Introduction

Alkaline soils occur where annual rainfall is less than about 20 inches per year, which includes most of Wyoming and the arid and semiarid West, as well as over 40 percent of Earth’s land surface (Millennium Ecosystem Assessment, 2005). As defined for this bulletin, alkaline soils include calcareous, saline, sodic, and saline-sodic soils that are either naturally occurring or created by various disturbances. They underlie unique and fragile desert, grassland, and shrub‑steppe ecosystems, as well as productive irrigated and dryland crops, but also pose management challenges that may require remediation practices to restore biodiversity and productivity.

Arid and semiarid ecosystems are fragile because plant growth often depends upon very thin topsoils, or A horizons, that are easy to disturb, difficult to restore, and overlay subsoils with higher concentrations of salts. Tillage, irrigation, erosion, heavy grazing, road building, energy

B-1358 | August 2020

Jay Norton

Ecosystem Science & Management, UW College of Agriculture and Natural Resources

Alkaline soils in Wyoming

Formation, ecology, and management of calcareous, saline, and sodic soils

development, and other activities that disturb the soil surface or alter drainage or surface hydrology can increase surface salt concentrations.

High concentrations of potassium (K), calcium (Ca), and magnesium (Mg) salts can limit the ability of plants to take up water, while sodium (Na) 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 bulletin covers the definitions, properties, assessment methods, effects on plant growth, and remediation strategies of alkaline soils, including calcareous, saline, sodic, and saline-sodic soils (Table 1).

Calcareous soils

Calcareous soils contain measureable amounts of calcium carbonate (CaCO 3), also known as lime, caliche, or calcite. Calcareous soils occur in arid and semiarid regions with less than 20 inches of precipitation, and therefore occur almost everywhere in Wyoming except in the mountains. At higher precipitation, CaCO 3 is carried below the rooting zone. Calcium carbonate is not as soluble as other salts, so is relatively stable in the soil and does not accumulate at the surface during hot, evaporative weather, nor does it rapidly leach from the soil with heavy rain or irrigation.

The source of CaCO 3 may be parent materials originating from limestone or limestone alluvium, but it often forms in place as “pedogenic” CaCO 3. Formation of pedogenic CaCO 3 requires Ca (Ca +2), which is supplied by mineral weathering and inputs with dust and precipitation. The Ca combines with carbonate (CO 3 -2) in the soil, which forms when carbon dioxide (CO 2) from plant and microbial respiration reacts with high-pH water. The formation of pedogenic CaCO 3 is a very slow process, but over time it can accumulate to high levels that, on stable surfaces over 1 million years old, can form rock-like, root-limiting petrocalcic horizons (Figure 1). Most calcareous soils are much younger and have a calcic horizon where CaCO 3 accumulates below the long-term leaching limit of water (the long-term average infiltration depth). This is called the Bk horizon and, in Wyoming, typically occurs within 6–12 inches

Figure 1. The surface of a petrocalcic horizon that occurs beneath shallow soils on 2-million year old, flat-topped terraces west of Laramie. With time in semiarid environments, accumulated CaCO 3 fills soil pores and solidifies into a rock-like soil horizon. This consolidated horizon can form an erosion‑resistant layer and lead to development of table‑mountain like landforms.

of the soil surface. Soils that are strongly calcareous have visible filaments and concentrations of white CaCO 3.

A quick and easy field method for estimating CaCO 3 content, commonly known as the “fizz test”, relies on placing a drop or two of dilute hydrochloric acid (HCl) on a soil sample and observing the reaction, or effervescence (Figure 2; Table 2). The concentration of dilute HCl should be approximately one Molar, or about one part concentrated HCl to nine parts distilled water (Soil Survey Division Staff, 2018). A slightly more complicated method uses common vinegar instead of dilute HCl (Zhu et al., 2015).

Calcium carbonate does not directly harm plant productivity unless it’s present at extremely high levels that form root‑limiting horizons. The supply of CO 3 -2 reduces the activity of hydrogen (H +) ions in the soil solution to keep soil pH in the alkaline range (about 7.2). Many calcareous soils in Wyoming have higher pH levels because of the presence of other salts. Plants that prefer acidic soils, such as blueberries, will not thrive in calcareous soils. The abundant Ca and the high pH can interfere with availability of phosphorus (P) by forming Ca-P minerals.

Phosphorus availability in calcareous soils is a big concern for Wyoming crop producers. The notion that the soil contains a great amount of P, and even adsorbs expensive fertilizers, is frustrating to farmers. A common question fielded by agronomists is “how can I reduce my pH to tap into my reserve of soil P?” Unfortunately, there is not an easy answer. The large amount of CaCO 3 in Wyoming agricultural soils makes acidifying the soil expensive and impractical. For example, elemental sulfur is commonly used to acidify soils because as it oxidizes to sulfate (SO 4 -2), reacting with water (H 2O) to release H + ions that decrease the pH. But to change the pH in

calcareous soil, CaCO 3 must be solubilized. A soil containing just two percent CaCO 3 would require 6.4 tons of sulfur per acre (Havlin et al., 2011), at up to $800 per ton. Even then the pH would stay alkaline because abundant basic cations on exchange in the soil would buffer the soil solution (would dissolve to replace H + in solution). As you will note if you start doing the fizz test around Wyoming, many soils have much greater than two percent CaCO 3 at or near the surface.

The better approach is to plant crops tolerant of somewhat alkaline conditions (pH ~ 8.0) and to carefully manage P and micronutrients by frequently testing the soil. In calcareous soils, P is usually available to the plant for a very short time compared with neutral or slightly acidic soils, so annual application is a good recommendation, rather than once per rotation, for instance. 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. Therefore, banding P slows down adsorption and fixation reactions.

Increasing soil organic matter content with organic amendments (compost) or conservation tillage creates organic acids as mineralization of organic material releases H +, as does conversion of ammonium fertilizers to nitrate. But this does not produce enough acid to neutralize calcareous soils. However, management practices and amendments that increase surface soil organic matter content serve to dilute the effects of CaCO 3 and also improve soil water infiltration so that, over time, CaCO 3 can be moved below the surface, improving P and micronutrient availability during germination and establishment. Because of the low solubility of CaCO 3 relative to other salts, changes to soil pH and structure are slow and require long time commitments.

Saline soils

Saline soils have an electrical conductivity (EC) in the soil solution greater than four decisiemens per meter (dS/m) and an exchangeable sodium percentage (ESP) of less than 15 (Table 1). Electrical conductivity is used to indicate salinity because the more ions are in solution, the more electricity water conducts. It is directly related to total dissolved solids (ppm TDS = 640 × EC), but EC is easier to measure.

Salts accumulate in regions where potential evapotranspiration exceeds precipitation. Salts released during mineral weathering, or added with irrigation water, fertilizer, or manure, are not leached from the soil profile. They may accumulate as white salt deposits during periods of soil water evaporation and were formerly called “white alkali”.

Soluble salts contain the cations Na +, K +, Ca +2, and Mg +2, and the anions Cl , SO 4 -2, HCO 3-, and CO 3 -2. The main detrimental effects on plant growth are physiological drought and ion imbalances that cause the plant to require more energy. Physiological drought occurs when elevated salt concentration in the soil increases the tension with which water is held by soil particles and decreases its availability to plants. Chlorosis in older leaves and stunting are early signs of salt stress. Plants suffering salt stress have necrotic “burned” leaf margins. Conifer needles die from the tips back. This type of salt stress is common in potted house plants because dissolved salts in tap water, even at low concentration, accumulate unless the pot is occasionally overwatered to leach them out.

The tolerance of plants varies greatly by species. It pays for agricultural producers in saline conditions to select salt tolerant crops and to understand how soil EC affects yields of crops being grown. Table 3, page 7, gives the salt tolerance rating, the threshold soil EC, and the yield decrease per unit EC increase for some common Wyoming crops.

Management of saline soils requires two general strategies: reducing upward soil water movement by slowing evaporation at the surface, or increasing downward soil water movement (leaching). With rainfed or limited irrigation situations, organic mulches or crop residues on the soil surface, such as in reduced- or no-till management, reduce evaporation and increase water infiltration and leaching. Incorporating organic materials can serve to reduce salinity in surface soil due to

both dilution and sorption of some salts, and also improve infiltration and leaching.

Leaching salts below the root zone is not difficult if abundant, low‑salt irrigation water is available and soils are well drained. In contrast to sodic soils, saline soils usually have good drainage because salts are dominated by highly charged Ca +2 and Mg +2 ions that aggregate, or flocculate, soil particles, creating good soil structure and porosity, in contrast to the weakly charged Na +, which tends to disperse soil particles, destroying soil structure and drainage (Figure 3).

The proportion of excess water required to leach salts is called the leaching requirement and can be calculated as follows:

Leaching Requirement=EC iw

5(EC target) − EC iw

Where EC iw is the EC of irrigation water and the EC target is the target soil‑solution EC. For example, if EC iw = 1.5 dS/m and EC target = 1 dS/m, the leaching requirement would be 0.43, or 43% more water than would saturate the soil.

To estimate the total amount of water required, we must know the saturated soil water holding capacity (WHC), which is the

same as the amount of pore space in the soil, or soil porosity. Porosity is calculated from the bulk density of the soil as:

Porosity (%)= 1 - bulk density × 100

2.65

Where 2.65 g/cm 3 is the estimated particle density of mineral soils (the density of quartz). Porosity ranges from around 35 percent for sandy soils to over 60 percent for clayey or organic matter rich soils. A good estimate for fine-textured soils with low organic matter content is 50 percent. At 50 percent porosity, each inch of soil would require about ½ inch of water to become saturated, plus additional water equal to the leaching requirement to remove salts. So each inch of soil to be leached would require:

Total water required = (Depth to be leached) × (saturated WHC) × (1 + Leaching Requirement)

For irrigated agriculture, the goal is often to leach salts below the crop root zone. For reclamation, there is sometimes limited water available, though it may be feasible to leach the surface two or three inches to facilitate germination of native, salt-tolerant plants. This essentially speeds up the natural process of removing salts from a shallow A horizon.

Sodic and saline-sodic soils

Sodic soils have an ESP greater than 15 percent and EC less than 4 dS/m, while saline-sodic soils have chemical characteristics of both: ESP greater than 15 percent and EC greater than 4 dS/m (Table 1). 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. Soil morphological descriptions denote a Bn, or natric horizon, often within two to six inches of the soil surface. Often, the natric horizon co-occurs with a calcic and clay-enriched horizon, a Btkn horizon. The thin A horizon over the natric horizon is very important for plant germination and establishment and is easily destroyed by erosion or during development activities.

Sodium is especially problematic because, with its monovalent (+1) charge, large hydrated ionic radius (size), and low charge density due in part to a tendency to attract water molecules as a “water jacket,” it disperses soil particles, forcing them apart and destroying soil structure and water movement (Figure 3). Sodic conditions are also toxic to plants because Na can replace Ca in cell membranes, increasing permeability and loss of ions. Sodium accumulation causes leaf-edge necrosis, especially in grasses. Large amounts of Na also overwhelm the pH buffering effects of CaCO 3, so that pH can increase to nine or higher, causing micronutrient deficiencies. Soils with high contents of shrink-swell clays, also common in Wyoming, are especially prone to problems and may become almost impervious to water, forming “slick spots” on the soil surface.

Management of Na in the soil requires reducing the ratio of Na to the other cations (K, Ca, and Mg). That’s why it is

Figure 3. Simplified schematic diagrams of (A) a dispersed soil in which the thick shell of water molecules of strongly hydrated Na ions force negatively charged soil particles apart, destroying soil aggregates and reducing porosity as individual particles plug pores, and (B) a flocculated soil in which less strongly hydrated Ca ions are closer to clay surfaces, forming a strong attraction and binding aggregates together to form inter-aggregate porosity. Larger silt and sand particles could also be bound within aggregates formed by the flocculated clay particles. Mg and K ions have intermediate hydration strength, so flocculate particles less strongly than Ca.

measured as ESP or sodium adsorption ratio (SAR). These two measurements are related, but ESP is derived from the cations on exchange, or held on negatively charged surfaces of clay and SOM particles and requires extraction of Na, Ca, and Mg using an extractant that displaces the ions on the cation exchange. ESP is calculated as:

ESP=Exchangeable Na × 100

CEC

Where units of exchangeable Na and CEC are meq/100g soil. SAR is derived from cations in the soil solution and not held on exchange surfaces, so extractable with water. SAR is calculated as follows:

SAR=soil solution Na

√ solution Ca+Mg

2

Where all units are meq/L.

The ESP value is needed to estimate the rates of amendments necessary to displace exchangeable sodium

in order to remediate sodic or saline-sodic soils. Both the solution and exchangeable concentrations are needed to calculate ESP because the extraction process for exchangeable ions also extracts the solution ions, so exchangeable = total extractable – solution. But the exchangeable and solution concentrations are closely related, so ESP can be estimated from SAR if only solution concentrations are available. In cases where only the SAR is available, ESP can be calculated as follows:

ESP=1.5 SAR

1+(0.015 SAR)

There are several chemical amendments that can be used to displace Na on the cation exchange with Ca or Mg, including gypsum (CaSO 4·2H 2O) and langbeinite (MgKSO 4). This reduces the ESP and improves soil structure so that salts can then be leached. The problem is that most of our saline soils are also calcareous. By adding more salts, salinity (EC) can be increased to harmful levels, but usually temporarily.

In calcareous soils, most Ca is present as CaCO 3 so, in theory, by acidifying the soil it should release Ca that will displace Na on exchange so that Na can be leached and sodic conditions alleviated. In practice, however, the high concentrations of Na in saline-sodic soils means that large amounts of acid are needed. Elemental S can be used because it forms sulfuric acid as it oxidizes in the soil, but adding enough to reduce sodicity is usually cost prohibitive, and the effect is temporary.

University of Wyoming Extension bulletin B-1231, Reclamation on Salt/Sodium-affected Soils (Norton and Strom, 2012), discusses options for reclaiming soils drastically disturbed by development activities. Such activities often move Na and other salts to the soil surface where they cause crusting and salinity that makes establishment of desirable plant communities very difficult. In recent research, we evaluated the effects of chemical amendments and compost in reclamation of soils that became saline-sodic during the development and reclamation process. We found that langbeinite can mobilize and reduce the amount of Na in the soil (Figure 4), improving soil structure and drainage, but temporarily increasing EC to levels that may be unacceptable. Gypsum, elemental S, and compost were not as effective at alleviating saline-sodic conditions in the short term. Since the reactions required for these amendments to be effective require water, they may require more time.

Figure 4. ESP at three depths with four amendments one year after application. Initial ESP was 38.6 at all depths. Cntrl, untreated control; S, elemental sulfur; G, gypsum; L, langbeinite. Horizontal line represents least significant difference.

Figure 5. Graphs for determining application rates of selected amendments. Determine amount of sodium to be replaced using lab data and Step 1 (previous page), then find the sodium to be replaced on the horizontal axis of the correct graph and use the linear relationship to determine the rate on the vertical axis.

Definitions

anionElemental ion with a negative charge.

cationElemental ion with a positive charge.

cation exchangeBase cations (mainly Ca, Mg, K, and Na) that can be held electrostatically on negatively charged clay and organic matter particles.

CECCation Exchange Capacity. Amount of base cations that can be held electrostatically on negatively charged clay and organic matter particles, expressed in meq/100 g soil

DispersionGenerally undesirable process in which excess weakly charged sodium ions (low charge per ionic radius, or charge density) force soil particles apart, destroying aggregates, plugging soil pores, forming dense crusts, and impeding soil water infiltration and movement.

ESPExchangeable Sodium Percentage. Exchangeable sodium divided by the cation exchange capacity times 100.

Exchangeable ionsIons held on cation exchange, extractable with potassium chloride, ammonium acetate, and other extractants that provide cations to displace those on exchange. These extractants remove both exchangeable and solution (water extractable) ions, so: exchangeable = extractable – solution.

FlocculationGenerally desirable process in which soil particles adhere to one another to form aggregates that increase porosity and water holding, infiltration, and movement. Flocculation is facilitated by organic matter and strongly charged cations, including Ca, Mg, and K (relatively high charge per ionic radius, or charge density), which adhere to negatively charged particle surfaces to hold them together.

pHNegative log of [H +], which indicates the acidity (pH < 7) or alkalinity (pH > 7) of a soil.

SAR Sodium adsorption ratio. Soil solution sodium concentration divided by one-half the square root of Ca plus Mg concentrations when expressed in meq/L.

soil solutionSoil water and dissolved ions. Ions extractable with water.

Some common conversion factors

References & further reading

Bischoff, J., and H. Werner. 1999. SALT/SALINITY TOLERANCE of Common Agricultural Crops in South Dakota: Forages and Grasses/Grains and Field Crops. South Dakota Extension Fact Sheet 903. Available from: https://openprairie.sdstate.edu/cgi/viewcontent.cgi?article=1082&context=extension_fact.

Bischoff, J., and H. Werner. 1999. SALT/SALINITY TOLERANCE of Common Agricultural Crops in South Dakota: Garden and Vegetable/Woody Fruit Crops. South Dakota Extension Fact Sheet 904. Available from: https://water-research.net/Waterlibrary/Stormwater/salinitytolerance.pdf.

Food and Agriculture Organization. 2006. Guidelines for soil description, Fourth Edition. FAO, Rome. Available at: http://www.fao.org/3/a-a0541e.pdf.

Havlin, J.L., S.L. Tisdale, W.L. Nelson, and J.D. Beaton. 2011. Soil fertility and fertilizers, 8th edition. Pearson, Inc.

Hopkins, B.G., D.A. Horneck , R.G. Stevens , J.W. Ellsworth , and D.M. Sullivan. 2007. Managing Irrigation Water Quality for crop production in the Pacific Northwest. Pacific Northwest Extension publication. PNW 597-E, August 2007. Available from: https://catalog.extension.oregonstate.edu/sites/catalog/files/project/pdf/pnw597.pdf.

Horneck, D.A., J.W. Ellsworth, D.M. Sullivan, and R.G. Stevens. 2007. Managing Salt-Affected Soils for Crop Production. Pacific Northwest Extension publication. PNW 601-E, November 2007. Available from: http://ir.library.oregonstate.edu/xmlui/bitstream/handle/1957/20788/pnw601-e.pdf.

McCauley, A., and C. Jones. 2005. Salinity and sodicity management. Soil & Water Management Module 2. Montana State University Bulletin 4481-2. Jan. 2005.

Millennium Ecosystem Assessment 2005, Ecosystems and Human Well-being: Desertification Synthesis, World Resources Institute, Washington. Available at: https://www.millenniumassessment.org/documents/document.356.aspx.pdf.

Norton, J.B., and C. Strom. 2012. Successful restoration of severely disturbed Wyoming lands: Reclamation on salt-affected soils. UW Extension Bulletin B-1231. Available at: http://www.wyoextension.org/publications/.

Zhu, Q. M. Ozores-Hampton, and Y. Li. 2015. Determination of carbonate concentrations in calcareous soils with common vinegar test. University of Florida Extension publication HS1262. Available at: http://edis.ifas.ufl.edu/pdffiles/HS/HS126200.pdf (accessed on October 3, 2018).

Soil Survey Division Staff. 2018. Assessing carbonates in the field with a dilute hydrochloric acid (HCl) solution. Soil Survey Technical Note 5. Available at: https://www.nrcs.usda.gov/wps/portal/nrcs/detail/soils/ref/?cid=nrcs142p2_053572 (accessed on August 17, 2018).

Issued in furtherance of extension work, acts of May 8 and June 30, 1914, in cooperation with the U.S. Department of Agriculture. Kelly Crane, director, University of Wyoming Extension, University of Wyoming, Laramie, Wyoming 82071.

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B-1358 | August 2020

Jay Norton, Ecosystem Science & Management, UW College of Agriculture and Natural Resources

Editor: Katie Shockley, UW Extension

Design: Tanya Engel, UW Extension

We appreciate careful review of this document by:

Clain Jones, Professor and Soil Fertility Extension Specialist, Montana State University

Karen Vaughan, Assistant Professor of Pedology, University of Wyoming

Mike Zhu, Associate Professor of Soil and Environmental Biogeochemistry, University of Wyoming