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id="mw-content-text" class="mw-body-content"><div class="mw-content-ltr mw-parser-output" lang="en" dir="ltr"><style data-mw-deduplicate="TemplateStyles:r1251242444">.mw-parser-output .ambox{border:1px solid #a2a9b1;border-left:10px solid #36c;background-color:#fbfbfb;box-sizing:border-box}.mw-parser-output .ambox+link+.ambox,.mw-parser-output .ambox+link+style+.ambox,.mw-parser-output .ambox+link+link+.ambox,.mw-parser-output .ambox+.mw-empty-elt+link+.ambox,.mw-parser-output .ambox+.mw-empty-elt+link+style+.ambox,.mw-parser-output .ambox+.mw-empty-elt+link+link+.ambox{margin-top:-1px}html body.mediawiki .mw-parser-output .ambox.mbox-small-left{margin:4px 1em 4px 0;overflow:hidden;width:238px;border-collapse:collapse;font-size:88%;line-height:1.25em}.mw-parser-output .ambox-speedy{border-left:10px solid #b32424;background-color:#fee7e6}.mw-parser-output .ambox-delete{border-left:10px solid #b32424}.mw-parser-output .ambox-content{border-left:10px solid #f28500}.mw-parser-output .ambox-style{border-left:10px solid #fc3}.mw-parser-output .ambox-move{border-left:10px solid #9932cc}.mw-parser-output .ambox-protection{border-left:10px solid #a2a9b1}.mw-parser-output .ambox .mbox-text{border:none;padding:0.25em 0.5em;width:100%}.mw-parser-output .ambox .mbox-image{border:none;padding:2px 0 2px 0.5em;text-align:center}.mw-parser-output .ambox .mbox-imageright{border:none;padding:2px 0.5em 2px 0;text-align:center}.mw-parser-output .ambox .mbox-empty-cell{border:none;padding:0;width:1px}.mw-parser-output .ambox .mbox-image-div{width:52px}@media(min-width:720px){.mw-parser-output .ambox{margin:0 10%}}@media print{body.ns-0 .mw-parser-output .ambox{display:none!important}}</style><table class="box-Disputed plainlinks metadata ambox ambox-content ambox-disputed" role="presentation"><tbody><tr><td class="mbox-image"><div class="mbox-image-div"><span typeof="mw:File"><span><img src="//upload.wikimedia.org/wikipedia/commons/thumb/1/17/System-search.svg/45px-System-search.svg.png" decoding="async" width="45" height="45" class="mw-file-element" srcset="//upload.wikimedia.org/wikipedia/commons/thumb/1/17/System-search.svg/68px-System-search.svg.png 1.5x, //upload.wikimedia.org/wikipedia/commons/thumb/1/17/System-search.svg/90px-System-search.svg.png 2x" data-file-width="48" data-file-height="48" /></span></span></div></td><td class="mbox-text"><div class="mbox-text-span">This article's <b>factual accuracy is <a href="/wiki/Wikipedia:Accuracy_dispute" title="Wikipedia:Accuracy dispute">disputed</a></b>.<span class="hide-when-compact"> Relevant discussion may be found on the <a href="/wiki/Talk:Residual_sodium_carbonate_index#Disputed" title="Talk:Residual sodium carbonate index">talk page</a>. Please help to ensure that disputed statements are <a href="/wiki/Wikipedia:Reliable_sources" title="Wikipedia:Reliable sources">reliably sourced</a>.</span> <span class="date-container"><i>(<span class="date">October 2019</span>)</i></span><span class="hide-when-compact"><i> (<small><a href="/wiki/Help:Maintenance_template_removal" title="Help:Maintenance template removal">Learn how and when to remove this message</a></small>)</i></span></div></td></tr></tbody></table> <p>The <b>residual sodium carbonate (RSC) index</b> 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 <a href="/wiki/Irrigation" title="Irrigation">irrigation</a> in <a href="/wiki/Clay" title="Clay">clay</a> soils which have a high <a href="/wiki/Cation_exchange_capacity" class="mw-redirect" title="Cation exchange capacity">cation exchange capacity</a>. When dissolved sodium in comparison with dissolved calcium and magnesium is high in water, clay soil swells or undergoes <a href="/wiki/Dispersion_(geology)" title="Dispersion (geology)">dispersion</a> which drastically reduces its <a href="/wiki/Infiltration_capacity" class="mw-redirect" title="Infiltration capacity">infiltration capacity</a>.<sup id="cite_ref-oregonstateedu_1-0" class="reference"><a href="#cite_note-oregonstateedu-1"><span class="cite-bracket">[</span>1<span class="cite-bracket">]</span></a></sup> </p><p>In the dispersed<sup class="noprint Inline-Template" style="margin-left:0.1em; white-space:nowrap;">[<i><a href="/wiki/Wikipedia:Please_clarify" title="Wikipedia:Please clarify"><span title="The text near this tag may need clarification or removal of jargon. (February 2015)">clarification needed</span></a></i>]</sup> <a href="/wiki/Soil_structure" title="Soil structure">soil structure</a>, the <a href="/wiki/Plant_roots" class="mw-redirect" title="Plant roots">plant roots</a> are unable to spread deeper into the soil due to lack of moisture. However, high RSC index water does not enhance the <a href="/wiki/Osmotic_pressure" title="Osmotic pressure">osmotic pressure</a> to impede the off take of water by the plant roots unlike high <a href="/wiki/Salinity" title="Salinity">salinity</a> water. Clay soils irrigation with high RSC index water leads to fallow <a href="/wiki/Alkali_soils" class="mw-redirect" title="Alkali soils">alkali soils</a> formation.<sup id="cite_ref-2" class="reference"><a href="#cite_note-2"><span class="cite-bracket">[</span>2<span class="cite-bracket">]</span></a></sup><sup id="cite_ref-3" class="reference"><a href="#cite_note-3"><span class="cite-bracket">[</span>3<span class="cite-bracket">]</span></a></sup><sup id="cite_ref-4" class="reference"><a href="#cite_note-4"><span class="cite-bracket">[</span>4<span class="cite-bracket">]</span></a></sup> </p> <figure class="mw-default-size" typeof="mw:File/Thumb"><a href="/wiki/File:Exposed_mango_tree_roots.jpg" class="mw-file-description"><img src="//upload.wikimedia.org/wikipedia/commons/thumb/2/29/Exposed_mango_tree_roots.jpg/220px-Exposed_mango_tree_roots.jpg" decoding="async" width="220" height="162" class="mw-file-element" srcset="//upload.wikimedia.org/wikipedia/commons/thumb/2/29/Exposed_mango_tree_roots.jpg/330px-Exposed_mango_tree_roots.jpg 1.5x, //upload.wikimedia.org/wikipedia/commons/thumb/2/29/Exposed_mango_tree_roots.jpg/440px-Exposed_mango_tree_roots.jpg 2x" data-file-width="2000" data-file-height="1477" /></a><figcaption>Cross section of a <a href="/wiki/Mango" title="Mango">mango</a> tree with shallow roots in partially formed alkali soil</figcaption></figure> <meta property="mw:PageProp/toc" /> <div class="mw-heading mw-heading2"><h2 id="RSC_index_formula">RSC index formula</h2><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=Residual_sodium_carbonate_index&action=edit&section=1" title="Edit section: RSC index formula"><span>edit</span></a><span class="mw-editsection-bracket">]</span></span></div> <p>RSC is expressed in <a href="/wiki/Meq/L" class="mw-redirect" title="Meq/L">meq/L</a> units. RSC should not be higher than 1 and preferably less than +0.5 for considering the water use for irrigation.<sup id="cite_ref-Handbook_5-0" class="reference"><a href="#cite_note-Handbook-5"><span class="cite-bracket">[</span>5<span class="cite-bracket">]</span></a></sup> The formula for calculating RSC index is: </p> <ul><li>RSC index = [HCO<sub>3</sub> + CO<sub>3</sub>] − [Ca + Mg]</li> <li>RSC index = HCO<sub>3</sub>/61 + CO<sub>3</sub>/30 – Ca/20 – Mg/12 (in case the ionic concentrations are measured in mg/L or <a href="/wiki/Parts_per_million" class="mw-redirect" title="Parts per million">ppm</a> as salts)</li></ul> <p>While calculating RSC index, the water quality present at the <a href="/wiki/Rhizosphere" title="Rhizosphere">root zone</a> of the crop should be considered which would take into account the <a href="/wiki/Soil_salinity_control" title="Soil salinity control">leaching</a> factor in the field.<sup id="cite_ref-6" class="reference"><a href="#cite_note-6"><span class="cite-bracket">[</span>6<span class="cite-bracket">]</span></a></sup> Calcium present in dissolved form is also influenced by the <a href="/wiki/Partial_pressure" title="Partial pressure">partial pressure</a> of dissolved CO<sub style="font-size: 80%;vertical-align: -0.35em">2</sub> at the plants root zone in the field water.<sup id="cite_ref-7" class="reference"><a href="#cite_note-7"><span class="cite-bracket">[</span>7<span class="cite-bracket">]</span></a></sup> </p> <div class="mw-heading mw-heading2"><h2 id="Natural_water_contamination">Natural water contamination</h2><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=Residual_sodium_carbonate_index&action=edit&section=2" title="Edit section: Natural water contamination"><span>edit</span></a><span class="mw-editsection-bracket">]</span></span></div> <figure class="mw-halign-right" typeof="mw:File/Thumb"><a href="/wiki/File:World_Soil_pH.svg" class="mw-file-description"><img src="//upload.wikimedia.org/wikipedia/commons/thumb/b/b8/World_Soil_pH.svg/500px-World_Soil_pH.svg.png" decoding="async" width="500" height="254" class="mw-file-element" srcset="//upload.wikimedia.org/wikipedia/commons/thumb/b/b8/World_Soil_pH.svg/750px-World_Soil_pH.svg.png 1.5x, //upload.wikimedia.org/wikipedia/commons/thumb/b/b8/World_Soil_pH.svg/1000px-World_Soil_pH.svg.png 2x" data-file-width="940" data-file-height="477" /></a><figcaption>Global variation in soil pH. <b><span style="color:red;">Red</span></b> = acidic soil. <b><span style="color:yellow;">Yellow</span></b> = neutral soil. <b><span style="color:blue;">Blue</span></b> = alkaline soil. <b>Black</b> = no data.</figcaption></figure> <p><a href="/wiki/Soda_ash" class="mw-redirect" title="Soda ash">Soda ash</a> [Na<sub>2</sub>CO<sub>3</sub>] can be present in natural water from the <a href="/wiki/Weathering" title="Weathering">weathering</a> of <a href="/wiki/Basalt" title="Basalt">basalt</a> which is an <a href="/wiki/Igneous" class="mw-redirect" title="Igneous">igneous</a> rock. <a href="/wiki/Calcium_hydroxide" title="Calcium hydroxide">Lime</a> [Ca(OH)<sub>2</sub>] can be present in natural water when rain water comes in contact with <a href="/wiki/Calcined" class="mw-redirect" title="Calcined">calcined</a> minerals such as ash produced from the burning of <a href="/wiki/Calcareous" title="Calcareous">calcareous</a> <a href="/wiki/Coal" title="Coal">coal</a> or <a href="/wiki/Lignite" title="Lignite">lignite</a> in boilers. <a href="/wiki/Anthropogenic_metabolism" title="Anthropogenic metabolism">Anthropogenic</a> use of soda ash also finally adds to the RSC of the river water. </p><p>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.<sup id="cite_ref-8" class="reference"><a href="#cite_note-8"><span class="cite-bracket">[</span>8<span class="cite-bracket">]</span></a></sup> The salinity of water need not be high. </p> <div class="mw-heading mw-heading2"><h2 id="Softened_water">Softened water</h2><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=Residual_sodium_carbonate_index&action=edit&section=3" title="Edit section: Softened water"><span>edit</span></a><span class="mw-editsection-bracket">]</span></span></div> <p>In industrial water treatment terminology, water quality with high RSC index is synonymous with the <a href="/wiki/Soft_water" class="mw-redirect" title="Soft water">soft water</a> but is chemically very different from naturally soft water which has a very low ionic concentration.<sup id="cite_ref-9" class="reference"><a href="#cite_note-9"><span class="cite-bracket">[</span>9<span class="cite-bracket">]</span></a></sup> 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 <a href="/wiki/Water_hardness" class="mw-redirect" title="Water hardness">water hardness</a>. </p><p>The following chemical reactions take place in <a href="/wiki/Soft_water" class="mw-redirect" title="Soft water">lime soda softening</a> process which precipitates the calcium and magnesium salts as calcium carbonate and magnesium hydroxide which have very low <a href="/wiki/Solubility" title="Solubility">solubility</a> in water. </p> <ul><li>CaSO<sub>4</sub> + Na<sub>2</sub>CO<sub>3</sub> ---> CaCO<sub>3</sub>↓ + Na<sub>2</sub>SO<sub>4</sub></li> <li>CaCl<sub>2</sub> + Na<sub>2</sub>CO<sub>3</sub> ---> CaCO<sub>3</sub>↓ + 2NaCl</li> <li>MgSO<sub>4</sub> + Ca(OH)<sub>2</sub> + Na<sub>2</sub>CO<sub>3</sub> ---> Mg(OH)<sub>2</sub>↓ + CaCO<sub>3</sub>↓ + Na<sub>2</sub>SO<sub>4</sub></li> <li>MgCl<sub>2</sub> + Ca(OH)<sub>2</sub> + Na<sub>2</sub>CO<sub>3</sub> ---> Mg(OH)<sub>2</sub>↓ + CaCO<sub>3</sub>↓ + 2NaCl</li> <li>2NaHCO<sub>3</sub> + Ca(OH)<sub>2</sub> ---> CaCO<sub>3</sub>↓ + Na<sub>2</sub>CO<sub>3</sub> + 2H<sub>2</sub>O</li> <li>Na<sub>2</sub>CO<sub>3</sub> + Ca(OH)<sub>2</sub> ---> CaCO<sub>3</sub>↓ + 2NaOH</li> <li>Ca(HCO<sub>3</sub>)<sub>2</sub> + Ca(OH)<sub>2</sub> ---> 2CaCO<sub>3</sub>↓ + 2H<sub>2</sub>O</li> <li>Mg(HCO<sub>3</sub>)<sub>2</sub> + 2Ca(OH)<sub>2</sub> ---> Mg(OH)<sub>2</sub>↓ + 2CaCO<sub>3</sub>↓ + 2H<sub>2</sub>O</li> <li>MgCO<sub>3</sub> + Ca(OH)<sub>2</sub> ---> Mg(OH)<sub>2</sub>↓ + CaCO<sub>3</sub>↓</li></ul> <p>The excess soda ash after precipitating the calcium and magnesium salts is in carbonates & bicarbonates of sodium which imparts high <a href="/wiki/PH" title="PH">pH</a> or <a href="/wiki/Alkalinity" title="Alkalinity">alkalinity</a> to soil water. </p> <div class="mw-heading mw-heading2"><h2 id="Soda_lakes">Soda lakes</h2><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=Residual_sodium_carbonate_index&action=edit&section=4" title="Edit section: Soda lakes"><span>edit</span></a><span class="mw-editsection-bracket">]</span></span></div> <p>The <a href="/wiki/Endorheic" class="mw-redirect" title="Endorheic">endorheic</a> basin lakes are called soda or <a href="/wiki/Soda_lake" title="Soda lake">alkaline lakes</a> when the water inflows contain high concentrations of Na<sub>2</sub>CO<sub>3</sub>. The pH of the <a href="/wiki/Soda_lake" title="Soda lake">soda lake</a> water is generally above 9 and sometimes the <a href="/wiki/Salinity" title="Salinity">salinity</a> is close to brackish water due to depletion of pure water by solar evaporation. </p><p>Soda lakes are rich with algal growth due to enhanced availability of dissolved CO<sub>2</sub> 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 </p> <ul><li>Na<sub>2</sub>CO<sub>3</sub> + H<sub>2</sub>O <=> 2NaOH + CO<sub>2</sub></li> <li>NaHCO<sub>3</sub> <=> NaOH + CO<sub>2</sub></li></ul> <p>During day time when sun light is available, Algae undergoes <a href="/wiki/Photosynthesis" title="Photosynthesis">photosynthesis</a> process which absorbs CO<sub>2</sub> to shift the reaction towards NaOH formation and vice versa takes place during night time with the release of CO<sub>2</sub> from the <a href="/wiki/Cellular_respiration" title="Cellular respiration">respiration</a> process of Algae towards Na<sub>2</sub>CO<sub>3</sub> and NaHCO<sub>3</sub> formation. In soda lake waters, carbonates of sodium act as catalyst for the algae growth by providing favourable higher concentration of dissolved CO<sub>2</sub> during the day time. Due to fluctuation in dissolved CO<sub>2</sub>, the pH and alkalinity of the water also keep varying.<sup id="cite_ref-10" class="reference"><a href="#cite_note-10"><span class="cite-bracket">[</span>10<span class="cite-bracket">]</span></a></sup> </p><p>. </p> <div class="mw-heading mw-heading2"><h2 id="See_also">See also</h2><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=Residual_sodium_carbonate_index&action=edit&section=5" title="Edit section: See also"><span>edit</span></a><span class="mw-editsection-bracket">]</span></span></div> <ul><li><a href="/wiki/Soil_pH" title="Soil pH">Soil pH</a></li> <li><a href="/wiki/Environmental_impact_of_irrigation" title="Environmental impact of irrigation">Environmental impact of irrigation</a></li> <li><a href="/wiki/Index_of_soil-related_articles" title="Index of soil-related articles">Index of soil-related articles</a></li> <li><a href="/wiki/Salsola_soda" class="mw-redirect" title="Salsola soda">Agreti green vegetable</a></li> <li><a href="/wiki/Algae_fuel" title="Algae fuel">Algae fuel</a></li> <li><a href="/wiki/Algaculture" title="Algaculture">Algaculture</a></li> <li><a href="/wiki/Gravitropism" title="Gravitropism">Gravitropism</a></li></ul> <div class="mw-heading mw-heading2"><h2 id="References">References</h2><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=Residual_sodium_carbonate_index&action=edit&section=6" title="Edit section: References"><span>edit</span></a><span class="mw-editsection-bracket">]</span></span></div> <style data-mw-deduplicate="TemplateStyles:r1239543626">.mw-parser-output .reflist{margin-bottom:0.5em;list-style-type:decimal}@media screen{.mw-parser-output .reflist{font-size:90%}}.mw-parser-output .reflist .references{font-size:100%;margin-bottom:0;list-style-type:inherit}.mw-parser-output .reflist-columns-2{column-width:30em}.mw-parser-output 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