The roots of green plants absorb a large number of elements from the
soil. Some of these elements are absolutely necessary for normal growth
and development of the plant. These elements, said to be macronutrients,
are potassium, calcium, magnesium, iron, carbon, hydrogen, oxygen,
nitrogen, sulphur and phosphorus. Macronutrients must be present in
plant tissues in concentration of at least one milligram per gram of dry
matter. There are other elements which are specifically required by the
plant in very minute quantities, and these elements are known as
micronutrients. Manganese, copper, molybdenum, zinc and boron are
essential micronutrients.