Soils: February 2004

All too often, the importance of potassium is overlooked in bermudagrass forage production systems. Potassium is second only to nitrogen in plant tissue concentration. It has often been stated that there is no need to apply potassium fertilizer in Oklahoma due to the soils' naturally high concentration of potassium. This statement may have been true thirty years ago, but it now appears that potassium levels are declining. This is due to lack of replacing soil potassium removed by the crop with fertilizer.

The role of potassium in the plant is indirect, meaning that it does not make up any plant part. Instead, it acts as a catalyst regulating enzymatic processes in the plant that are necessary for plant growth. Potassium is important for a plant's ability to withstand extreme cold and hot temperatures, drought and pests. Another responsibility in the plant is the regulation of water use. Potassium affects water transport in the plant, maintains cell pressure and regulates the opening and closing of stomates (small openings found on the leaf responsible for cooling and taking in carbon dioxide for photosynthesis).

Symptoms of potassium deficiency include yellowing of the lower leaves and, in severe cases, leaf-tip dieback. Once symptoms are present, the plant's ability to withstand stress conditions, such as high heat, drought and pests, is diminished. Factors that cause soil potassium deficiency range from leaching from sandy soils with low cation exchange capacity (CEC — additional information regarding CEC will be in a future issue) to fields under irrigation high in sodium. There are three types of potassium found in the soil. The first is found in soil minerals. This type of potassium makes up more than 90 to 98 percent of soil potassium. It is tightly bound and most is unavailable for plant uptake. The second is nonexchangeable potassium. Nonexchangeable potassium acts as a reserve to replenish potassium taken up or lost from the soil solution. It makes up approximately 1 to 10 percent of the soil potassium. The last type is the exchangeable or plant available potassium at 1 to 2 percent. It is found either in the soil solution or as part of the cation exchange.

Soil type and environmental conditions have an effect on the amount of potassium available for plant use. Potassium availability is highest under warm, moist conditions in soils that are well aerated with a neutral or slightly acidic pH. Too much water in the soil profile will lower oxygen levels, which in turn decreases plant respiration reducing potassium uptake. In clay soils, potassium availability can be affected due to its competition with calcium and magnesium for sites on the cation exchange. Both calcium and magnesium can easily displace potassium from the cation exchange.

The amount of potassium needed is dependent upon the level of management. For instance, there is a high demand for potassium in a hayed bermudagrass field compared to one that is grazed. This is due to the amount of potassium removed in the hay. About 50 pounds of potassium is in a ton of bermudagrass hay. Determine potassium fertilizer needs by establishing production goals for the site. Then establish a realistic yield goal and, as always, soil test to determine current potassium levels. Then apply the recommended amount of potassium fertilizer to meet the yield goal.

Remember that soil type and CEC determine the amount of potassium that is available for plant uptake. It is difficult to build soil potassium levels especially in soils with a high percentage of clay. Clay provides hiding places for potassium to bind and become unavailable for plant uptake. Also, a plant can take up more potassium than it needs with no additional yield. This is called "luxury consumption." Therefore, it is wise to apply only the amount needed to meet the yield goal for the growing season. Of course, there are exceptions to this rule. Bermudagrass hay pastures under irrigation may require high amounts of potassium. It should not all be applied in the spring — instead spoon-feed potassium after each cutting.

Potassium is extremely important in many ways to the productivity of bermudagrass. It not only performs the important physiological functions as discussed above, but it improves nitrogen use efficiency. As we know, nitrogen is directly related to yield. However, if potassium is the limiting nutrient, forage production will decrease. It is like having a full tank of gas in your truck, but having a flat tire. In both cases, you will not get too far.