Disclosed is a stable fertilizer composition including: a sulfur-containing compound; and a reaction product of a potassium-containing compound and a humectant selected from the group consisting of polyhydric alcohols, esters of polyhydric alcohols, polyalkylene glycols, and hydroxylated carboxylic acids. One version of the fertilizer composition also includes a nitrogen-containing compound; a phosphorous-containing compound; and at least one more additional macronutrient or micronutrient. The macronutrient may be selected from the group consisting of calcium and magnesium, and the micronutrient may be selected from the group consisting of zinc, manganese, iron, copper, calcium, magnesium, cobalt, boron, nickel, and molybdenum. The fertilizer composition may be essentially free of chelated metals and chelated metal salts. Methods of using a solution of the fertilizer composition for supplying plant nutrients to a plant are also disclosed.