Sulfur deficiency is often mistaken as nitrogen deficiency, says Judith Hooper, president, Pima Research Co. of Tucson, Ariz. Sulfur chemistry is Hooper's specialty.

“Sulfur is an incredible element. It's an absolute requirement for every living system. If your plants don't have it, they die,” Hooper says.

Although sulfur plays many roles in plant growth and health, its job in protein synthesis is especially vital. If we remember our Biology 101, proteins are the building blocks of the cell and they provide that cell with much of its structure. To create a protein requires up to 20 amino acids, two of which are sulfur dependent: cysteine and methionine. Those two amino acids are found in nearly every protein. If sulfur is not available, cysteine and methionine disappear, proteins are not made and new cells with good cell walls do not get produced. The plant suffers and can die.

When raising a crop, Hooper says it doesn't matter how much nitrogen you apply if there is not enough sulfur to make the two amino acids. Because the plant can't adequately build proteins, “you end up with a backlog of nitrogen in the plant,” she says.

Scientists are finding that sulfur is critical for a plant to achieve optimal health, Hooper says. Sulfur is important to the immune system of the plant, actually helping it reduce its reactions to stress. Sulfur improves a plant's resistance to dehydration and disease, and it helps it remove toxic metals. Finally, like calcium, sulfur is important for creating strong cell walls and that ultimately translates into quality fruit.