Salinity: The Water Problem

By Travis Goebel, PAg

Conservation Agrologist

The word salinity is a scary word in many areas of the province. Salinity is widespread in Saskatchewan but the severity varies due to many management and natural factors. A common misconception concerning soil salinity is referring salinity as alkalinity. Saline soils are high in soluble salts whereas alkali soils are low in soluble salts but high in Sodium (Na) and have a pH over 8.5.

Saline soils are formed from the accumulation of salts. There are different ways in which salts accumulate in soil and this will be pointed out in a later section. There are many different types of salt and they vary in ability to create saline soils. Basically, the more soluble a salt is the more it can contribute in forming saline soil. Some common salts are listed in Table 1 along with their corresponding solubility. Gypsum and lime are present in most saline soils but their low solubility indicates they are not as damaging as other salts such as Glaubers's and Epsom salts. In many cases, it is possible to visually see the salts in the soil when they precipitate out of solution. White streaking throughout the soil profile and a white crusting on the surface can be visible. Salts are not always visible. For example, lime is quite difficult to see in the soil. Soil can easily be tested for lime by applying a dilute acid such as HCl to the soil; fizzing and bubbling indicates its presence. The salt content of a soil can be estimated by measuring the electrical conductivity (EC) of the soil (EC is expressed in deciSiemens/meter, dS/m). EC can be measured in a laboratory by preparing a water-soil solution from a sample of soil. A soil is considered saline if the EC is greater than 4 dS/m. There are also hand-held devices available that are much quicker and easier than sending away a soil sample to a laboratory. When using handheld devices, measurements can be obtained quickly in the field. One device used is called an EM 38, which is placed on the surface of the soil and it then immediately takes a reading. It is important to realize that when using the quick field method there are many factors that may skew results. The operator of an EM 38 must know and understand properties of soil such as texture and moisture. The soil texture and soil moisture can affect the reading obtained. For example, the reading from a non-saline sandy soil could measure 15-30 and a measurement from a non-saline clay soil at field capacity 70-80.

Table 1. Solubility of salts in water

There are many visual indicators of salinity in affected areas. The absence of crop or poor crop in seeded areas can be a good indicator that salts are present. Another indicator of soil salinity is the presence of a "Bathtub ring" around sloughs or depressions; this is an area around a slough where it is easy for salts to accumulate under the right conditions. Vegetation is also a good indicator of salinity problems. In areas where salt concentrations are high halophytes, salt-tolerant plants, thrive. Some examples of halophytes, in cultivated land, include: kochia, Russian thistle, and foxtail barley. Red samphire, salt grass, and greasewood are halophytes common on uncultivated land. It is quite common for weeds that are halophytes to become a severe problem and completely take over saline areas of the landscape due to the absence of crop competition. A common symptom of plants affected by salinity is a bluish appearance.

Table 2 shows various crops and their tolerance levels to saline soil. Even though many field crops do not grow well in saline soil there are other cropping options, such as forages, that will grow well. Crop selection is a valuable tool in a salinity management program.

Table 2. Relative Tolerance of Annual Field Crops and Forages

*Crops are in order of increasing salt tolerance
*Conductivity is in dS/m of saturated paste

What happens when salt is present in the soil solution?

It is interesting to know what causes the problem with plant growth when salt is present in the soil solution. When plants take in water, nutrients are also present in the water and are taken up. Plants naturally have salt present in their rooting systems which pulls water into the plant from this difference in osmotic pressure. Salt in the soil solution decreases the osmotic potential of the system and slows or even stops the uptake of water. As the difference in concentration decreases, the osmotic potential decreases. When the concentration of salt in the soil increases and approaches that of the plant attempting to grow, the osmotic potential decreases. As the osmotic potential decreases, the movement of soil solution into the plant decreases. Salt sensitive plants basically perish from water deprivation. The plant will express symptoms of drought even though the soil is saturated with water. The water is present but is unavailable to the plant.

Where do salts come from and how do they get into my field?

This is a common question that is quite simple and will be investigated here. Originally salts came from the weathering of rocks that contain salt. Salinity is seldom produced as a result of the weathering of rocks but rather the redistribution and accumulation of salts. Salt accumulates by water entering the soil at a "recharge area"; this water flows through the soil profile and into aquifers in the bedrock. The water flows through these aquifers accumulating salts into solution, as the water flows through areas that have high concentrations of salt, the salt concentration in the water increases. Eventually, due to bedrock formation, the water in the aquifer is forced close to the soil surface and the water table is elevated. There are different mechanisms that cause an elevated water table. Once the water table is within 2 meters of the soil surface, it is possible for the salt infected water to creep up to the surface by capillary action. The location where the water creeps to the surface is called the discharge area. This upward flow of water, accompanied by evaporation, leaves high concentrations of salt on or near the soil surface. There are two vectors acting on the salt infected water, the upward pull from evaporation and capillary action and the downward force of infiltration. Whenever the net flow is up, a saline soil will result. It is important to realize that any factor that increases downward infiltration in a recharge area or any practice that increases evaporation and decreases downward percolation in a discharge area will increase the potential for having a saline soil.

What causes an elevated water table?

There are three different underground mechanisms that cause an elevated water table. Artesian discharge is where water enters through a recharge area and travels through layers of bedrock to a discharge area in lower lands. The distance from recharge area to the discharge area can be greater than 10 kilometers. There is pressure that forms at the discharge area, this pressure pushes water toward the surface. These areas have a high water table. A good indicator of artesian discharge is the presence of free flowing water wells. The extent to which this causes salinity depends on the pressure, salt content, and the extent of upward water movement. Evaporitic rings occur in low-lying potholes. In these areas it is difficult for the surface water to drain. The combination of failure to drain, high water table, and evaporation causes salinity around the slough, sometimes referred to as a "bathtub ring". Side hill seeps are another mechanism that causes saline soils on the side of hills. For this type of problem, water enters an upland recharge area and travels through the bedrock then is discharged at a side hill. This occurs due to an impermeable layer in bedrock close to the surface. The water is pinched off and forced to exit the system at the side of a hill.

How can saline soils be managed?

The mechanisms of saline soils are important but what is even more important is the management of the infected soil and how to slow the formation of these soils. As mentioned previously, there are two areas of concern of saline soils; recharge and discharge areas. It should be realized that salinity is a water problem not a soil problem. Excess water at the recharge area is what causes most salinity problems. Preventing the accumulation and resulting deep percolation of water to the bedrock is important. Excess water in recharge areas may arise as a result of man made ponding, excess accumulation of snow, excessive summerfallowing, excess annual cropping, and decreased forage and perennial cropping. Control of water accumulation in recharge areas can be established by drainage. Care should be taken when attempting any type of drainage as it may result in causing salinity elsewhere. Continuous cropping or planting alfalfa is a strategy that helps decrease soil water content. Alfalfa does exceptionally well at using up moisture because of its deep rooting system. Summerfallowing should be avoided in recharge area because there is no crop to utilize available soil moisture.

There are also different management strategies for saline discharge sites. The goal of discharge management should not be to remove salts completely, rather decrease the salt concentration in the top 12 inches of the soil. Practicing direct seeding in these areas reduces evaporation and increases deep percolation of water. This is achieved because the trash layer insulates the soil and consequently reduces evaporation. The trash layer also decreases water runoff which increases deep percolation.

Crop selection is required to find a crop that will grow in a saline area. Table 2 shows various crops and their tolerance to salt; there are more extensive lists available and different forage blends available that can be very productive. It is important to have good fertility management for the crops grown to have a better chance of successful stand establishment. When seeding forages into saline soil they should be seeded as shallow as possible and early when the salt concentration may be lower in the top portion of the soil.

There is no magical soil additive that will neutralize the effects of salt on soil. Water management is the key to successful salinity management. Although saline soils are not as easy to manage as healthy non-saline soils it is possible, with proper management, to grow productive crops on saline soil.