Yield and Quality Response of Last-Year Alfalfa to Topdressed K Fertilizer.

Potassium (K) fertilizer prices are higher now than in the past, and thus have a greater effect on economic returns for alfalfa (Medicago sativa L.) growers. Potassium supports plant stress tolerance and plays a critical role in alfalfa yield by moving sugars from shoots to roots; however, current K recommendations in Minnesota do not take into account the age of the alfalfa stand. Last-year K fertilizer recommendations for alfalfa in its third or fourth full year of production were tested on ten farms with medium soil-test K (STK) levels (80-120 mg kg^-1) in 2008 and 2009. Treatments were 0, 22, 56, 112, and 224 kg K₂O ha^-1 as KCl applied in the early spring at three farms and after the first harvest at seven farms. Alfalfa yield was not improved by topdressed potash and forage quality changes were slight. When potash rate increased from 0 to 224 kg K₂O ha^-1, neutral detergent fiber digestibility increased from 48.0 to 50.1% when applied in the early spring, and from 47.1 to 49.2% when applied after first harvest. Acid detergent fiber also increased with K rate from 26.2 to 27.2% with early spring application and from 27.2 to 27.8% with potash applied after the first harvest. However, relative feed value (mean= 196) and relative feed quality (mean= 205) were not changed by potash additions. Fertilizer K availability was demonstrated by increased alfalfa K uptake as potash rate increased. End of season plant population was not related to initial (pre-treatment) STK or to STK the following spring. Over 10 site-years in Minnesota, there was no economic benefit to applying potash in the last year of alfalfa production when topsoil exchangeable K was greater than 80 mg kg^-1 at the beginning of that growing season.