tailieunhanh - Handbook of Plant Nutrition - chapter 20

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. | 6 30 2006 3 37 PM Page 569 20 Sodium John Gorham Tottori University Tottori Japan University of Wales Bangor United Kingdom CONTENTS Sodium in Soils and Salinity .570 Sodium as an Essential Beneficial Growth Interaction with Other Sodium in Sodium Metabolism in Effects on C4 Species .573 Toxicity of Intracellular and Intercellular Sodium in Various Plant Species .574 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 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 by weight compared with only for potassium and for calcium. Chloride although present at only 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 w v compared with about for potassium calcium and magnesium and chloride 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 .