Alkali Soils - Chemistry

Chemistry

Soil alkalinity is associated with the presence of sodium carbonate or washing 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 CO₃2- (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>8.5).

Notes:

• Water (H₂O) is partly dissociated into H+ (hydrogen) and OH– (hydroxyl) ions. The ion H+ has a positive electric charge (+) and the hydroxyl group OH– has a negative charge (–). In pure, neutral water, the concentration of H+ and OH– ions equals 10–7 eq/l each (respectively 10–7 g/l and 17x10–7 g/l), a very small concentration.
• 1 eq = 1 equivalent weight corresponds to as many grams of the substance as its molecular weight divided by its valence. It is also known as gram-molecule or mole per unit of valence. For the hydrogen ion (H+ ) the molecular weight equals 1, and for the hydroxyl group (OH–) it equals 17. As they are both monovalent (or univalent) their equivalent weights are the same. Substances with the same equivalent weight have equal positive or negative electric charge.
• In neutral water, 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+ y OH– ions may change, but their sum remains constant, namely 7 + 7 = 14. A pH of 7 therefore corresponds to a pOH of 7, and a pH of 9 with a pOH of 5.
• Formally it deserves preference to express the ion concentrations in terms of chemical activity, but this does hardly affect the value of pH.
• 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 H₂O can be represented as follows:

• 2Na+ + CO₃2- + 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 CO₃2- ions, causes CaCO₃ (which is only slightly soluble) to precipitate as solid calcium carbonate (limestone). Hence, the calcium ions Ca++ are immobilized.

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, officially called diffuse double layer) 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.

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% 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:

1) The sodium adsorption ratio (SAR, )

The formula for calculating sodium adsorption ratio is:

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

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.

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

The formula for calculating residual sodium carbonate is:

RSC = ‑

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

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

The above expression recognizes the presence of bicarbonates (HCO₃–), 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 in to 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.