In recent years, the price of potash (K2O) has risen to high levels and is characterized by more volatility. During the years of extremely high potash prices and challenging grain or milk prices, growers have pulled back on applying full rates of potash.
Role of potassium in alfalfa and corn
Alfalfa plants require potassium for proper growth and development. Potassium plays an indirect role by acting as a catalyst, which regulates enzymatic processes in the plant. Photosynthesis, cell division, carbohydrate production, protein synthesis, root development and tolerance to temperature extremes are all influenced by potassium. It can also improve the crop's tolerance to drought by regulating water use and can minimize susceptibility to disease.
In alfalfa, potassium is responsible for helping the plant withstand extreme cold temperatures during winter. It also helps with the critical processes of nitrogen fixation and nitrogen utilization. For alfalfa production, proper potassium fertility should be teamed with a soil pH management program to take full advantage of applied nutrients.
Adequate and balanced nutrition maintains a plant’s vigor and reduces vulnerability to stress. When in proper balance with nitrogen, phosphorus, sulfur and other micronutrients, sufficient potassium helps ensure high yields of quality forage.
Soil availability of potassium
Three types of potassium exist in soil. The first is found in soil minerals and makes up 90 to 98 percent of soil potassium. It is tightly bound and most is unavailable for plant uptake. The second is nonexchangeable potassium, which 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 third is the exchangeable, or plant-available, potassium, making up 1 to 2 percent of the soil potassium. It is found either in the soil solution or as part of the cation exchange.
Soil type and environmental conditions affect 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 lowers 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.
Managing potassium for alfalfa
For alfalfa, potassium is best applied based on yields and harvest schedule. Potassium application needs to increase with harvest frequency and in high-yield situations. Young plants are higher in potassium content and protein level. With bud-stage harvest practices, the high-quality feed is removing up to 25 percent more potassium from the field than at early- to mid-flower stage.
New seedings
It is important to build up soil-test potassium to the optimum or high range before seeding; this is the only opportunity during the life of the stand to mix nutrients through the topsoil.
Established stands
The time at which alfalfa needs the greatest amount of potassium is in preparation for winter. To boost the crop’s winterhardiness, a good supply of potassium needs to be added before the critical fall-growth period to enhance storage of soluble carbohydrates in the roots.
Applications are best made prior to the last six weeks of the growing season. Following the first hay cutting is another convenient time to apply potassium. Avoid applying high levels of potassium in the spring prior to first cut. This can lead to excessively high potassium concentration of first-growth alfalfa and can cause milk fever in dairy cattle.
When high nutrient application rates are needed to boost soil fertility, splitting the total required amount into two or more applications is recommended to avoid salt injury and luxury consumption beyond the alfalfa nutritional requirement. Multiple applications ensure potassium availability during the most critical growth periods of the season.
Maintain alfalfa potassium concentration between 2 and 3 percent for maximum yield potential and winterhardiness. The highest-yielding alfalfa crops have potassium concentrations as high as 3 percent. When potassium concentration drops below 2 percent, alfalfa plants are much more susceptible to winter injury. A simple forage test will provide this insight.
As alfalfa stands age, the response to potassium fertilization increases. Fewer plants per acre combined with less-efficient root systems means that access to potassium is more limited. To maintain maximum productivity, it is critical to continue to fertilize aging stands at a high level.
Conclusion
Many growers experience alfalfa issues that result from insufficient potassium fertility. To maintain very high yields of alfalfa requires optimum to high soil-test levels of potassium. This is best maintained through a renewed emphasis on soil testing and adjustments to field potassium removal rates. Finally, a potassium fertility program requires a balanced approach. It is important to provide all the essential nutrients in the proper amount. FG
Dan Wiersma
Livestock Information Manager
DuPont Pioneer
