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.
Micronutrients are key players in many of the processes important for plant growth.
Boron deficiency can be overcome by periodically applying boron when topdressing alfalfa. Even rarer is zinc deficiency, sometimes observed on corn, particularly when phosphorus levels become excessive from overapplication of phosphorus fertilizer. Symptoms on corn appear as a rough stripping on either side of the midrib of the corn leaf. Zinc deficiency can be corrected with a periodic broadcast application of zinc or by applying zinc in the starter fertilizer. Routine application of zinc is not recommended.