 | This article mayrequirecleanup to meet Wikipedia'squality standards. The specific problem is:General layout and markup, especially with respect to chemical formulas and equations. Please helpimprove this article if you can.(February 2018) (Learn how and when to remove this message) |
| Alkali soil | |
|---|---|
| Alkaline soils | |
 Rice cultivation /paddyfield in alkali soils | |
| Clay soil | |
| Key minerals | Sodium carbonate and sodium bicarbonate |
| Key process | Lime softening |
| pH | > 8.5 |
Alkali, or alkaline,soils areclaysoils with highpH (greater than 8.5), a poorsoil structure and a low infiltration capacity. Often they have a hardcalcareous layer at 0.5 to 1 metre depth. Alkali soils owe their unfavorablephysico-chemical properties mainly to the dominating presence ofsodium carbonate, which causes the soil to swell[1] and to be difficult to clarify/settle. They derive their name from thealkali metal group of elements, to whichsodium belongs, and which can inducebasicity. Sometimes these soils are also referred to as alkalinesodic soils. Alkaline soils arebasic, but not all basic soils arealkaline.
The causes of soil alkalinity can be natural or man-made:
The natural cause is the presence of soil minerals producingsodium carbonate (Na2CO3) andsodium bicarbonate (NaHCO3) uponweathering.
Coal-fired boilers / power plants, when using coal or lignite rich inlimestone, produceash containingcalcium oxide. CaO readily dissolves in water to formslaked lime, Ca(OH)2, and carried by rain water to rivers / irrigation water.Lime softening process precipitates Ca2+ and Mg2+ ions / removes hardness in the water and also converts sodium bicarbonates in river water into sodium carbonate.[2] Sodium carbonates (washing soda) further reacts with the remaining Ca2+ and Mg2+ in the water to remove / precipitate the totalhardness. Also water-soluble sodium salts present in the ash enhance the sodium content in water. The globalcoal consumption in the world was 7.7 billion tons in the year 2011.[citation needed] Thus river water is made devoid of Ca2+ and Mg2+ ions and enhanced Na+ by coal-fired boilers.[clarification needed]
Many sodium salts are used in industrial and domestic applications such assodium carbonate,sodium bicarbonate (baking soda),sodium sulphate,sodium hydroxide (caustic soda),sodium hypochlorite (bleaching powder), etc. in huge quantities. These salts are mainly produced fromsodium chloride (common salt). All the sodium in these salts enter into the river / ground water during their production process or consumption enhancing water sodicity. The total global consumption ofsodium chloride is 270 million tons in the year 2010. This is nearly equal to thesalt load in the mightyAmazon River. Man made sodium salts contribution is nearly 7% of total salt load of all the rivers. Sodium salt load problem aggravates in the downstream of intensively cultivated river basins located in China, India, Egypt, Pakistan, west Asia, Australia, western US, etc. due to accumulation of salts in the remaining water after meeting various transpiration and evaporation losses.[3]
Another source of man-made sodium salts addition to the agriculture fields / land mass is in the vicinity of thewet cooling towers using sea water to dissipatewaste heat generated in various industries located near the sea coast. Huge capacity cooling towers are installed in oil refineries, petrochemical complexes, fertilizer plants, chemical plants, nuclear & thermal power stations, centralizedHVAC systems, etc. The drift / fine droplets emitted from the cooling towers contain nearly 6% sodium chloride which would deposit on the vicinity areas. This problem aggravates where the national pollution control norms are not imposed or not implemented to minimize the drift emissions to the best industrial norm for the sea water based wet cooling towers.[4]
The man-made cause is the application ofsoftened water inirrigation (surface or ground water) containing relatively high proportion ofsodium bicarbonates and less calcium and magnesium.[1]
Alkaline soils are difficult to take into agricultural production. Due to the lowinfiltration capacity, rain water stagnates on the soil easily and, in dry periods, cultivation is hardly possible without copious irrigated water and good drainage. Agriculture is limited to crops tolerant to surfacewaterlogging (e.g.rice, grass) and the productivity is lower.
Soil alkalinity is associated with the presence ofsodium carbonate (Na2CO3) orsodium bicarbonate (NaHCO3) in the soil,[5] either as a result of naturalweathering of the soil particles or brought in by irrigation and/or flood water.
This salt is extremely soluble, when it undergoeshydration, it dissociates in:
The carbonate anionCO2−
3, is a weakbase accepting a proton, so ithydrolyses in water to give thebicarbonate ion and ahydroxyl ion:
which in turn givescarbonic acid and hydroxyl:
Seecarbonate for the equilibrium of carbonate-bicarbonate-carbon dioxide.
The above reactions are similar to the dissolution ofcalcium carbonate, the solubility of the two salts being the only difference. Na2CO3 is about78000 times more soluble than CaCO3, so it can dissolve far larger amounts ofCO2−
3, thus rising the pH to values higher than 8.5, which is above the maximum attainable pH when the equilibrium between calcium carbonate and dissolvedcarbon dioxide are in equilibrium in soil solution.
H2CO3 (carbonic acid) is unstable and produces H2O (water) and CO2 (carbon dioxide gas, escaping into the atmosphere). This explains the remainingalkalinity (or ratherbasicity) in the form of solublesodium hydroxide and the highpH or lowpOH.
Not all the dissolved sodium carbonate undergoes the above chemical reaction. The remaining sodium carbonate, and hence the presence ofCO2−
3 ions, causes CaCO3 (which is only slightly soluble) to precipitate as solidcalcium carbonate (limestone), because the product of theCO2−
3 concentration and the Ca2+ concentration exceeds the allowable limit. Hence, the calcium ions Ca2+ are immobilized.
The presence of abundant Na+ ions in the soil solution and the precipitation of Ca2+ ions as a solid mineral causes theclay particles, which have negative electric charges along their surfaces, to adsorb more Na+ in thediffuse adsorption zone (DAZ, also more commonly calleddiffuse double layer (DDL), orelectrical double layer (EDL), see the corresponding figure)[6] and, in exchange, release previously adsorbed Ca2+, by which theirexchangeable sodium percentage (ESP) is increased as illustrated in the same figure.
Na+ is more mobile and has a smaller electric charge than Ca2+ so that the thickness of the DDL increases as more sodium ions occupy it. The DDL thickness is also influenced by the total concentration of ions in the soil moisture in the sense that higher concentrations cause the DDL zone to shrink.
Clay particles with considerable ESP (> 16), in contact with non-saline soil moisture have an expanded DDL zone and the soil swells (dispersion).[6]The phenomenon results in deterioration of thesoil structure, and especially crust formation and compaction of the top layer.Hence the infiltration capacity of the soil and the water availability in the soil is reduced, whereas the surface-water-logging orsurface runoff is increased. Seedling emergence and crop production are badly affected.
Alkalinity problems are more pronounced inclay soils than in loamy, silty or sandy soils. The clay soils containingmontmorillonite orsmectite (swelling clays) are more subject to alkalinity problems thanillite orkaolinite clay soils. The reason is that the former types of clay have largerspecific surface areas (i.e. the surface area of the soil particles divided by their volume) and highercation exchange capacity (CEC).
The quality of the irrigation water in relation to thealkalinity hazard is expressed by the following two indexes:
| RSC | = [HCO− 3 +CO2− 3] − [Ca2+ + Mg2+] |
| = {HCO− 3/61 +CO2− 3/30} − {Ca2+/20 + Mg2+/12} |
which must not be much higher than 1 and preferably less than 0.5.
The above expression recognizes the presence ofbicarbonates (HCO−
3), the form in which most carbonates are dissolved.
While calculating SAR and RSC, the water quality present at the root zone of the crop should be considered which would take into account theleaching factor in the field.[7] The partial pressure of dissolved CO2 at the plants root zone also decides the calcium present in dissolved form in the field water.USDA follows the adjusted SAR[8] for calculating water sodicity.
Alkaline soils with solid CaCO3 can be reclaimed withgrass cultures, organic compost, waste hair / feathers, organic garbage, waste paper, rejected lemons/oranges, etc. ensuring the incorporation of muchacidifying material (inorganic ororganic material) into the soil, and enhancing dissolved Ca in the field water by releasing CO2 gas.[9] Deepploughing and incorporating the calcareoussubsoil into the top soil also helps.
Many times salts' migration to the top soil takes place from the underground water sources rather than surface sources.[10] Where the underground water table is high and the land is subjected to high solar radiation, ground water oozes to the land surface due to capillary action and gets evaporated leaving the dissolved salts in the top layer of the soil. Where the underground water contains high salts, it leads to acute salinity problem. This problem can be reduced by applyingmulch to the land. Using poly-houses or shade netting during summer for cultivating vegetables/crops is also advised to mitigate soil salinity and conserve water / soil moisture. Poly-houses filter the intense summer solar radiation in tropical countries to save the plants from water stress and leaf burns.
Where the ground water quality is not alkaline / saline and ground water table is high, salts build up in the soil can be averted by using the land throughout the year for growing plantation trees / permanent crops with the help of lift irrigation. When the ground water is used at requiredleaching factor, the salts in the soil would not build up.
Plowing the field soon after cutting the crop is also advised to prevent salt migration to the top soil and conserve the soil moisture during the intense summer months. This is done to break the capillary pores in the soil to prevent water reaching the surface of the soil.
Clay soils in high annual rain fall (more than 100 cm) areas do not generally suffer from high alkalinity as the rain water runoff is able to reduce/leach the soil salts to comfortable levels if properrainwater harvesting methods are followed. In some agricultural areas, the use of subsurface "tile lines" are used to facilitate drainage and leach salts. Continuousdrip irrigation would lead to alkali soils formation in the absence of leaching / drainage water from the field.
It is also possible to reclaim alkaline soils by adding acidifying minerals likepyrite or cheaperalum oraluminium sulfate.
Alternatively,gypsum (calcium sulfate,CaSO
4 · 2H
2O) can also be applied as a source of Ca2+ ions to replace thesodium at the exchange complex.[9] Gypsum also reacts with sodium carbonate to convert intosodium sulphate which is a neutral salt and does not contribute to high pH. There must be enough natural drainage to the underground, or else an artificial subsurface drainage system must be present, to permit leaching of the excess sodium by percolation ofrain and/orirrigation water through thesoil profile.
Calcium chloride is also used to reclaim alkali soils. CaCl2 converts Na2CO3 into NaCl precipitating CaCO3. NaCl is drained off by leaching water.Calcium nitrate has a similar effect, withNaNO3 in the leachate.Spent acid (HCl, H2SO4, etc.) can also be used to reduce the excess Na2CO3 in the soil/water.
Whereurea is made available cheaply to farmers, it is also used to reduce the soil alkalinity / salinity primarily.[11] Theammonium (NH+
4) cation produced by ureahydrolysis which is a strongly sorbingcation exchanges with the weakly sorbing Na+ cation from the soil structure and Na+ is released into water. Thus alkali soils adsorb / consume more urea compared to other soils.
To reclaim the soils completely one needs prohibitively high doses of amendments. Most efforts are therefore directed to improving the top layer only (say the first 10 cm of the soils), as the top layer is most sensitive to deterioration of thesoil structure.[9] The treatments, however, need to be repeated in a few (say 5) years' time. Trees / plants followgravitropism. It is difficult to survive in alkali soils for the trees with deeperrooting system which can be more than 60 meters deep in good non-alkali soils.
It will be important to refrain from irrigation (ground water or surface water) with poor quality water. In viticulture, adding naturally occurring chelating agents such as tartaric acid to irrigation water has been suggested, to solubilize calcium and magnesium carbonates in sodic soils.[12]
One way of reducing sodium carbonate is to cultivateglasswort orsaltwort orbarilla plants.[13] These plants sequester the sodium carbonate they absorb from alkali soil into their tissues. The ash of these plants contains good quantity of sodium carbonate which can be commercially extracted and used in place of sodium carbonate derived from common salt which is highly energy intensive process. Thus alkali lands deterioration can be checked by cultivating barilla plants which can serve as food source, biomass fuel and raw material for soda ash andpotash, etc.
Saline soils are mostly also sodic (the predominant salt issodium chloride), but they do not have a very highpH nor a poor infiltration rate. Upon leaching they are usually not converted into a (sodic) alkali soil as the Na+ ions are easily removed. Therefore, saline (sodic) soils mostly do not need gypsum applications for their reclamation.[14]
While traditional reclamation often requires extensive chemical amendments and drainage infrastructure, aquaculture offers an alternative or complementary strategy by using water bodies to leach salts and problematic soil compounds, introduce organic matter, and create conditions for biological soil improvement. Since the 1990s, research and experimentation have been conducted in China and elsewhere for remediation and utilization of alkali land via combined agriculture and aquaculture practices, with considerable success and gains in experience.[15][16][17] Despite technical, infrastructure and management challenges[18][19], aquaculture technology of utilizing inland saline-alkali water and non-arable land for seafood production is becoming mature, covering wide range of seafood species including shrimps, crabs, shellfish and fish such as sea bass and grouper.[20][21]
In recent years,aquaculture (orsalt-alkali land aquaculture) has been recommended by theMinistry of Agriculture and Rural Affairs of China as an economically viable and environmentally beneficial model for the transformation and utilization of saline-alkali land.[15][22][23][16]FAO noted in a recent newsletter that alkaline land is one area that there are innovative ways and opportunities for aquaculture to expand.[24]
{{cite journal}}: CS1 maint: numeric names: authors list (link){{cite journal}}: CS1 maint: numeric names: authors list (link)Aquaculture development today is imbalanced and there are multiple opportunities for the expansion of aquaculture in less developed areas with suitable natural resources, particularly in Africa, and specifically in areas not otherwise farmable such as arid zones, alkaline land or open oceans.
| General | |
|---|---|
| Property | |
| Related fields | |
| |
