Potassium Fertilizer for Alfalfa - - - how much, what source and when?

Alfalfa requires more potassium (K) than any other nutrient. Because phosphorus (P) soil test levels are often in a range where additional topdress P fertilizer is not needed, it is often the case where we see K as the only component of an alfalfa fertilizer program (with the possible exception of boron). Potassium influences yield, forage quality, disease resistance, and overwinter survival. It is important to apply enough K to maintain optimum economic yields, however, applying too much can be detrimental from both an economic and forage quality standpoint. Recently, an extensive alfalfa K fertilizer study was completed by Keith Kelling, UW Extension Soils Specialist, that examined fertilizer rates, timing, source, and forage quality. The following is a discussion highlighting some of the results.

Rates

Potassium fertilizer was applied annually to alfalfa stands at six different rates from 0 to 350 lbs. K2O per acre on soils that tested from less than 70 to greater than 150 ppm K. Alfalfa yields were measured for three years (1994-96). In 1996, Verticillium wilt became a problem and reduced yields of all treatments significantly.

The results from this research trial confirmed that there is no reason to build soil test K levels beyond about 120 ppm (the top end of the optimum range). Beyond 150 ppm K, there was little or no benefit from topdressing any K fertilizer. Once soils reach the high end of the optimum or low end of the high range, annual applications of about 200 pounds of K per acre is all that is needed to maintain soil tests and maximize yields.

Timing of K fertilizer on alfalfa

In this component of the experiment, 350 pounds of K2O per acre was applied to field plots at greenup in early spring, after 1st cut, after 3rd cut, or split after 1st and 3rd cut. The 3-year average dry matter yields by cutting are presented in Table 1.

Application of K fertilizer at greenup in the early spring was never better than applying topdress K after 1st cutting. In fact, it was sometimes worse. Driving over alfalfa stands during this time of year can result in crown damage to the plant and subsequent disease infection. Beyond not applying K fertilizer in early spring, there appears to be no significant yield advantage to any one of the other application times. However, when applying less than 500 pounds of actual fertilizer (as would be the case in most situations), there certainly is no need to invest the added time and expense of making two separate applications.

K Source

The recommendation for alfalfa K fertilizer source is relatively straight forward. From an agronomic standpoint, there is no difference in performance between K sources when equal amounts of the nutrient are applied. Thus, cost per unit of nutrient is the driving force behind a K fertilizer buying decision for alfalfa. Usually, potassium chloride (0-0-60) is the cheapest and most widely available source. Sometimes KCl is criticized for having a high salt content, however, at amounts normally applied to alfalfa (less than 500 pounds of actual fertilizer) research confirms no negative impact to either the plant or soil organisms. Producers should also be cautious of liquid forms of K fertilizers. These can be extremely expensive on a cost per unit basis and are no more "available" than dry forms.

Forage quality

More K fertilizer does not necessarily mean more yield when soil tests rise into the high and excessively levels. However, more K fertilizer or excessive soil K levels can translate into luxury consumption of K by the plant and higher K plant tissue levels (Table 2 - Arlington data, avg. of two years).

High potassium forages are largely responsible for metabolic disorders like milk fever in dairy cattle.

For more information contact Mike Rankin

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