Saline, Sodic, and Saline-Sodic Soils
Management of Saline Soils
Some of the options available to growers in managing saline soils include leaching the soluble salts below the root zone, drainage, irrigation timing, adopting various furrow bed configurations, frequent irrigations, planting, residue management, and salt-tolerant crops.
Leaching
Soluble salts that accumulate in soils may be driven below the root zone by temporary leaching. Leaching is the basic management tool for controlling salinity. No chemical amendment is needed here. Water is applied in excess of the total amount used by the crop and lost to evaporation. The strategy is to keep the salts in solution and flush them below the root zone. The amount of water needed is referred to as the leaching requirement or the leaching fraction. The salt content of the soil, salinity level to achieve, depth to which reclamation is desired, how the water is applied, and soil characteristics are important factors determining the amount of water needed for reclamation.
Leaching Methods
Continuous Ponding. Continuous ponding involves the application of large volumes of standing water until enough salt has been removed from the root zone. The final desired salinity depends on the salinity tolerance of the crop. The actual amount of water required depends on soil type and the initial EC.
Intermittent Ponding or Sprinkler/Drip Irrigation. Intermittent ponding or sprinkler/drip irrigation can be used to save water while reclaiming soils. Instead of ponding as a continuous application, several smaller amounts of water can be applied as sprinkler- or drip-irrigation, followed by periods of dry down. The wetting and drying cycles efficiently leach salts from smaller soil pores using one-third to two-thirds less water than continuous ponding.
Timing of Leaching Events
The time interval between leachings does not appear to be critical provided that crop tolerances are not exceeded. Hence, leaching can be accomplished with each irrigation, every few irrigations, once yearly, or even longer depending on the severity of the salinity problem and salt tolerance of the crop. An annual leaching event where water is ponded on the surface is an easy and effective method for controlling soil salinity. Planned annual leaching events might include a post-harvest irrigation to push salts below the root zone to prepare the soil (especially the seedbed/surface zone) for the following spring.
Determining Leaching Requirement
Salts are added to the soil as irrigation water is applied. If no water leaches below the root zone, salts will accumulate and eventually reduce yields as discussed. The amount of water required to control salt and maintain root-zone soil salinity low enough to produce an acceptable yield of a specific crop is the leaching requirement (LR). The leaching fraction is dependent upon the salt load of the applied water and the salt tolerance of the crop.
Estimating Applied Water for a Desired Leaching Requirement
After determining LR, the total amount of water required (WR) by the crop can be estimated by knowing the crop’s evapotranspiration (ET) rate:
Irrigation Strategies
Irrigation Timing
The timing of irrigation is extremely important on saline soils, particularly early in the growing season. Germinating seeds and young seedlings are especially sensitive to salts. Therefore, irrigation should precede or immediately follow planting to move the salts downward and away from the seedling roots. The irrigator can use high-quality water to keep root-zone salinity low during the sensitive early growth stages and then switch to lower-quality water as the maturing plants become more salt-tolerant. Sprinkler irrigation is often used to germinate salt-sensitive vegetable crops such as lettuce.
Furrow Bed Systems
In addition to leaching salt below the root zone, salts can also be moved to areas away from the primary root zone with certain crop bedding and surface irrigation systems. The goal is to ensure the zones of salt accumulation stay away from germinating seeds and plant roots. With a double-row raised planting bed, the seeds are placed near the shoulders and away from the area of greatest salt accumulation.
Frequent Irrigations
Salt concentrations increase in the soil as water is extracted by the crop. Typically, salt concentrations are lowest following an irrigation and higher just before the next irrigation. Increasing irrigation frequency maintains a more constant moisture content in the soil. Keeping soil moisture levels higher between irrigation events effectively dilutes salt concentrations in the root zone, thereby reducing the salinity hazard.
Residue Management
The common saying “salt loves bare soils” refers to the fact that exposed soils have higher evaporation rates than those covered by residues. Crop residue at the soil surface reduces evaporative water losses, thereby limiting the upward movement of salt (from shallow, saline groundwater) into the root zone.
Growing Salt-Tolerant Crops
Although several treatments and management practices can reduce salt levels in the soil, there are some situations where it is either impossible or too costly to attain desirably low soil salinity levels. In some cases, the only viable management option is to plant salt-tolerant crops. Salt tolerance is not an exact value, but rather depends upon many factors, such as salt type, climate, soil conditions, and stage of plant growth. In general, perennial plants, especially some grass forages, possess the highest tolerance to salts, while legumes are typically the most sensitive to salts.
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