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 cations.

Some sodium occurs in most soils, but in temperate climates, the concentrations are often similar to, or lower than, those of potassium. Excessive amounts of sodium may be present in the soil in arid and semi-arid areas, and where evapotranspiration is similar to or greater than precipitation. The excess may be in the form of high concentrations of sodium ions in solution, usually accompanied by chloride and sulfate (saline soils), or where sodium is the main cation associated with cation-exchange sites (sodic soils). There is no absolute division of salt-affected soils into these two categories, saline or sodic, as there is a range from purely saline to purely sodic, with most saltaffected soils falling somewhere between the two extremes. The FAO estimated that in 2000, 3.1% of the Earth's land area was affected by salinity and a further 3.4% had sodic soils (2). These figures include 19.5% of irrigated land and 2.1% of land under dry-land agriculture. Detailed properties of these soils are presented in a number of monographs (3-9).

Salinity

A widely accepted definition of a saline soil is one that gives a saturated paste extract with an electrical conductivity (ECe) of >4 dS m^-1 (mmho cm^-1). Seawater is about 55 dS m^-1. These saline soils will also have an exchangeable sodium percentage (ESP) of <15 and a pH of <8.5. Saline soils are a problem for most plants because of the high concentrations of soluble salts in the soil solution. Soil salinity usually involves other ions in addition to those of sodium and chloride, particularly calcium, magnesium, and sulfate. The proportions of these ions depend on the chemistry and hydrology of the soil, but all saline soils have high concentrations of salts that may be harmful in three ways. First, the high concentrations result not only in higher electrical conductivity, but also in high osmotic pressures (more negative osmotic potentials). This action makes it more difficult for plants to establish a continuous gradient of water potential between the soil solution and the atmosphere- the driving force for transpiration and water uptake by osmosis. Plants must make their own tissue solutions more concentrated (higher osmotic pressure) in order to draw water into their tissues. This response is called osmotic adjustment, and in a strict sense, it refers to an increase in solutes on a dry weight basis (a higher osmotic pressure can also be achieved to some extent by a reduction in the amount of water). The simplest and energetically the cheapest way to achieve osmotic adjustment is by the accumulation of inorganic ions (10). This action can lead to the second problem-the toxicity of high concentrations of inorganic ions in plant tissues (11). Toxicity, in this context, can result from direct interference with cellular metabolism or from an osmotic imbalance caused by the accumulation of salts in the leaf apoplast, known as the Oertli effect (12,13). The third problem is that high concentrations of salts can inhibit the uptake of other nutrients such as potassium and nitrate.

Sodicity

In contrast, soils with little soluble sodium, and hence a low ECe (<4 dS m^-1), but with a substantial proportion of the exchangeable cations in the form of sodium (ESP>15) and a pH of >8.5, are called sodic soils. In purely sodic soils, a substantial osmotic problem does not occur, since the concentrations of free ions in the soil solution are low. Nutrition is a problem because of the replacement of nutrient cations (K⁺, Ca²⁺, and Mg²⁺) at ion-exchange sites in the soil by sodium (Na⁺) and because of the high pH. Sodic soils have poor physical structure and may be impermeable to water and to plant roots, so that there are often secondary problems such as waterlogging and hypoxia.

Primary salinization is the result of geological processes such as the deposition of salt from drying lakes and seas. The large areas of salt-affected soil in parts of Hungary, Australia, and the western United States of America are the result of such natural events. Secondary salinization refers to the impact of man, mainly resulting from unsustainable irrigation for agriculture and rising water tables. Secondary salinization has played a role in the decline of several civilizations. The Sumerian civilization in Mesopotamia is probably the best known. This civilization was initially based on irrigated wheat farming, but lack of adequate drainage and excessive use of irrigation water with an appreciable salt content led to accumulation of salts in the irrigated lands. Wheat (Triticum aestivum L.) was replaced gradually by the more tolerant cereal barley (Hordeum vulgare L.), until it was abandoned completely in about 1700 BC (6). Eventually, the salinity reached levels at which not even barley would grow. Clearly, this presentation is a simplification of a complex series of events, but the pattern of irrigation without adequate drainage or control of salt fluxes in the soil has been repeated in other civilizations such as the Hohokam of the Sonoran Desert and the Indus civilization of Pakistan. The mistakes of ancient civilizations have, unfortunately, been repeated in more modern times. Examples are the vast irrigation systems in the Indian subcontinent and central Asia. In the former case, remedial civil engineering is tackling the problem (6). In the former Soviet Union, largescale irrigation schemes built in the 1950s abstracted water from the Amu Darya and Syr Darya rivers for the cultivation of cotton (Gossypium hirsutum L.) and other crops. These rivers flow into the Aral Sea, and with the reduction in river flows, the level of the sea dropped by more than 10 m; and its area decreased by over 40% in the latter half of the 20th century and is still decreasing. Even the United States of America, with all of its technological and financial resources, is not immune to the impact of secondary salinization, as in the San Joachim valley and the Salton Sea.

Secondary salinization is most severe in arid and semi-arid regions, where potential evapotranspiration rates are high, as in parts of the United States, the Indian subcontinent, Australia, the Middle East, and South America.