Dan Wiersma, Livestock Information Manager, DuPont Pioneer
Dan Wiersma, Livestock Information Manager, DuPont Pioneer

Good forage crops place a high demand for potassium on the soil’s nutrient supplying power. Potassium is critical for growing high yield, high quality alfalfa, helping maximize animal performance and profitability.

1. What is the role of potassium in alfalfa growth?

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 help minimize susceptibility to disease.

2. How is potassium available to alfalfa in the soil?

Three types of potassium exist in soil. The first is found in soil minerals, and makes up 90-98% 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 1-10% of the soil potassium. The third is the exchangeable, or plant-available, potassium, making up 1-2% of the soil potassium. It is found either in the soil solution or as part of the cation exchange.

3. How should potassium be managed 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 high in potassium content and protein level. With bud-stage harvest practices, the high-quality feed is removing up to 25% 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 should be added before the critical fall-growth period to enhance storage of soluble carbohydrates in the roots. 

4. What is the optimal level of potassium for alfalfa stands?

Maintain alfalfa forage potassium concentration between 2- 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%, alfalfa plants are much more susceptible to winter injury. A simple forage test will provide this insight.

5. How should a potassium program be managed overall?

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 for field potassium removal rates.