Good forage crops place a high demand for soil potassium, a nutrient critical for growing high-yield, high-quality alfalfa. Dan Wiersma, DuPont Pioneer, livestock information manager, offers some suggestions on managing potassium levels in alfalfa.
Why Potassium is Important
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. Potassium influences photosynthesis, cell division, carbohydrate production, protein synthesis, root development and tolerance to temperature extremes. It also can 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.
In recent years, the price of potash (K2O) has risen and is more volatile. During extremely high potash prices and challenging grain or milk prices, growers often pull back on applying full rates of potash.
Soil Availability of Potassium
Three types of potassium exist in soil. The first is found in soil minerals and makes up 90% to 98% of soil potassium. It's mostly 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% of soil potassium. The third is the exchangeable, or plant-available, potassium, making up 1% to 2% of the soil potassium. It's 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 lowers oxygen levels, which decreases plant respiration, reducing potassium uptake. In clay soils, calcium and magnesium can compete with potassium for uptake.
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, growers get a high quality but are removing up to 25% more potassium from the field than at early- to mid-flower stage.
New seedings: It's 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: Alfalfa needs the greatest amount of potassium as it prepares for winter. To boost winterhardiness, the crop needs a good supply of potassium before the critical fall-growth period.
Applications are best made prior to the last six weeks of the growing season. It's also convenient to apply potassium following the first hay cutting. Avoid applying high levels of potassium in the spring prior to first cut. This can lead to excessive potassium concentration in first-growth alfalfa and can cause milk fever in dairy cattle.
When high rates are needed to boost soil fertility, splitting into two or more applications can help avoid salt injury and luxury consumption beyond alfalfa's nutritional requirement. Multiple applications ensure potassium availability during the most critical growth periods.
Maintain alfalfa potassium concentration between 2% and 3% for maximum yield potential and winterhardiness. The very highest-yielding alfalfa crops have potassium concentrations as high as 3%. When potassium concentration drops below 2%, 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.
Many alfalfa growers experience issues that result from insufficient potassium fertility. Maintaining very high yields requires optimum to high soil-test levels of potassium.
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