Alfalfa requires potassium in the greatest amount to facilitate its maximum growth potential. For a healthy stand and optimum hay yields, alfalfa absorbs potassium more than any other nutrient from the soil.

Being a luxury consumer of potassium, alfalfa takes up more potassium than needed when potassium is available at the root zone. However, there are feedback mechanisms that maintain a balance between demand and uptake.

During the growing season, about 100% of the aboveground portion of alfalfa is cut and baled several times. This results in substantial amounts of potassium removal from the soil each year.

For example, cutting 1 ton of alfalfa removes about 65 pounds potassium per acre from the soil, which implies that in a single growing season, a fairly obtainable 3 tons of alfalfa removes about 195 pounds potassium per acre.

This may lead to potassium depletion in soils. Therefore, when soil potassium levels are limiting, alfalfa’s potential to maximize growth and yield decreases long before any visual symptoms of potassium deficiency are observed, such as stunted growth, white spots on leaf margins and defoliation.

How alfalfa utilizes potassium

Generally, alfalfa seeds begin to germinate in two to three days after planting. A germinating alfalfa seed gets its nutrients from the carbohydrates, proteins and fats stored in the cotyledons until the true leaves begin to photosynthesize.

After germination, the young roots penetrate the surrounding soil to search for nutrients and water. As the roots move through the soil, nutrients adjacent to the roots are absorbed for plant growth.

Potassium absorption by alfalfa’s roots occurs from the soil solution, as an ion (K+) and the rate of uptake is closely related to the concentration of the soil solution.

Consequently, the roots tend to grow and space themselves uniformly in the soil to allow potassium to diffuse through the soil to the roots and then transport to the tissues of the growing plant. This helps strengthen the photosynthetic system of the plant to boost its physiological and biochemical processes for increasing growth and development.

As a result of growth increase, forage yield increases. Since alfalfa roots penetrate deep depths in the soil, it is important to meet the soil’s moisture requirement to increase the mobility of potassium.

Factors that affect potassium uptake in alfalfa

Potassium fertilization makes significant contribution to alfalfa’s productivity through the improvement of plant metabolism, stress response and regulation of water loss. Several factors that affect potassium uptake in alfalfa include:

Soil factors

• Moisture: Moisture is required to supply potassium to plant roots for uptake. In water-deficit soils, the path in which K+ moves through becomes more tortuous, which hinders diffusion of K+ from the soil to the roots. Increasing soil moisture levels to about 30% facilitates potassium diffusion, resulting in high potassium uptake by alfalfa.
  
  
• Temperature: Soil temperature greatly affects potassium uptake via changes in root activity. Generally, low temperature slows down plant processes, rate of potassium uptake and plant growth by changing the structure of water and aqueous solution in the soil. Supplying high levels of potassium is a practical approach to offset issues of potassium uptake related to low temperatures.
  
  
• Aeration: Normal root function is dependent on adequate supply of oxygen from the soil, which is required for potassium uptake through plant root respiration. Under waterlogged or compacted soils, reduced soil air space reduces oxygen supply and restricts root growth and potassium uptake by alfalfa.
  
  
• pH: In low-pH soils, high amounts of aluminum (Al3+) and manganese (Mn2+) ions create unfavorable root environment for potassium uptake due to competition involving K+, calcium (Ca2+) and magnesium (Mg2+) ions for plant uptake. Increasing soil pH results in less exchangeable Al3+ and Mn2+ to reduce competition and allows K+ to compete more effectively against Ca2+ and Mg2+ for vacant exchange sites. This leads to uptake of high quantities of potassium by alfalfa for satisfactory growth because uptake of potassium is more dependent on potassium concentration relative to Ca2+ and Mg2+ than the total quantity of potassium present in the soils.

Plant factors

• Cultivar differences: Variations among alfalfa cultivars relate to the genetics of the plant. The difference arises through plant breeding, which is associated with the type of root system, root density and metabolic activities, which influences uptake of potassium by alfalfa. While improved genetics of alfalfa results in new cultivars with novel traits of higher yield potential, it also optimizes the efficiency of potassium uptake by the plant to maximize its ability for high growth and yields.
  
  
• Plant population and yield targets: As the population of the alfalfa plant increases, demand for soil potassium increases to attain high growth and yields. Thus, uptake of potassium increases because of competition among plants for uptake. This, therefore, depletes potassium in the soil for subsequent uptake, especially in soils with low potassium reserves.
  
  
• Plant age: Younger plants have higher metabolic activities than older plants. Consequently, plants in early growth stage require higher amounts of potassium than advanced growth stage. This implies that management practices, such as cutting schedules, may have influence on potassium uptake and growth in alfalfa.

Potassium and cutting schedules

Because of the trade-off between forage yield and quality of alfalfa, the time of cut is crucial for maximizing alfalfa’s yield, quality and stand longevity.

Consequently, cutting times can influence alfalfa’s uptake of potassium. Generally, frequently cutting alfalfa at an early time lowers the plant’s root reserve and vigor prior to the next cut.

Therefore, high quantities of potassium are absorbed by the plant to boost the plant’s metabolic processes and facilitate the translocation of photosynthates in its root and crown to accelerate growth until future cuts.

In contrast, in frequently late cutting times, the plant’s root reserve is reduced moderately with a minimal decline in its vigor.

As a result, moderate quantities of potassium are absorbed by the plants to boost regrowth rate for satisfactory plant growth until the next cut. We observed these traits in our recent study in Wyoming (Table 1).

The uptake of potassium by alfalfa has great impact on alfalfa’s productivity. When we regressed potassium uptake against forage yield, we saw a direct positive relationship between potassium uptake and alfalfa forage yield (Figure 1).

These observations suggest that cutting schedules play a major role in determining the amount of potassium required by alfalfa to optimize growth and yields.

Alfalfa fertilized with high amounts of potassium at early cutting and with moderate amounts of potassium at late cutting schedules resulted in high potassium uptake by alfalfa (Table 1). This trait eventually produced high forage yields in respective potassium application rates (Table 2).

This indicates that delaying cut for a few days in alfalfa reduces the requirement of potassium for high forage yields and vice versa.

The study is ongoing; however, based on two years of observations, we encourage alfalfa growers to consider cutting schedules when making decisions on potassium fertilization for sustaining high yield in the longer term.