In the last issue of Grainews I discussed the physical and chemical characteristics of sodic soils. In this issue, I’ll discuss managing those soils.
Solonetzic soils in the brown or dark brown soil zones of southern Alberta or southern Saskatchewan, that are in native grassland may be best left in their native condition and used for carefully managed livestock grazing. For more information about improving or reclaiming Solonetzic soil, refer to Alberta Agriculture Agdex 518-8 Management of Solonetzic Soils available at Alberta Agriculture’s website.
Sodic soils have limited crop production potential. The relatively high sodium and pH levels restrict the growth and yield potential of most annual crops and even a number of forage crops. Farmers with sodic soils are faced with decisions on how to best manage their land.
Unfortunately, the options are limited.
1. Leave it alone
Option 1: Leave it in its native state.
Sodic soils in native grassland are often best left in their native state and utilized for carefully managed livestock grazing.
2. Work with it
Option 2: Grow sodium-tolerant crops.
Barley is the most tolerant annual crop to sodium, and crops with moderate sodium tolerance are wheat, oat and rye. However, none of these crops will be very productive at SAR levels above eight to 12. The crops most sensitive to sodium toxicity are pulse crops, including pea and lentil; therefore, these crops should not be grown on soils with moderate to higher levels of sodium.
A good option is to establish a sodium-tolerant forage mixture. The most sodium-tolerant forages include various wheat grasses and alfalfa. Fescue grasses have moderate sodium tolerance. Soils that have higher levels of sodium are probably best seeded to a tolerant grass mixture and used for livestock grazing.
3. Reclaim it
Option 3: Reclaim sodic soils.
Reclamation or improvement of sodic soils can be very expensive, and improvement will take time. Reclamation involves careful soil sampling and analysis to determine the severity of the problem and then calculating how much calcium must be added to the soil. Often, several tons of a calcium product must be applied and well incorporated into the soil to modify the sodic condition.
To improve a sodic soil, most of the exchangeable sodium must be removed by leaching it downward, below the root zone. To accomplish this result, the sodium on the soil exchange complex must be replaced by calcium. If free lime (calcium hydroxide) is present in the soil (determined by a soil test), applying elemental sulfur will reduce the soil pH, which will also dissolve the calcium hydroxide that naturally occurs in the soil, to free up the calcium and the calcium will displace the sodium, for gradual soil improvement.
If free lime is not present in the soil, calcium must be added with application of a chemical soil amendment. Soil amendments are calcium-containing materials such as gypsum (calcium sulphate) or calcium chloride. Calcium carbonate is normally not recommended as an amendment due to its lower solubility. Calcium carbonate is ideal for improvement of acidic soils but not for sodic soils.
The calcium amendment is normally broadcast onto the soil surface, followed by thorough incorporation with cultivation. Then, adequate moisture is necessary to dissolve the calcium to initiate the displacement of sodium from the soil exchange complex.
This takes considerable time (many months to years) for natural precipitation to leach the sodium from the root zone. In drier regions, this process could take a number of years and may not be completely successful. However, greater success can result with the application of significant amounts of good quality irrigation water to leach the sodium from the root zone.
Additional organic matter such as livestock manure or green manure will help to improve the physical soil condition (e.g. tilth and water infiltration). However, care must be taken to ensure that any material added to the soil (e.g. manure) does not contain sodium.
The amount of a chemical amendment used to replace the exchangeable sodium in soil will depend on the amount of sodium in the soil, the desired level of soil improvement, the type of amendment used and the volume of soil to be reclaimed. If chemical amendment application is considered, a landowner should work with a qualified agrologist with a specialty in soil science. To learn how to determine gypsum application rates refer to: Alberta Agriculture Agdex 518-20 Management of Sodic Soils.
Once the amendment has been applied, it must be thoroughly mixed into the soil by cultivation. The soil must be very moist for the exchange process to take place. The sodium must then be leached down the soil profile by rainfall or by the application of good quality irrigation water. Short, frequent irrigations will give the best results.
If a sub-surface hard pan layer is preventing internal soil drainage, the soil may have to be deep-ripped to break up the hard pan to allow the sodium to be flushed downward.
The installation of a sub-surface drainage system may be necessary to permanently move the sodium salts from the soil root zone; however, drainage systems are expensive and safe disposal of the high sodium drainage water is essential. Your provincial Environment Department must be contacted prior to installation of a subsurface drainage system to determine the process of approval and licensing of the drainage system.
The amendment of sodic soils can be a long and laborious process and is not always successful or permanent. The occurrence of large rainfall events can cause the water table to rise, moving the sodium salts back into soil root zone, and leading to deflocculation of the surface soil. Therefore, the management of a sodic soil must be considered as an ongoing process.
It is important to note that calcium-containing amendments should never be added to saline or saline-sodic soils because this will only increase the amount of soluble salts in the soil and worsen the salinity problems.
If you are getting technical assistance from a consultant, be sure that person has proper soils expertise to assist you.