8.3 Saline & Sodic Soil Diagnosis, Reclamation & Management

Saline & Sodic Soil Diagnosis, Reclamation & Management

When poor quality irrigation water is used, or when high water tables bring dissolved salts to the surface, soils can become salt-affected. Managing these soils requires precise diagnosis because treating a saline soil is fundamentally different from treating a sodic soil. Misdiagnosis can severely worsen the problem.

Diagnosing Salt-Affected Soils

Laboratory analysis of soil samples is required to classify salt-affected soils. The standard procedure involves creating a saturated soil paste extract and measuring specific parameters.

Key Diagnostic Parameters

1. Electrical Conductivity of the Extract (ECe): This measures the total concentration of soluble salts in the soil. It is measured in dS/m (or mmho/cm). Higher ECe means higher salinity.
2. Exchangeable Sodium Percentage (ESP): This measures the percentage of the soil's cation exchange capacity (CEC) that is occupied by sodium.
   • Formula: $ESP = (Exchangeable Na / Total CEC) * 100$ (using meq/100g)
   • Sometimes SAR of the soil extract is used as a proxy for ESP, as they are highly correlated.
3. Soil pH: The acidity or alkalinity of the soil. Sodic soils often have very high pH values due to the presence of sodium carbonates.

Soil Classification

Based on these parameters, soils are classified into three distinct categories:

Understanding the Impacts

Saline Soils ("White Alkali")

Saline soils have an excess of soluble salts (chlorides and sulfates of calcium, magnesium, and sodium).

• Visual Symptoms: Often exhibit a white salt crust on the surface when dry.
• Plant Impact: High salt concentration increases the osmotic potential of the soil water. This means the plant must expend more energy to extract water. The result is "osmotic stress," where plants show symptoms of drought (stunting, wilting, dark green/blue foliage) even when the soil appears wet.

Sodic Soils ("Black Alkali")

Sodic soils have excessive exchangeable sodium but low total salinity.

• Visual Symptoms: Severe structural degradation. The soil disperses, pores collapse, and water cannot infiltrate, leading to ponding. Organic matter dissolves in the highly alkaline conditions and moves to the surface, leaving a dark, black crust (hence "black alkali").
• Plant Impact: Poor aeration, waterlogging, physical impedance to root growth, and sometimes direct sodium toxicity.

Reclamation Strategies

The approach to fixing the soil depends entirely on its classification.

Reclaiming Saline Soils

Saline soils are the easiest to reclaim, provided you have good drainage.

• The Solution: Leaching. You must apply enough high-quality irrigation water to push the excess soluble salts below the crop root zone.
• Requirement: Internal soil drainage is mandatory. If a hardpan or high water table exists, tile drainage must be installed before leaching can occur; otherwise, you just create a saline swamp.
• Note: No chemical amendments (like gypsum) are needed for purely saline soils.

Reclaiming Sodic Soils

Leaching a sodic soil with pure water will cause the structure to collapse completely, rendering it impermeable. The sodium must be displaced before it can be leached.

• Step 1: Apply Calcium. A soluble calcium amendment must be applied to replace sodium on the exchange sites.
• Step 2: Displace Sodium. The calcium pushes the sodium off the clay particle and into the soil solution.
• Step 3: Leaching. Irrigation water is then applied to leach the newly soluble sodium (often as sodium sulfate) below the root zone.

Gypsum Application: Gypsum (Calcium Sulfate, CaSO4·2H2O) is the most common and economical amendment for sodic soils. It is moderately soluble and provides the necessary calcium. The amount of gypsum required is determined by a soil test and is often massive (several tons per acre).

• Alternative Amendments: Elemental sulfur or sulfuric acid can be used only if the soil already contains free lime (calcium carbonate). The acid reacts with the lime to create soluble calcium in place. If there is no free lime, sulfur/acid will only lower the pH and will not solve the sodic problem.

Reclaiming Saline-Sodic Soils

These soils have both high salts and high sodium. They currently maintain their structure because the high salinity prevents the sodium from dispersing the clay.

• The Danger: If you try to leach a saline-sodic soil with fresh water without adding calcium, you wash away the stabilizing salts while leaving the sodium behind. You will convert the soil into a highly degraded sodic soil.
• The Solution: You must treat it exactly like a sodic soil. Apply heavy rates of gypsum first, and then leach the salts and displaced sodium together.

Management Practices

When complete reclamation is not economically feasible, management practices must adapt:

• Crop Selection: Grow salt-tolerant crops (barley, cotton, sugarbeets, Bermuda grass) and avoid sensitive crops (beans, many fruits).
• Irrigation Frequency: Irrigate more frequently to maintain higher soil moisture. This dilutes the salt concentration, reducing osmotic stress on the plants.
• Seedbed Management: Salts accumulate at the top and center of raised beds as water evaporates. Planting seeds on the sloping side of the bed (where salt concentrations are lower) can improve germination.
• Leaching Fraction: Routinely apply a calculated percentage of water above the crop ET requirement to continually push accumulating salts downward.