The residual sodium carbonate (RSC) index of irrigation water or soil water is used to indicate the alkalinity hazard for soil. The RSC index is used to find the suitability of the water for

irrigation Irrigation (also referred to as watering of plants) is the practice of applying controlled amounts of water to land to help grow crops, landscape plants, and lawns. Irrigation has been a key aspect of agriculture for over 5,000 years and has bee ...

clay Clay is a type of fine-grained natural soil material containing clay minerals (hydrous aluminium phyllosilicates, e.g. kaolinite, ). Most pure clay minerals are white or light-coloured, but natural clays show a variety of colours from impuriti ...

soils which have a high cation exchange capacity. When dissolved sodium in comparison with dissolved calcium and magnesium is high in water, clay soil swells or undergoes dispersion which drastically reduces its infiltration capacity.Managing irrigation water quality, Oregon State University, USA
Retrieved on 2012-10-04. In the dispersed

soil structure In geotechnical engineering, soil structure describes the arrangement of the solid parts of the soil and of the Pore space in soil, pore space located between them. It is determined by how individual soil granules clump, bind together, and Soil a ...

, the

plant roots In vascular plants, the roots are the organs of a plant that are modified to provide anchorage for the plant and take in water and nutrients into the plant body, which allows plants to grow taller and faster. They are most often below the surfa ...

are unable to spread deeper into the soil due to lack of moisture. However, high RSC index water does not enhance the

osmotic pressure Osmotic pressure is the minimum pressure which needs to be applied to a Solution (chemistry), solution to prevent the inward flow of its pure solvent across a semipermeable membrane. It is also defined as the measure of the tendency of a soluti ...

to impede the off take of water by the plant roots unlike high

salinity Salinity () is the saltiness or amount of salt (chemistry), salt dissolved in a body of water, called saline water (see also soil salinity). It is usually measured in g/L or g/kg (grams of salt per liter/kilogram of water; the latter is dimensio ...

water. Clay soils irrigation with high RSC index water leads to fallow alkali soils formation. Exposed mango tree roots

RSC index formula

RSC is expressed in meq/L units. RSC should not be higher than 1 and preferably less than +0.5 for considering the water use for irrigation.US Salinity Lab Handbook 60
/ref> The formula for calculating RSC index is: * RSC index = CO₃ + CO₃− a + Mg* RSC index = HCO₃/61 + CO₃/30 – Ca/20 – Mg/12 (in case the ionic concentrations are measured in mg/L or ppm as salts) While calculating RSC index, the water quality present at the root zone of the crop should be considered which would take into account the leaching factor in the field. Calcium present in dissolved form is also influenced by the

partial pressure In a mixture of gases, each constituent gas has a partial pressure which is the notional pressure of that constituent gas as if it alone occupied the entire volume of the original mixture at the same temperature. The total pressure of an ideal g ...

of dissolved at the plants root zone in the field water.

Natural water contamination

Soda ash Sodium carbonate (also known as washing soda, soda ash, sal soda, and soda crystals) is the inorganic compound with the formula and its various hydrates. All forms are white, odourless, water-soluble salts that yield alkaline solutions in water ...

a₂CO₃can be present in natural water from the

weathering Weathering is the deterioration of rocks, soils and minerals (as well as wood and artificial materials) through contact with water, atmospheric gases, sunlight, and biological organisms. It occurs '' in situ'' (on-site, with little or no move ...

basalt Basalt (; ) is an aphanite, aphanitic (fine-grained) extrusive igneous rock formed from the rapid cooling of low-viscosity lava rich in magnesium and iron (mafic lava) exposed at or very near the planetary surface, surface of a terrestrial ...

which is an

igneous Igneous rock ( ), or magmatic rock, is one of the three main rock types, the others being sedimentary and metamorphic. Igneous rocks are formed through the cooling and solidification of magma or lava. The magma can be derived from partial ...

rock. Lime a(OH)₂can be present in natural water when rain water comes in contact with calcined minerals such as ash produced from the burning of

calcareous Calcareous () is an adjective meaning "mostly or partly composed of calcium carbonate", in other words, containing lime (mineral), lime or being chalky. The term is used in a wide variety of Science, scientific disciplines. In zoology ''Calcare ...

coal Coal is a combustible black or brownish-black sedimentary rock, formed as rock strata called coal seams. Coal is mostly carbon with variable amounts of other Chemical element, elements, chiefly hydrogen, sulfur, oxygen, and nitrogen. Coal i ...

lignite Lignite (derived from Latin ''lignum'' meaning 'wood'), often referred to as brown coal, is a soft, brown, combustible sedimentary rock formed from naturally compressed peat. It has a carbon content around 25–35% and is considered the lowest ...

in boilers.

Anthropogenic Anthropogenic ("human" + "generating") is an adjective that may refer to: * Anthropogeny, the study of the origins of humanity Anthropogenic may also refer to things that have been generated by humans, as follows: * Human impact on the enviro ...

use of soda ash also finally adds to the RSC of the river water. Where the river water and ground water are repeatedly used in the extensively irrigated river basins, the river water available in lower reaches is often rendered not useful in agriculture due to high RSC index or alkalinity. The salinity of water need not be high.

Softened water

In industrial water treatment terminology, water quality with high RSC index is synonymous with the

soft water Hard water is water that has a high mineral content (in contrast with "soft water"). Hard water is formed when water percolates through deposits of limestone, chalk or gypsum, which are largely made up of calcium and magnesium carbonates, ...

but is chemically very different from naturally soft water which has a very low ionic concentration. When calcium and magnesium salts are present in dissolved form in water, these salts precipitate on the heat transfer surfaces forming insulating hard scaling / coating which reduces the heat transfer efficiency of the heat exchangers. To avoid scaling in water cooled heat exchangers, water is treated by lime and or soda ash to remove the

water hardness Hard water is water that has a high mineral content (in contrast with "soft water"). Hard water is formed when water percolates through deposits of limestone, chalk or gypsum, which are largely made up of calcium and magnesium carbonates, bicar ...

. The following chemical reactions take place in lime soda softening process which precipitates the calcium and magnesium salts as calcium carbonate and magnesium hydroxide which have very low

solubility In chemistry, solubility is the ability of a chemical substance, substance, the solute, to form a solution (chemistry), solution with another substance, the solvent. Insolubility is the opposite property, the inability of the solute to form su ...

in water. * CaSO₄ + Na₂CO₃ → CaCO₃↓ + Na₂SO₄ * CaCl₂ + Na₂CO₃ → CaCO₃↓ + 2NaCl * MgSO₄ + Ca(OH)₂ + Na₂CO₃ → Mg(OH)₂↓ + CaCO₃↓ + Na₂SO₄ * MgCl₂ + Ca(OH)₂ + Na₂CO₃ → Mg(OH)₂↓ + CaCO₃↓ + 2NaCl * 2NaHCO₃ + Ca(OH)₂ → CaCO₃↓ + Na₂CO₃ + 2H₂O * Na₂CO₃ + Ca(OH)₂ → CaCO₃↓ + 2NaOH * Ca(HCO₃)₂ + Ca(OH)₂ → 2CaCO₃↓ + 2H₂O * Mg(HCO₃)₂ + 2Ca(OH)₂ → Mg(OH)₂↓ + 2CaCO₃↓ + 2H₂O * MgCO₃ + Ca(OH)₂ → Mg(OH)₂↓ + CaCO₃↓ The excess soda ash after precipitating the calcium and magnesium salts is in carbonates & bicarbonates of sodium which imparts high pH or

alkalinity Alkalinity (from ) is the capacity of water to resist Freshwater acidification, acidification. It should not be confused with base (chemistry), basicity, which is an absolute measurement on the pH scale. Alkalinity is the strength of a buffer s ...

to soil water.

Soda lakes

The

endorheic An endorheic basin ( ; also endoreic basin and endorreic basin) is a drainage basin that normally retains water and allows no outflow to other external bodies of water (e.g. rivers and oceans); instead, the water drainage flows into permanent ...

basin lakes are called soda or alkaline lakes when the water inflows contain high concentrations of Na₂CO₃. The pH of the

soda lake A soda lake or alkaline lake is a lake on the strongly base (chemistry), basic side of neutrality, typically with a pH, pH value between 9 and 12. They are characterized by high concentrations of carbonate salts, typically sodium carbonate (and ...

water is generally above 9 and sometimes the

is close to brackish water due to depletion of pure water by solar evaporation. Soda lakes are rich with algal growth due to enhanced availability of dissolved CO₂ in the lake water compared to fresh water or saline water lakes. Sodium carbonate and sodium hydroxide are in equilibrium with availability of dissolved carbon dioxide as given below in the chemical reaction * Na2CO3 + H2O <=> 2NaOH + CO2 * NaHCO3 <=> NaOH + CO2 During day time when sun light is available, Algae undergoes

photosynthesis Photosynthesis ( ) is a system of biological processes by which photosynthetic organisms, such as most plants, algae, and cyanobacteria, convert light energy, typically from sunlight, into the chemical energy necessary to fuel their metabo ...

process which absorbs CO₂ to shift the reaction towards NaOH formation and vice versa takes place during night time with the release of CO₂ from the respiration process of Algae towards Na₂CO₃ and NaHCO₃ formation. In soda lake waters, carbonates of sodium act as catalyst for the algae growth by providing favourable higher concentration of dissolved CO₂ during the day time. Due to fluctuation in dissolved CO₂, the pH and alkalinity of the water also keep varying. .

References

{{Reflist, 2 Soil chemistry Types of soil Land reclamation Water quality indicators

RSC index formula

Natural water contamination

Softened water

Soda lakes

See also

References