Salt Balance & Reclamation of Saline and Alkaline Soils

Salt-affected soils

Salt-affected soils contain soluble salts or exchangeable sodium in amounts that harm crops. In irrigated regions of arid and semi-arid areas, they are a major cause of declining yields and land abandonment.

Causes

• Rising water table (from canal seepage, over-irrigation, no drainage) bringing salty groundwater near the surface; evaporation leaves salts behind.
• Irrigation with saline or sodic water without leaching.
• Poor drainage and low rainfall (salts are not flushed).
• Seawater intrusion and tidal flooding in coastal areas.
• Salts from parent material and natural salt-laden groundwater.
• Deficient management — shallow irrigation, no leaching, no drainage.

Classification (USDA Salinity Laboratory)

Measurements are made on the saturation extract of the soil:

• EC = electrical conductivity of the saturation extract ($\text{dS/m}$ at 25 °C).
• pH (of the saturated soil paste).
• ESP = exchangeable sodium percentage = $\frac{\text{exchangeable Na}}{\text{CEC}}\times 100$.
• SAR = sodium adsorption ratio = $\frac{{\text{Na}}^{+}}{\sqrt{({\text{Ca}}^{2+}+{\text{Mg}}^{2+})/2}}$ (concentrations in meq/L).

(The limits are the standard USDA values; some Indian practice takes pH > 8.2 for sodic soils.)

Effects on crops

1. Osmotic effect — a high salt concentration lowers the soil water potential, so roots cannot absorb water (physiological drought).
2. Specific-ion toxicity — sodium, chloride, boron damage plants.
3. Nutritional imbalance — deficiency of calcium, potassium and micronutrients.
4. Poor soil structure in sodic soils — dispersion of clay, crusting, low infiltration, poor aeration.
5. Reduced germination, stunted growth and lower yield.

Crop tolerance: sensitive — beans, citrus, most fruit trees; moderately tolerant — wheat, maize, rice (at high levels), sorghum; tolerant — cotton, barley, sugar beet, date palm, Sesbania. Yield reduction follows a threshold–slope relationship (Maas–Hoffman): $Y_r=100-b({\text{EC}}_e-{\text{EC}}_t)$.

Salt balance of the root zone

The salt balance of an irrigated field compares the salt that enters with irrigation water and the salt that is removed in the drainage water.

where $V_i$, $C_i$ are the volume and salt concentration of the irrigation water and $V_d$, $C_d$ those of the drainage water leaving the root zone. If the salts removed are less than those added, the soil accumulates salt.

Leaching fraction and leaching requirement

The leaching fraction (LF) is the fraction of the applied water that passes below the root zone to carry salts away:

The Leaching Requirement (LR) is the minimum leaching fraction needed to keep the root-zone salinity at or below the tolerance of the crop:

where ${\text{EC}}_{iw}$ = salinity of the irrigation water and ${\text{EC}}_e$ = the tolerable salinity of the saturation extract for the crop (for a given yield reduction); the factor 5 allows for a concentration factor of about 5 from the drainage water to the saturation extract. (Rhoades' formula.)

The gross irrigation requirement is then ${\text{ET}}_c/(1-\text{LR})$ (for small LR), or ${\text{ET}}_c/(1-\text{LR})$ with ${\text{ET}}_c$ the crop evapotranspiration.