Alkaline soil management in Wyoming
By Jeremiah Vardiman
Most of Wyoming has alkaline soils, often times referred to as alkali soils. Because of the misuse of this term, alkali soils are typically considered salt crusted, unvegetated spots. Alkaline soil is a broader term which includes calcareous, saline, sodic and saline-sodic soils. These soils naturally occur in areas with annual rainfall less than 20 inches per year, which occurs over 40 percent of the Earth’s land surface.
The characteristics of alkaline soils include pH levels greater than 7.0, which is caused by the buildup of water-soluble salts of potassium, calcium, magnesium, sodium and other cations. These salts accumulate due to the lack of precipitation to leach them deeper in the soil profile.
Today, these soils can be found in native rangelands, which include grasslands and shrub-steppe ecosystems, as well as productive irrigated and dryland cropland. Understanding alkaline soils helps producers determine when the soils could be limiting production and when remediation methods are needed. Most remediation efforts focus on planting tolerant crops which acts as a buffer in soil pH and increases soil infiltration to allow salts to leach deeper.
To better understand these soils, we must explore the types of alkaline soils which occur in Wyoming. The first is calcareous soils, which are soils that contain measurable amounts of calcium carbonate, better known as lime.
Calcareous soils are not directly harmful to plant productivity, unless they form root-limiting layers in the soil which are caused by extremely high levels. Estimating calcium carbonate is a quick and easy field test commonly called a “fizz test.” The fizz test observes the reaction of one or two drops of dilute hydrochloric acid on a soil sample – the more fizzing or effervescence the more calcium carbonate.
The largest concern for calcareous soils is the unavailability of phosphorus and the possibility of soil absorbing expensive fertilizers before plants can utilize them. The higher the soil pH is, the more abundant calcium there is and phosphorus becomes unavailable to the plant due to calcium phosphorus minerals being formed.
The application of elemental sulfur can be applied to reduce the soil pH, however to make large changes in soil pH, this is expensive and impractical because the change is buffered by the abundant amount of cations in the soil resulting in no change in the long term. Other approaches are planting alkaline tolerant crops and carefully managing phosphorus and micronutrients, which can be done by banding phosphorus fertilizers within the rooting zone and making annual applications.
An additional management technique would be increasing soil organic matter and improving soil water infiltration. This method is slow and requires a long-time commitment.
The next type is saline soils. These soils are high in salts with lower sodium cations. Salt levels exceed four decisiemens per meter (dS/m) when measured with electrical conductivity (EC) in the soil solution and less than 15 dS/m for exchangeable sodium percentage (ESP). Salt levels can be recorded in the field with an electroconductivity meter or in a laboratory. Exchangeable sodium percentage is only measured in the lab.
These soils are found in regions where evapotranspiration exceeds precipitation and may accumulate to the point where white salt deposits are on the soil surface. High salt accumulation is detrimental to plant growth by causing physiological drought, which means water is held tighter to the soil particles and decreases the availability to the plant, even though there may be adequate soil moisture. These conditions, along with ion imbalances force the plant to expend more energy to maintain normal growth and development.
The management strategies for saline soils are to select crops tolerant of salts, reduce upward movement of salts by slowing evaporation and increasing downward movement of water through the soil profile.
There is wide variety of salt tolerance in crops. For example, beans can grow at an EC level of one dS/m – a low tolerance – whereas barley can grow at eight dS/m. Increasing ground cover, crop residues or applying organic mulches can reduce evaporation rates while also increasing infiltration rates in the soil and leaching of salts. Be aware of other compounding factors which can counteract these management strategies, such as irrigation waters high in salts and organic amendments like manure that can contain high salts.
Sodic and saline-sodic soils
Sodic and saline-sodic soils occur naturally in Wyoming, specifically in the desert basins. Sodic soils have an exchangeable sodium percentage greater than 15 and salt less than four dS/m, while saline-sodic soils have both high characteristics, ESP greater than 15 and salts greater than four dS/m.
The high sodium levels in these soils destroys the soil structure, which impedes water movement. High sodic conditions are toxic to plants and the high sodium can cause leaf-edge necrosis. If these soil types are found in areas of shrink-swell clays, impervious areas are formed and slick spots are created on the soil surface.
To improve these soil conditions, chemical amendments – gypsum and langbeinite – need to be added to displace the sodium cations with either calcium or magnesium. These amendments improve soil structure and reduce sodium levels by allowing salts to be leached from the soil.
Saline soils and sodic-saline soils are also calcareous, which can result in the salt levels rising to temporarily harmful levels prior to them being leached out. If these soils are calcareous, adding elemental sulfur can also be utilized with other amendments to temporarily change the soil pH, allowing micronutrients to become available to plants.
Increasing organic matter, soil infiltration and soil structure provides the best long-term results for management of alkaline soils. Knowing what type of alkaline soil aides in knowing the best management strategies to achieve those longer results. For more detailed information and testing of alkaline soils, pick up a copy of the University of Wyoming Extension’s bulletin Alkaline Soils in Wyoming, B-1358 and bulletin Reclamation on Salt/Sodium-affected soils, B-1231.
Jeremiah Vardiman is a University of Wyoming Agriculture and Horticulture Extension educator. He can be reached at firstname.lastname@example.org.