Sodic soils have a high level of exchangeable sodium (Na+) which can have a negative effect on soil quality, crop growth and yield. These soils often develop on naturally occurring high-sodium glacial till parent materials. Soil enrichment of sodium by groundwater movement can also cause sodic soil development. Sodic soils often have very poor soil physical structure with a hard, cloddy structure and high sodium concentration causes toxicity to sensitive plants. Sodic soils occur mostly in Alberta and Saskatchewan as Solonetzic soils.
Sodic soils are one of three types of salt affected soils: sodic soils, saline soils and saline-sodic soils.
Diagnosing saline and sodic soils
To determine if a soil is saline or sodic, laboratory soil testing is required. The first step is taking composite samples of about 20 soil cores at zero to six inches, six to 12 inches, and 12 to 24 inches in depth from the area of concern in the affected field. For comparison, also take samples from unaffected areas, to assist with diagnosing the problem soil. Send the soil samples to an accredited laboratory — follow their procedures for handling and shipping.
Ask the lab to do an Electrical Conductivity (EC) soil test, which is used to determine the level of soluble salts in the soil sample. This will indicate if a soil is saline. EC is given in units of decisiemens per metre (dS/m). The classification levels of saline soils are shown in the table.
The Sodium Adsorption Ratio (SAR) is the laboratory soil test commonly used to determine the sodium level in soil. SAR is the ratio of the concentration of sodium cations in soil compared to the combined concentration of calcium (Ca++) and magnesium (Mg++) cations in soil. For interest, the formula used to calculate SAR is:
A soil SAR value below two is most desirable. A soil SAR level above 13 is considered very high, and the soil is classified as sodic (Figure 1). It is important to note, that sodium can cause soil structure deterioration and water infiltration problems at SAR values as low as four to five. When I see an SAR of >4, I am concerned but when the SAR is >13, sodium is a very serious problem in soil.
Exchangeable Sodium Percentage (ESP) is used by some labs to determine sodium levels in soil. An ESP of more than 15 per cent is considered the value for a soil classified as sodic, indicating that sodium occupies more than 15 per cent of the soil cation exchange capacity (CEC). However, sensitive plants can show toxicity injury or poor growth at an ESP of four to eight per cent.
Sodic Soil Structure
Solonetzic soils are characterized by a high level of sodium in the B soil horizon (the layer of soil below the top soil or A horizon. The B horizon may also have an accumulation of clay leached down from the A horizon into the B horizon.
The presence of a sodium-enriched B soil horizon causes unique physical and chemical characteristics that cause agronomic problems for crop production:
- poor physical soil structure that reduces crop emergence, root growth and water infiltration;
- toxic levels of sodium in the soil that reduce plant growth; and,
- very high soil pH.
The poor, cloddy soil structure is caused by the high levels of sodium in the soil. It occurs when positively charged sodium ions are unable to satisfy the negative charge of clay particles. This situation results in dispersion or repelling of clay particles when soils are wet. This process is called “deflocculation” of soil particles. When the soil dries, the result is a very hard, massive, cloddy soil with a columnar structure. The poor soil structure makes cultivation and seeding operations very difficult. These soils tend to crust after precipitation resulting in reduced crop emergence.
The hard B soil horizon will restrict root growth and penetration. Reduced crop emergence and poor root penetration will reduce crop growth and production. Sodic soils often have a relatively high clay content (> 30 per cent). When soils are wet, they tend to seal up as a result of the deflocculation process, causing very slow water infiltration into soil.
Reduced water infiltration is caused by the dispersion of clay particles and reduced soil pore size, which can lead to water runoff and soil erosion problems with sloping soils. When runoff occurs, the amount of water stored in the soil is reduced, which further limits crop yield potential. The increased sodium level causes soil pH to be very high, often above 8.4. A soil pH above 8.4 often indicates a sodic soil problem. A soil with a high pH is correctly referred to as “alkaline.”
However, the term “alkali ” is often incorrectly used to describe surface soils that appear white and are high in soluble salts. Soils high in soluble salts are correctly referred to as “saline soils.”
The term “black alkali” is sometimes used to refer to sodic soils. Sodic soil clods may have a somewhat black sheen or appearance caused by a coating of sodium enriched organic matter.
This briefly summarizes the physical and chemical characteristics of sodic soils. In the next issue, I’ll discuss the options for managing sodic soils.