Certain elements are essential for plant growth. If these elements are absent from the soil, or present in insufficient quantities, plants will exhibit limited or stunted growth, reduced yields, and sometimes deficiency symptoms. By the time deficiencies appear, plant growth and yield are usually irretrievably retarded. As well as reduced yields, food and fodder crops may also, if the soil is deficient, lack certain elements essential for animal and human health. Thus people and grazing animals may develop deficiency diseases through eating deficient crops.
Most higher green plants require the following elements: carbon, hydrogen, oxygen, nitrogen, potassium, phosphorus, sulphur, calcium, iron, magnesium, boron, manganese, zinc, copper, chlorine and molybdenum. Those elements must be present in the soil in sufficient quantities (though not in excess, as this may prove lethal to plants). The necessary quantities vary: copper, molybdenum, manganese, zinc, boron, iron, and chlorine are required in trace quantities only. A deficiency of any one of the 16 essential elements results in stunted growth and reduced yield. The elements whose lack is most likely to have a limiting effect on growth are nitrogen, phosphorus and potassium. Some generalizations can be made about the deficiency symptoms of these three main elements. When nitrogen moves out of the older, hence lower, leaves of a plant, the deficiency is generally characterized by yellowing of these leaves. Phosphorus deficiency is characterized by a purpling of the stem, leaf, or veins on the underside of the leaves. Potassium deficiency results in burn or scorch of the margin of the leaves, particularly the older, lower leaves.