Phosphorus also has the undesirable effect of enhancing the growth of broadleaf weeds, however. Because potassium is relatively immobile in the soil, it should be applied at seeding and incorporated into the soil. For established stands, apply K as a topdress treatment in the fall. Most soils in northern Idaho contain sufficient amounts of K for maximum plant growth and yields. Consequently K fertilizer will rarely be needed on fields in conservation programs in Idaho.
Potassium fertilizer needs based on a soil test.
Sulfur is second only to N in importance for grass production. Without adequate S, the grass will appear light green to yellowish-green in color. This is similar to the appearance of plants having a shortage of N. Plants require S to use N efficiently. Since the legumes in the mixture will symbiotically fix N for their own use as well as provide N for the use of the grasses in the mixture, the S fertility will be the most limiting factor. Since S is mobile in soils, it is subject to leaching.