Alkali or alkaline soils are clay soils with high pH (generally greater than 8.5), often accompanied by poor soil structure and low infiltration capacity. They frequently have a hard calcareous layer beneath the surface. Alkali soils owe their unfavorable physico-chemical properties mainly to the dominating presence of sodium carbonate, which causes the soil to swell and makes it difficult to clarify/settle. They derive their name from the alkali metal group of elements, to which sodium belongs, and which can induce basicity. Sometimes these soils are also referred to as alkaline sodic soils.
Causes
The causes of soil alkalinity can be natural or man-made. The natural cause is the presence of soil minerals producing sodium carbonate (Na2CO3) and sodium bicarbonate (NaHCO3) upon weathering.
Coal-fired boilers and power plants, when burning coal or lignite rich in limestone, produce a type of ash containing calcium oxide, known as fly ash. Calcium oxide readily dissolves in water to form slaked lime, Ca(OH)2, an effluent which is carried by runoff to rivers and irrigation systems which it pollutes unless clean coal technologies such as IGCC achieve zero water discharge. The process of lime softening precipitates Ca2+ and Mg2+ ions and thereby removes hardness in the water and also converts sodium bicarbonates in river water into sodium carbonate. Sodium carbonates (washing soda) further reacts with the remaining Ca2+ and Mg2+ in the water to remove or precipitate the total hardness.
Numerous sodium salts are used in industrial and domestic applications in huge quantities, such as sodium carbonate, sodium bicarbonate (baking soda), sodium sulfate, sodium hydroxide (caustic soda), sodium hypochlorite (chlorine bleach), etc. These salts are mainly produced from sodium chloride (common salt). The sodium in these salts enters into surface waters or groundwater in the process of production or consumption, enhancing water sodicity. The total global consumption of sodium chloride was 280 million tons in the year 2020. Sodium salt load problems are particularly egregious in downstream areas of intensively cultivated river basins in China, India, Egypt, Pakistan, West Asia, Australia, the Western United States, etc., due to accumulation of salts in the remaining water after meeting various transpiration and evaporation losses.
Another source of man-made sodium salts addition to the land is the wet cooling towers that use seawater to dissipate waste heat generated in various industries located near the coast. High-capacity cooling towers are installed in oil refineries, petrochemical complexes, fertilizer plants, chemical plants, nuclear and thermal power stations, centralized HVAC systems, etc. The drift and fine droplets emitted from the cooling towers contain nearly 6% sodium chloride, which deposits on surfaces in the vicinity. This problem is exacerbated by the fact that national pollution control norms are not imposed or not implemented to minimize the drift emissions to the optimal industrial norm for seawater-based wet cooling towers.
By replacing calcium and magnesium with sodium softened water used in irrigation (surface or groundwater) dissolves soil organic matter and disperses clay mineral particles, resulting in poor soil structure and low infiltration capacity typical of alkali soils.
Agricultural problems
Alkaline soils are difficult to use for agriculture, with considerable areas being salt-affected worldwide. Due to the low infiltration capacity, rainwater tends to stagnate on the soil surface and evaporate before it can be used by plant crops, and in dry periods, cultivation is hardly possible without copious irrigation and good drainage. Agriculture in alkaline soils is therefore usually limited to crops tolerant to surface waterlogging (e.g. rice).
Chemistry
Soil alkalinity is associated with the presence of sodium carbonate (Na2CO3) or sodium bicarbonate (NaHCO3) in the soil, either as a result of natural weathering of the soil minerals or brought in by irrigation and/or flood water.
Sodium carbonate is extremely soluble. When it undergoes hydration, it dissociates into sodium cations and carbonate anions:
→ 2 +
The carbonate anion, , is a weak base accepting a proton, so it hydrolyses in water to give the bicarbonate ion and a hydroxyl ion:
+ → +
This in turn is susceptible to further hydrolysis, giving carbonic acid and another hydroxyl ion:
+ → +
See carbonate for the carbonate-bicarbonate-carbon dioxide equilibrium.
The above reactions are similar to the dissolution of calcium carbonate, the solubility of the two salts being the only difference. Na2CO3 is about times more soluble than CaCO3, so it can dissolve far larger amounts of , thus increasing pH to values higher than 8.5, which is above the maximum attainable pH when calcium carbonate and dissolved carbon dioxide are in equilibrium in the soil solution.
Notes: Water (H2O) is partly dissociated into H3O+ (hydronium) and OH– (hydroxyl) ions. The ion H3O+ has a positive electric charge (+) and its concentration is usually written as [H+]. The hydroxyl ion OH– has a negative charge (−) and its concentration is written as [OH−]. In pure water, at 25 °C, the dissociation constant of water (Kw) is 10−14.Since Kw = [H+] × [OH–], then both the concentration of H3O+ and OH– ions equal 10−7 M (a very small concentration). In neutral water, the pH, being the negative decimal logarithm of the H3O+ concentration, it is 7. Similarly, the pOH is also 7. Each unit decrease in pH indicates a tenfold increase of the H3O+ concentration. Similarly, each unit increase in pH indicates a tenfold increase of the OH– concentration. In water with dissolved salts, the concentrations of the H3O+ and the OH– ions may change, but their sum remains constant, namely . A pH of 7 therefore corresponds to a pOH of 7, and a pH of 9 with a pOH of 5. Formally it is preferred to express the ion concentrations in terms of chemical activity, but this hardly affects the value of the pH. Water with excess H3O+ ions is called acid (), and water with excess OH– ions is called alkaline or basic (). Soil moisture with is called very acid and with very alkaline (basic).
H2CO3 (carbonic acid) is unstable and produces H2O (water) and CO2 (carbon dioxide gas, escaping into the atmosphere). This explains the remaining alkalinity (or rather basicity) in the form of soluble sodium hydroxide and the high pH or low pOH of sodium carbonate solutions.
Not all of the dissolved sodium carbonate undergoes the above chemical reaction. The remaining sodium carbonate, and hence the presence of ions, causes CaCO3 (which is only slightly soluble) to precipitate as solid calcium carbonate (limestone), because the product of the 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 the clay particles, which have negative electric charges along their surfaces, to adsorb more Na+ in the diffuse adsorption zone (DAZ, also more commonly called diffuse double layer (DDL), or electrical double layer (EDL), see the corresponding figure) and, in exchange, release previously adsorbed Ca2+, by which their exchangeable 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 (as when the soil dries out) cause the DDL zone to shrink and clay particles to adhere more firmly: the soil becomes more compact and desiccation cracks appear.
Clay particles with considerable ESP (> 16), in contact with non-saline soil moisture have an expanded DDL zone and the soil swells (dispersion). The phenomenon results in deterioration of the soil 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-waterlogging or surface runoff is increased. Seedling emergence and crop production are badly affected.
Note: Under saline conditions, the many ions in the soil solution counteract the swelling of the soil, so that saline soils usually do not have unfavorable physical properties. Alkaline soils, in principle, are not saline since the alkalinity problem is worse as the salinity is less.
Alkalinity problems are more pronounced in clay soils than in loamy, silty or sandy soils. The clay soils containing montmorillonite or smectite (swelling clays) are more subject to alkalinity problems than illite or kaolinite clay soils. The reason is that the former types of clay have larger specific surface areas (i.e. the surface area of the soil particles divided by their volume) and higher cation exchange capacity (CEC).
Note: Certain clay minerals with almost 100% 'exchangeable sodium percentage' (ESP) (i.e. almost fully sodium saturated) are called bentonite, which is used in civil engineering to place impermeable curtains in the soil, e.g. below dams, to prevent seepage of water.
The quality of the irrigation water in relation to the alkalinity hazard is expressed by the following two indexes: The sodium adsorption ratio (SAR,)
The formula for calculating sodium adsorption ratio is:
where: [ ] stands for concentration in milliequivalents/liter (briefly meq/L), and { } stands for concentration in mg/L.
It is seen that Mg (magnesium) is thought to play a similar role as Ca (calcium).
The SAR should not be much higher than 20 and preferably less than 10;
When the soil has been exposed to water with a certain SAR value for some time, the ESP value tends to become about equal to the SAR value. The residual sodium carbonate (RSC, meq/L):
The formula for calculating the residual sodium carbonate is:
RSC = [ + ] − [Ca2+ + Mg2+] = {/61 + /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 of bicarbonates (), 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 the leaching factor in the field. 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 for calculating water sodicity.
Soil improvement
Alkaline soils with solid CaCO3 can be reclaimed with grass cultures, compost, waste hair/feathers, organic garbage, waste paper, rejected lemons/oranges, etc. ensuring the incorporation of much acidifying material (inorganic or organic material) into the soil, and enhancing dissolved Ca in the field water by releasing CO2 gas.
Many times salts' migration to the top soil takes place from underground water sources rather than surface sources. 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 applying mulch to the land. Using polyhouses or shade netting (protected cutivation) during summer is also advised to mitigate soil salinity and conserve soil moisture for cultivating vegetables and flowers. Polyhouses filter the intense summer solar radiation in tropical countries to save the plants from water stress and leaf burns.