Alkali soils

Alkali soils are clay soils with a relatively high exchangeable sodium percentage, a relative high pH (> 9), a poor soil structure and a low infiltration capacity. Often they have a hard calcareous layer at 0.5 to 1 m. depth.

Akali soils are usually not saline, i.e. the total amount of soluble salts, especially sodium chlorides, is not excessive (ECe < 4 to 8 dS/m). Alkali soils are also called sodic soils. This does not differentiate alkali soils from saline soils. Saline soils have a high sodicity (i.e. a high sodium content that may be higher than that of alkali soils), but they are not alkali soils and have usually a good infiltration capacity.

In literature, the terms sodicity, alkalinity and basicity are not always clearly differentiated. The term "basic" for high pH is ambiguous as readers might think the word is derived from "basis". Therefore many scientists and authors prefer to speak of alkaline instead of basic.

The notion "alkali soil" is not unambiguous in itself, as the term "akali" corresponds to "sodic".

At present, we will have to live with the given definitions until a new classification comes up.

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Causes

The causes of soil alkalinity are natural or they can be man-made.

1. The natural cause is the presence soil minerals producing sodium carbonate (Na₂CO₃) upon weathering.
2. The man-made cause is the application of irrigation water (surface or ground water) containing a relatively high proportion of sodium bicarbonates.

Occurrence

The extent of alkaline soils is not precisely known (Brinkman, 1988) ^[1] .Important research on alkaline soils has mainly occurred in Central Europe, North India (above the Ganges river) and along the Indus River basin of Pakistan, where alkaline soils occur frequently.

Well known research institutes are: Indian Central Soil Salinity Research Institute (CSSRI), Mona Reclamation Research Station, Bhalwal, Punjab Pakistan, and University of Agriculture, Faisalabad, Pakistan.

Agricultural problems

Alkaline soils are difficult to take into agricultural production. Due to the low infiltration capacity, rain water stagnates on the soil easily and, in dry periods, irrigation is hardly possible. Agriculture is limited to crops tolerant to surface waterlogging (e.g. rice, grasses) and the productivity is low.

Chemistry

Reference: Handbook 60 ^[2] .

Soil alkalinity is associated with the presence of sodium carbonates or (soda) (Na₂CO₃) in the soil, either as a result of natu­ral weathering of the soil particles or brought in by irrigation and/or flood water.
The sodium carbonate, when dissolved in water, dissociates into 2Na⁺ (two sodium cations, i.e. ions with a positive electric charge) and CO3⁼ (a carbonate anion, i.e. an ion with a double negative electric charge).
The sodium‑carbonate can react with water to produce carbon dioxide (CO₂), escaping as a gas, and sodium hydroxide (Na⁺OH^–), which is alkaline (or rather basic) and gives high pH values (pH>9). To understand this reaction, we may consider the water (H₂O) as being partly dissociated into H⁺ (hydrogen) and OH^– (hydroxyl) ions. In pure, neutral water, the concentration of H⁺ and OH^– ions equals 10^–0.7 eq/l each (respectively 10^–7 g/l and 17x10^–7 g/l), a very small concentration.

The pH, being the negative log value of the H⁺ concen­tration in eq/l, is 7. Similarly, the pOH is also 7.

Each unit decrease in pH indicates a tenfold increase of the H⁺ concentration. Similarly, each unit increase in pH indicates a tenfold increase of the OH^– concentration.

In water with dissolved salts, the concentrations of the H⁺ and OH ^- ions may change, but the sum of pH and pOH remains equal to 14.

Water with excess H⁺ ions is called acid (pH < 6), and water with excess OH^– ions is called alkaline or rather basic (pH > 8). Soil moisture with pH < 4 is called very acid and with pH > 10 very alkaline (basic).

The reaction between Na₂CO₃ and H⁺O can represented as follows:

2Na⁺ + CO3^{- -} + 2H⁺ + 2OH ^- --> 2Na⁺ + 2OH ^- + H₂CO₃

The acid H₂CO₃ 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.
Not all sodium carbonate follows the above chemical reac­tion. The remaining sodium carbonate, and hence the presence of CO₃⁼ ions, causes CaCO₃ (which is only slightly soluble) to precipitate as solid calcium carbonate (lime­stone). Hence, the calcium ions Ca⁺⁺ are immobilized:

2Na⁺ + CO₃⁼ + Ca⁺⁺ --> 2Na+ + CaCO₃ (solid)

  The presence of abundant Na⁺ ions in the soil solution and the precipitation of Ca⁺⁺ 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, see figure) and, in exchange, release Ca⁺⁺, by which their exchangeable sodium percentage (ESP) is increased as illustrated in the figure:

Na⁺ is more mobile and has a smaller electric charge than Ca⁺⁺ so that the thickness of the DAZ increases as more sodium is present. The thickness is also influenced by the total concentration of ions in the soil moisture in the sense that higher concentrations cause the DAZ zone to shrink.

Clay particles with considerable ESP (> 16), in contact with non-saline soil moisture have an expanded DAZ 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-water-logging or runoff is increased. Seedling emergence and crop production are badly affected.

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 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).

Certain clay minerals with 100% ESP (i.e. 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.

Reference for the description of the DAZ (officially called diffuse double layer): ^[3] .

Solutions

Reference : Chhabra ^[4]

Alkaline soils with solid CaCO₃ can be reclaimed with grass cul­tures, ensuring the incorporation of much acidifying organic material into the soil, and leaching of the excess sodium. Deep plowing and incorporating the calcareous subsoil into the top soil also helps.

It is also possible to reclaim alkaline soils by adding acidifying minerals like pyrite.

Alternatively, gypsum (calcium sulphate, CaSO₄. 2H₂O) can also be applied as a source of Ca⁺⁺ ions to replace the sodium at the exchange complex. 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 of rain and/or irrigation water through the soil profile.

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 the soil structure. The treatments, however, need to be repeated in a few (say 5) years time.

It will be important to refrain from irrigation with poor quality water.

The quality of the irrigation water in relation to the alkalinity hazard is expressed by the following two indexes:

1) The sodium adsorption ratio (SAR)

                    [Na⁺]                         {Na⁺/23}
SAR = ───────────── = ──────────────
           √[Ca⁺⁺/2 + Mg⁺⁺/2]     √{Ca⁺⁺/40 + Mg⁺⁺/24}

where: [] stands for concentration in 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.

2) The residual sodium carbonate content (RSC, meq/l):

RSC = [HCO₃^– + CO₃⁼] ‑ [Ca⁺⁺+ Mg⁺⁺]

        = {HCO₃^–/61 + CO₃⁼/30} ‑ {Ca⁺⁺/20 + Mg⁺⁺/12}

which must not be much higher than 1 and preferably less than 0.5.

Note that the above expression recognizes the presence of bicarbonates (HCO₃^–), the form in which most carbonates are dis­solved.

Leaching sodic saline soils

Saline soils are mostly also sodic (the predominant salt is sodium chloride), but they do not have a very high pH nor a poor infiltration rate. Upon leaching they are usually not converted into a (sodic) alkali soil as the Na⁺ ions are easily removed. Mostly, the saline (sodic) soils do not need gypsum applications for their reclamation. (See the Chacupe [case study])

References

1. ^ R.Brinkman, 1988. Saline and sodic soils. In: Land Reclamation and Water Management, ILRI publication 27, p.62-68, International Institute for Land Reclamation and Improvement, Wageningen, The Netherlands. ISBN 90 70 26062 1
2. ^ http://www.ars.usda.gov/Services/docs.htm?docid=10158 US Salinity Lab Handbook 60
3. ^ G.H.Bolt (ed.), 1981. Soil chemistry: A. basic elements. Vol 5a, Elsevier, Amsterdam, The Netherlands
4. ^ Chhabra, R. 1996. Soil Salinity and Water Quality. pp 284. Oxford&IBH Publishing Co. Pvt. Ltd., New Delhi (South Asian edition) and A.A. Balkema Uitgevers BC, Rotterdam (edition elsewhere). ISBN 81 204 1049 1.