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) 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 been devel ...

clay Clay is a type of fine-grained natural soil material containing clay minerals (hydrous aluminium phyllosilicates, e.g. kaolin, Al2 Si2 O5( OH)4). Clays develop plasticity when wet, due to a molecular film of water surrounding the clay pa ...

soils which have a high

cation exchange capacity Cation-exchange capacity (CEC) is a measure of how many cations can be retained on soil particle surfaces. Negative charges on the surfaces of soil particles bind positively-charged atoms or molecules (cations), but allow these to exchange with ot ...

. When dissolved sodium in comparison with dissolved calcium and magnesium is high in water, clay soil swells or undergoes

dispersion Dispersion may refer to: Economics and finance *Dispersion (finance), a measure for the statistical distribution of portfolio returns *Price dispersion, a variation in prices across sellers of the same item *Wage dispersion, the amount of variatio ...

which drastically reduces its

infiltration capacity Infiltration is the process by which water on the ground surface enters the soil. It is commonly used in both hydrology and soil sciences. The infiltration capacity is defined as the maximum rate of infiltration. It is most often measured in meter ...

.Managing irrigation water quality, Oregon State University, USA
Retrieved on 2012-10-04. In the dispersed

soil structure Soil structure describes the arrangement or the way of soil in the solid parts of the soil and of the pore space located between them. It is determined by how individual soil granules clump, bind together, and aggregate, resulting in the arrangem ...

, 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 sur ...

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 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 solution to take in a pure ...

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 Alkali, or Alkaline, soils are clay soils with high pH (greater than 8.5), a poor soil structure and a low infiltration capacity. Often they have a hard calcareous layer at 0.5 to 1 metre depth. Alkali soils owe their unfavorable physico ...

formation. Exposed mango tree roots

RSC index formula

RSC is expressed in

meq/L An equivalent (symbol: officially equiv; unofficially but often Eq) is the amount of a substance that reacts with (or is ''equivalent'' to) an arbitrary amount (typically one mole) of another substance in a given chemical reaction. It is an arc ...

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 + CO₃/60 – Ca/40 – Mg/24 (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 CO2 at the plants root zone in the field water.

Natural water contamination

Soda ash Sodium carbonate, , (also known as washing soda, soda ash and soda crystals) is the inorganic compound with the formula Na2CO3 and its various hydrates. All forms are white, odourless, water-soluble salts that yield moderately alkaline solutions ...

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, and biological organisms. Weathering occurs '' in situ'' (on site, with little or no movement ...

basalt Basalt (; ) is an 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 surface of a rocky planet or moon. More than 90 ...

which is an

igneous Igneous rock (derived from the Latin word ''ignis'' meaning fire), or magmatic rock, is one of the three main rock types, the others being sedimentary and metamorphic. Igneous rock is formed through the cooling and solidification of magma or ...

rock. Lime a(OH)₂can be present in natural water when rain water comes in contact with

calcined Calcination refers to thermal treatment of a solid chemical compound (e.g. mixed carbonate ores) whereby the compound is raised to high temperature without melting under restricted supply of ambient oxygen (i.e. gaseous O2 fraction of air), gener ...

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 or being chalky. The term is used in a wide variety of scientific disciplines. In zoology ''Calcareous'' is used as an ad ...

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 elements, chiefly hydrogen, sulfur, oxygen, and nitrogen. Coal is formed when ...

lignite Lignite, 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 rank of coal due to its relatively low heat ...

in boilers.

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

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 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, bicarbo ...

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 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, bica ...

. 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 substance, the solute, to form a solution with another substance, the solvent. Insolubility is the opposite property, the inability of the solute to form such a solution. The extent of the solubi ...

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 ar, القلوي, al-qaly, lit=ashes of the saltwort) is the capacity of water to resist acidification. It should not be confused with basicity, which is an absolute measurement on the pH scale. Alkalinity is the strength ...

to soil water.

Soda lakes

The

endorheic An endorheic basin (; also spelled endoreic basin or endorreic basin) is a drainage basin that normally retains water and allows no outflow to other external bodies of water, such as rivers or oceans, but drainage converges instead into lakes ...

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 alkaline side of neutrality, typically with a pH value between 9 and 12. They are characterized by high concentrations of carbonate salts, typically sodium carbonate (and related salt complex ...

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 process used by plants and other organisms to convert light energy into chemical energy that, through cellular respiration, can later be released to fuel the organism's activities. Some of this chemical energy is stored in ...

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 Respiration may refer to: Biology * Cellular respiration, the process in which nutrients are converted into useful energy in a cell ** Anaerobic respiration, cellular respiration without oxygen ** Maintenance respiration, the amount of cellul ...

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

{{DEFAULTSORT:Alkali Soils Soil chemistry Types of soil Land reclamation

RSC index formula

Natural water contamination

Softened water

Soda lakes

See also

References