A recently completed study by UW Soil Scientists once again underscores the importance of managing soil test K levels on fields where alfalfa is grown. In a summary format, here are some of the results of this study:

Finding #1: Alfalfa yields plateau at about 120 ppm soil test K. At this level, approximately 200 lbs. of K need to be applied annually as topdress to maintain soil test levels.

Finding #2: Little yield response to topdress K was seen when soil test K exceeded 150 ppm. Below 150 ppm K, some topdress K was always needed to obtain maximum yields.

Finding #3: At soil tests above 120 ppm K, adding more than 160 lb. K₂O/acre/year resulted in forage K levels above 3.5% with no response in yield. Thus, over applications of K₂O result in the double whammy of a high K forage and no additional yield.

Finding #4: For the first time in an Arlington research trial, a small but significant effect to applied sulfur was reported (these fields had not had previous manure applications prior to establishment).

Finding #5: There was no advantage to splitting applications of topdress K after 1^st and 3^rd cut versus applying all topdress K after 1^st cut unless more than 400 to 500 lbs. of actual fertilizer are applied in a single application. There was a negative impact on yield when fertilizer was applied at greenup in the spring.