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20 Sodium Sodium and potassium, being adjacent elements in Group 1 of the Periodic Table, have similar chemical properties. In the biology of higher organisms, however, these two elements have very different roles and are treated very differently by mechanisms involved in short- and long-range transport. | CRC_DK2972_Ch020.qxd 6 30 2006 3 37 PM Page 569 20 Sodium John Gorham Tottori University Tottori Japan University of Wales Bangor United Kingdom CONTENTS 20.1 Sodium in Soils and Water.569 20.1.1 Salinity .570 20.1.2 Sodicity.570 20.2 Sodium as an Essential Element.571 20.3 Beneficial Effects.571 20.3.1 Growth Stimulation.571 20.3.2 Interaction with Other Nutrients.572 20.4 Sodium in Fertilizers.573 20.5 Sodium Metabolism in Plants.573 20.5.1 Effects on C4 Species .573 20.5.2 Toxicity of Sodium.573 20.6 Intracellular and Intercellular Compartmentation.574 20.7 Sodium in Various Plant Species .574 References.575 20.1 SODIUM IN SOILS AND WATER Sodium and potassium being adjacent elements in Group 1 of the Periodic Table have similar chemical properties. In the biology of higher organisms however these two elements have very different roles and are treated very differently by mechanisms involved in short- and long-range transport. Estimates of the percentages of sodium and potassium in the Earth s crust vary between 2.5 and 3 by weight with slightly more sodium than potassium 1 and these concentrations are similar to the percentages of calcium and magnesium. Much of the sodium is in seawater to the extent of 30.6 by weight compared with only 1.1 for potassium and 1.2 for calcium. Chloride although present at only 0.05 in the Earth s crust makes up 55 of the mass of seawater salts. For humans and most animals physiological solutions are dominated by sodium around 0.8 w v compared with about 0.02 for potassium calcium and magnesium and chloride 0.9 and both elements are essential for animals. Thus when we think of sodium we think first of common salt sodium chloride. In soils the situation is more complex than in bulk solutions and concentrations of cations as experienced by the plant root are influenced by ion exchange diffusion and mass-flow processes. The osmotic effects of excessive salts are also influenced by the exact amounts and proportions of anions and .
