A saline patch of soil in a wheat field clearly hinders plant growth. Source: Food and Agriculture Organization of the United Nations
Salt-tolerant species are able to grow in saline soils. Photo credit: Fool-On-The-Hill Flickr CC BY-NC 2.0
When it comes to growing gardens, flower beds, turf, and crops, there are a number of characteristic problems with many soils in the arid west. These problems include high salinity, high pH and low organic matter. This article addresses high-salinity soils and their treatment.
Salinity is a measure of the soluble salts in soil. Salinity is measured in a water-extract of the soil. Saline soils pose a problem for plants. The more saline the soil, the more difficult it is for plants to absorb and extract water from the soil.
Plants can concentrate solutes in their roots to increase water absorption from the soil (via osmosis), but plants cannot compete for water with highly saline soils.
Soils may be saline because of the presence of salts naturally occurring in the soil’s parent material (geologic material out of which the soil formed). They may be saline because of irrigation/watering with high-salt water, including water that has been run through a water softener.
Frequent but short irrigation events can lead to accumulation of salts, as the water never percolates below the root zone, carrying excess salts with it.
Excessive fertilizer application or application of salt-containing organic materials (such as manure and sludges) can create saline soils. Run-off from roads and sidewalks may contain de-icing materials that contribute salts to the soil.
Soil salinity can be tested easily and inexpensively. County Extension Agents can give advice on how to sample the soil and where to have the sample(s) analyzed. Salinity can drastically hamper plant growth, and because it is often readily treatable, soils should be analyzed before planting time, whether in lawns, landscapes, or gardens.
Soil salinity levels below 1 dS/m (deciSeimens per meter) are normal in the arid west. Salinity values above 2 dS/m will cause difficulties for salt-sensitive plants, such as beans, carrots, corn, lettuce, sugar maples, and Scotch pine. Soil salinity levels above 4 dS/m are problematic for a large variety of landscape and garden plants.
There are plants that can tolerate some soil salinity; these plants include tall fescue, buffalograss, bermudagrass, asparagus and beets, among others. Salt-tolerant plants can be grown in lieu of treating moderately-saline soils if those particular plants meet the needs of the grower.
The salinity of soils formed out of saline parent materials, such as some old lake beds, may be impossible to change. The minerals in the soil are inherently high in salts, and as the minerals weather and/or are leached with water, they will continue to release salts into the soil solution. However, for soils that have become saline over time due to reasons stated above, such as improper irrigation, reclamation is feasible.
Most reclamation approaches to treating saline soils involve leaching (flushing) of the soil with clean/relatively pure water. Sufficient water must be applied to dissolve the excess salts that have accumulated and cause them to percolate/flow out of the soil profile, particularly the root zone. To accomplish this leaching of salts, adequate drainage is requisite. Once good drainage is assured, the soil can be irrigated with clean water. Run-off should be avoided to prevent erosion.
The rate of infiltration or flow of water into the soil will determine how quickly water can be applied. The rate will be dependent on the type of soil. Fine-textured soils, such as clayey soils, will have slower infiltration rates than coarse-textured soils. Any restrictive layer, such as a plow pan, will slow the flow of water down through the soil, as will compaction. In all limiting cases, measures must be taken to improve drainage. The rate of infiltration will be faster initially, but will reach a constant rate. Observation and monitoring will be required to achieve leaching of salts while avoiding run-off.
As a starting point, apply 6 inches of water to reduce salinity by 50% and 12 inches of water to reduce salinity levels by 80%. 24 inches of water may need to be applied to reduce salinity levels by 90%. Irrigation via sprinklers is best for sloped areas, but if necessary, flood irrigation may be used on level areas if berms or basins are used to contain the water.
Testing initial soil salinity levels will enable determination of how much water should be applied to reduce salt concentrations to acceptable levels. Post-leaching soil salinity tests will ensure that saline-soil reclamation has been successful.