a little higher than what we would normally expect.

That soil test may indicate your fertilizer needs would be less, but in some cases, producers have been under fertilizing their Bermuda grass pastures and hay fields, and they would actually need more potassium.

Your Oklahoma Cooperative Extension Service ag educator can help you with your fertilizer needs and also assist with your soil test recommendations. Stop by for a discussion about your potassium needs.

Gribble is Oklahoma Cooperative Extension Service northwest area agronomist.