Alkaline soils in Wyoming
Published on Feb. 8, 2020
Alkaline soils occur where the amount of potential evaporation is greater than the average annual rainfall, which includes almost everywhere in Wyoming except higher elevations. “Alkaline” refers to pH over 7.0, in the basic, not acidic, range. As soil pH approaches 8.0 or higher, which is common in Wyoming, the availability of some plant nutrients is constrained and special management strategies might be beneficial.
Alkaline soils include many soil types across a broad range of potential productivity and are naturally nutrient rich because weathering rocks and sediments release phosphorus, potassium, calcium, magnesium, sulfur and micronutrients, though availability can be constrained.
In contrast to areas with higher rainfall, there is not enough moisture to wash the nutrients out of the root zone. They accumulate below the soil surface tied up in soluble salts. While naturally alkaline soils underlie productive rangelands and croplands, as well as fragile desert ecosystems, disturbances from development or intensive land use can increase near-surface salinity to detrimental levels.
Moisture from rainfall and snowmelt in semiarid regions penetrates only the top two or three inches of soil, leaching soluble salts, reducing pH and forming a thin topsoil or A horizon that is crucial to plant establishment and growth. But it’s easy to disturb, difficult to restore and overlays saltier subsoils.
Tillage, irrigation, erosion, heavy grazing, road building, energy development and other activities that disturb the soil surface or alter hydrology can increase surface salt concentrations.
High concentrations of potassium, calcium, and magnesium salts limit the ability of plants to take up water, while sodium salts cause soil aggregates to disintegrate, or disperse, plugging pores, forming hard crusts, and limiting water infiltration and movement.
Identification and management of soils with high concentrations of soluble salts is important for maintenance and restoration of sustainable crop and forage production and wildlife habitat management in arid and semiarid regions. This article discusses different types of alkaline soils and their management. It summarizes a University of Wyoming Extension Bulletin that will be released soon.
Calcareous soils contain calcium carbonate (CaCO3), which is also known as lime, caliche or calcite, and they occur almost everywhere in Wyoming. Calcium carbonate is relatively stable in the soil and accumulates below the long-term leaching limit of water, typically six to 12 inches deep.
Soils that are strongly calcareous have visible chalky filaments and concentrations, and the presence of those at the surface can indicate erosion and loss of the A horizon.
Calcium carbonate does not directly harm plant productivity, but can interfere with availability of phosphorus, which is a big concern for Wyoming crop producers.
The notion that the soil contains a great amount of phosphorous (P), and even adsorbs expensive fertilizers, is frustrating to farmers. Unfortunately, there is not an easy solution. The large amount of CaCO3 makes acidification expensive and impractical.
A better approach is to plant crops tolerant of alkaline conditions, those with a pH around 8.0, and to carefully manage P and micronutrients by frequently testing the soil. In calcareous soils, P is available to the plant for a very short time compared with neutral or slightly acidic soils. Banding P fertilizer is also a good approach because it places P where it is accessible to growing roots and creates less contact with the soil than broadcasting.
Increasing soil organic matter content with compost or conservation tillage can dilute the effects of CaCO3 and improve soil water infiltration so that, over time, CaCO3 can be moved below the surface, improving P and micronutrient availability during germination and establishment. Because of the low solubility of CaCO3 relative to other salts, changes to soil pH and structure are slow and require long time commitments.
Saline soils contain enough soluble salts to reduce growth in most plants by increasing the tension with which water is held by soil particles and decreasing its availability to plants, though both range plants and crops have broad ranges of tolerance.
Salts may accumulate as white salt deposits during periods of soil water evaporation and were formerly called “white alkali.” Chlorosis in older leaves and stunting are early signs of salt stress. It pays for agricultural producers in saline conditions to select salt tolerant crops.
Management of saline soils requires two strategies: reducing upward soil water movement (evaporation), or increasing downward soil water movement (leaching).
Maintaining or restoring soil cover and healthy soil surface horizons supports both strategies. Abundant plant residues left on the surface, with careful grazing management or limited tillage, acts as mulch that reduces evaporation.
It also adds organic material to the A horizon, which improves soil structure and water infiltration, so soluble salts can be carried to deeper depths.
Sodic and saline-sodic soils
Sodic and saline-sodic soils occur naturally in landscape depressions in desert basins in Wyoming, especially those underlain by sodium-containing marine shale parent materials. Soils a sodium-rich horizon, often within 2 to 6 inches of the soil surface.
The thin A horizon over the sodium-rich horizon is very important for plant germination and establishment, and is easily destroyed by erosion or disturbance. Changes in surface or groundwater hydrology, such as from irrigation return flows or disruption of natural drainage by roads or trails, can increase surface-soil sodium.
Sodium is especially problematic because it tends to disperse soil aggregates, forcing particles apart and destroying soil structure.
Management of sodium in the soil requires reducing the ratio of sodium relative to calcium, magnesium, and potassium. Amendments like gypsum can increase calcium content, which causes soil particles to form aggregates, improving water movement so that excess sodium can then be leached deeper into the soil. Unless abundant, low-salt, irrigation water is available, this is a very slow and difficult process.
Jay Norton is an Extension Soils Specialist at University of Wyoming. For more information on alkaline soils, contact Norton at firstname.lastname@example.org or see sources posted online at soilmanagement.wordpress.com.