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The essential elements of plant food are ten in number. Of these carbon and oxygen are obtained by the plant from the air, and hydrogen from the water. Sulphur, calcium, iron and magnesium are required by plants in small quantities and are not likely to be deficient in soils. The three remaining ones--nitrogen, phosphorus, and potassium--are likely to be present in soils in smaller quantities and are used by plants in larger amounts than any of the other elements taken from the soil. In addition to these ten elements already named, five other elements, viz., silicon, aluminum, sodium, chlorine, and manganese are commonly present in plants, but these latter five are generally not considered to be essential to plant growth. In the soils of arid regions nitrogen is very low, while phosphorus and potassium are usually present in larger quantities.