The Ionic Environment of Plant Roots

Abstract

In the previous chapter the solutes contained in the water that fills the soil pore space were neglected, except for their effect on the soil moisture potential. Soil water, even in nonsaline soils is, however, not pure water but an electrolyte solution. This solution is dilute: the sum of the concentrations of all the ions is usually in the molar range 10⁻³ to 10⁻² whereas the concentration of the most abundant ions are commonly of the order of 10⁻⁴ to 10⁻³ molar. The composition of the solution is very different not only among soils, but also varies with time and depth of sampling within any one profile. The composition of the soil solution of a wide range of soils is shown in table 3.1. Apart from carbon, oxygen and hydrogen, which occur in the plant environment as carbon dioxide and water respectively, thirteen elements are known to be essential for higher plants (nitrogen, phosphorus, sulphur, potassium, calcium, magnesium, iron, manganese, zinc, copper, boron, molybdenum and chlorine). All these elements occur in ionic form in the soil solution. Among those listed in table 3.1, calcium, magnesium and potassium are cations, whereas chloride, sulphate and phosphate are anions; nitrogen occurs in the soil solution either as the ammonium cation or as the nitrate anion. The remaining six elements not listed in the table are usually present and are required by plants only in trace quantities.