Understanding Saline and Sodic Soils

by Jim Bauder

Salinization of soils is common in Montana. Although this process occurs naturally, without careful management of irrigated and dryland soils, it is possible for salinization to increase. At present, more than 280,000 acres of land in Montana are characterized as sodium or salt affected. Understanding saline and sodic soils, their causes, and management will help land managers reduce future salinization.

Where does salt come from?
All waters and parent rock contain some salts, the amount of of which depends on several factors, the most important being parent material, the conditions under which the soil formed, the drainage of the soil, and the predominant weather conditions. The term saline refers to more than just sodium or chloride. Ions contributing to salinity include magnesium, calcium, carbonate, bicarbonate, and sulfate. As water evaporates from a soil surface or is used by plants, the salts in the water are left behind. This causes salt to accumulate in the soil. If this salt accumulation is not balanced or offset by downward leaching, due either to rainfall or irrigation, salinity will occur. If the predominant ion is sodium, then the soil can also be sodic.

Where do saline and sodic soils occur in Montana?
The most common locations to find saline soils are in the eastern and central parts of Montana and in poorly drained areas north of the Missouri River. Naturally saline soils are found along many stream terraces and bottoms, while saline seeps can be found throughout most of the glaciated plains region. Sodic soils are unlike saline soils, although they occur in many of the same locations and can form together. Sodic soils are most common in eastern and north central Montana and along irrigated flood plains of many rivers.

     Saline soils contain excess soluble salts, which make it difficult for plants to take up water and nutrients. A saline soil (see Table 1) has an electrical conductivity (EC) more than four mmhos/cm. Saline soil causes spotty bare areas in a crop field, due to poor emergence. In severe cases, the soil will have a white residue at the surface. Irrigated saline soils can be improved by leaching and good drainage. Dryland saline seep areas can be reclaimed by planting deep rooted perennials such as alfalfa, sweet clover, and grasses in the recharge areas.

     Sodic soils contain excess exchangeable sodium; this sodium is not harmful to plants, but it does make fine textured soil extremely impermeable to water and difficult for roots to penetrate. Sodic soils have an exchangeable sodium percentage (ESP) more than 15% or a sodium adsorption ratio (SAR) more than 12. Sodic soils generally occur as localized pan spots. The subsoil of sodic soils is usually very compact, moist, and sticky and is composed of soil columns with rounded caps. To improve sodic soils, the sodium must be replaced with calcium and the sodium leached from the soil. Hence, it is not possible to reclaim a sodic soil without good drainage. The sodium can be replaced by adding calcium in the form of gypsum or calcium chloride or by adding materials which will release the calcium already present (sulfur, sulfuric acid, organic matter).

     Saline sodic soils have both excess soluble salts and exchangeable sodium. To improve these soils, amendments and drainage are essential. Leaching a saline sodic soil without amendments will result in a sodic soil and may worsen the soil structure.

Table 1. Conditions of saline, sodic, and saline-sodic soils.
Soil condition
EC (mmhos/cm)
ESP (%)
SAR
Saline
<4
0-15
0-12
Sodic
0-4
>15
>12
Saline sodic
>4
>15
>12
Non-saline, non-sodic
0-4
0-15
0-12

     Things to remember: Saline soils: the problem is lack of available water to plants and toxicity; the solution is leaching and improved drainage. Sodic soils: the problem is poor soil structure, drainage, and impermeability; the solution is addition of soil amendments and improved drainage. Saline sodic soils: the problem is lack of available water, and poor drainage; the solution is addition of amendments, leaching, and improved drainage.

     How do I know if the problem is there? Often the problem is obvious. The presence of a permanent or seasonal high water table will often be a sign of saline or sodic soils. Poorly drained potholes in glacial landscapes often have localized areas that have temporary high water tables. Excess soluble salts will often crystallize on the surface of fallow fields. Thick continuous crusts form in saline seeps. Thin patchy salt crusts will form under clods or on the shady side of clods where marginal salt problems are found. Patterns of growth in cropped fields will be poor, spotty stand establishment. Saline soils tend to inhibit germination and emergence of cereal grains. Under severe salt stress, herbaceous crops appear bluish green; leaf tip burn and die off of older leaves in cereal grains can result from salinity or related drought stress.
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