Since potassium is mobile in the plant, the symptoms appear on the older leaves first. Another indication of potassium deficiency is reduced straw or stalk strength, which results in lodging problems, reduced disease resistance, and reduced winter hardiness of perennial or winter annual crops. The secondary nutrients, calcium, magnesium, and sulfur play a variety of roles in plants. Calcium deficiency is rare among agronomic crops under Pennsylvania conditions. When a soil is properly limed to maintain an optimum pH level, calcium is usually adequate for agronomic crops.
Magnesium deficiency is characterized by white stripes between the leaf veins.
Magnesium is best supplied by a limestone that contains this nutrient. Sulfur-deficient plants have a general yellowing and are very spindly. Symptoms of sulfur deficiency are similar to those of mild nitrogen deficiency, except that they appear sooner in new growth than in old growth, since sulfur is not mobile in the plant. Under Pennsylvania conditions, sulfur deficiency is not common. Rainfall supplies significant amounts of sulfur, and it is recycled efficiently through the manure applications that much of the cropland in the state receives.