Restoration of Alkaline Soils
We are going to discuss soil restoration in the context of reclaiming alkaline soils. This is certainly one of the major challenges we face today, particularly in the western United States.
Factors that Contribute to Soil Alkalinity
Typically soils in the arid regions contain less clay. Clay containing soil contains numerous negatively charged exchange sites for cation nutrients. Therefore when there is little or no clay that means that there are fewer sites in the soil to hold hydrogen (H⁺) ions.
Consequently there is a far less chance of the soils being acidic or neutral. And with less rainfall to incorporate H⁺ ions into the soil there is little chance for any Acid:Base reaction to occur at all in the soil.
Irrigation water further compounds the problem by adding more alkaline salts.
Water evaporation rates are so high in arid regions that it causes the hydroxyl (OH⁻) ions to combine with alkaline earth metals such calcium, magnesium, iron, zinc, and manganese. The compounds formed are virtually insoluble in water, creating deposits on the surface.
Problems Associated with High pH Soils
Alkaline soils typically have the following characteristics and impacts:
- Accumulation of excessive salt in irrigated soils can reduce crop yields, reduce the effectiveness of irrigation, ruin soil structure, and other soil properties.
- Common salts may include calcium, magnesium, sodium, potassium, sulfate, chloride, carbonate, and bicarbonate.
- High exchangeable sodium, high pH, and low calcium and magnesium disperse clay soils.
- Generally when soil pH is above 8.5 it indicates sodium problems.
- Infiltration through the surface and root zone can be severely impacted in alkaline soils.
- Drainage in the soil is required to allow the alkaline salts to leach below the rootzone.
- Since saline soils increase the osmotic potential of soil water, plants have difficulty absorbing water.
Bicarbonates (HCO₃⁻ ) can be common in irrigation water. The presence of substantial amounts of bicarbonates will cause the calcium to precipitate, especially in high pH and low moisture soils. What happens? Sodium increases relative to the available calcium.
It is worth noting that bicarbonates are toxic to plant roots, reduce plant growth, and inhibits phosphorous and micronutrient uptake.
The Effects of Alkaline Salts on Soil Structure
Management of alkaline soils is a challenge, because the salts affect plant growth, soil structure, and water percolation. Salts can be leached through the profile by irrigation. The problem is that if the irrigation water contains more than 50 ppm of sodium, it could impact soil structure. This is dependent upon how much free calcium (Ca²⁺) and magnesium (Mg²⁺) ions are in the water or soil.
Sodium Absorption Ratio (SAR):
Soil structure depends on the balance of Ca²⁺and Mg²⁺relative to sodium (Na⁺). Soil particles aggregate when Ca²⁺and/or Mg²⁺ions are increased relative to Na⁺thereby decreasing the sodium absorption ratio (SAR). However, soil particles disperse if the concentrations of [Ca²⁺] + [Mg²⁺] decrease relative to the concentration of [Na⁺].
SAR is the expression of the relative amount of sodium ions within the soil. Mathematically this expressed as SAR = [Na⁺] / [Ca²⁺] + [Mg²⁺] units expressed in mmoles / liter.
The Role of Calcium and Magnesium:
Therefore the soil restoration protocol that would involve improving these soils is by releasing Calcium (Ca²⁺) and Magnesium (Mg²⁺) ions into the soil solution. Why? Because these cations join together negatively charged clay soil particles forming water stable aggregates.
This is great except for one problem. The calcium and magnesium in high pH soils are locked up thus preventing them from performing its “magic”. The base cations are tied up as insoluble salts.