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Response of Two Alfalfas (Medicago sativa L. and M. falcata L.) to Time and Rate of Potassium Application in the Subarctic¹

Annual topdressings of K at 0, 42, and 166 pounds per acre, applied either in spring or mid-summer, were compared with 2 alfalfas—Medicago sativa (var. ‘Vernal’) and M. falcata—for 3 seasons in Alaska's Tanana Valley.

Stands of both alfalfas thinned rapidly during the study but stands and stand losses showed virtually no relationship to K treatments. Winterkill accounted for most of the reduction of stands and Vernal winterkilled more than M. falcata.

K treatments resulted in large differences in yield. Both rates of K topdressing resulted in higher alfalfa yields than no topdressing; and 166 pounds of K per acre were superior to 42 pounds per acre.

Crude protein content of forage was seldom influenced to a significant extent by K applications. Where differences existed, crude protein content was lowest with the highest rate of K application. M. falcata contained higher levels of crude protein than Vernal.

K content of forage, virtually always lowest where no K was topdressed, was usually higher in alfalfa topdressed with 166 pounds of K per acre than at the lower rate. K content was lower in Vernal than in M. falcata.

Highest percentage recovery in forage of applied K occurred with no topdressing. There, recovery exceeded the amount of K applied at planting. K recovery with the higher rate of topdressing was poorer than with the lower rate. M. falcata was superior to Vernal in K recovery.

Amounts of available K in the soil did not differ among treatments until the third harvest season, when K levels were higher in most plots that had received the higher rate of K topdressing.

² Research agronomist, Alaska Agricultural Experiment Station, Palmer, Alaska, and Crops Research Division, ARS, USDA; and formerly agronomist (now retired), Alaska Agricultural Experiment Station, College, Alaska. The authors acknowledge and thank Paul F. Martin, soil scientist, for soil analyses, Margaret H. Blom, chemist, for Kjeldahl N determinations, and R. L. Taylor, research agronomist, for advice in statistical analyses.

Received for publication March 16, 1966.