Managing Salt-affected Soils for Crop Production

There are three types of salt-affected soils:

1. Saline soils contain high concentrations of soluble salts. Soluble salts present in the root zone can make it difficult for plants to extract water from the soil.
2. Sodic soils have too much sodium.
3. Saline-sodic soils have too much sodium and other soluble salts.

Management practices differ for saline, sodic and saline-sodic soils:

• Saline soils are reclaimed by leaching salts below the root zone with the application of low-salt irrigation water. Plants are most susceptible to soil salinity at germination. In some cases, moving salt away from the germinating seed is all that is necessary.
• Sodic soils require the addition of soluble calcium, followed by leaching of sodium. Calcium promotes soil aggregation. Gypsum is the most common material used to supply calcium for sodic soil reclamation. Elemental S is useful for sodic soil reclamation only when the soil contains free lime.
• Saline-sodic soils must be treated for sodicity first. These soils first require calcium additions to correct sodium, followed by leaching to remove salts.

When developing a plan for salt management, keep in mind these concepts:

• Irrigation water can create or worsen salt-affected soil conditions, making irrigation water management important. Test your irrigation water for salt before using.
• Some crops are more sensitive to salts than others, and plant sensitivity varies depending on crop growth stage and weather.
• High concentrations of chloride and boron can reduce crop yield independent of saline or sodic soil conditions.
• Reclamation of salt-affected soils requires adequate drainage. If necessary, improve drainage before attempting reclamation.

This publication is designed to assist in evaluating the kind and amount of salts present in soils and to select appropriate management practices to maintain soil health and crop productivity.

The “For more information” section provides additional resources on this topic. A companion publication, Managing Irrigation Water Quality for Crop Production, PNW 597, provides additional details on interpretation of irrigation water analyses.

Salts

Salts are composed of positively charged ions (cations) and negatively charged ions (anions). They can be dissolved in water (soluble salts) or be present as solids. Salts in soil can originate from soil parent material; from irrigation water; or from fertilizers, manures, composts or other amendments. The predominant salts that accumulate in soils are calcium, magnesium, sodium, potassium, sulfate, chloride, carbonate and bicarbonate. Any salt that accumulates in excess amounts in soil can cause plant growth problems.

Salts in the root zone can reduce crop yield by making it difficult for roots to extract water from the soil.

Excess sodium causes problems related to soil structure. As sodium percentage increases, so does the risk of dispersion of soil aggregates (Figures 1 and 2).

High concentrations of certain salts in the soil may also be toxic to plants (see “Beyond salt and sodium: Specific ion toxicities”).

Salt is exported from the soil via leaching through the soil profile or by crop removal. Salts accumulate when inputs exceed exports. To successfully manage salts over the long term, inputs and exports must be balanced.

Knowing how much salt you are applying and how much can be removed is critical for long-term management.

Three general categories of salt-affected soils have been identified for management purposes:

• Saline soils: soils affected by salt problems in general.
• Sodic soils: soils affected by sodium problems.
• Saline-sodic soils: soils affected by problems with sodium and other salts.

These categories are defined in soil classification literature by the Natural Resources Conservation Service (NRCS) (Table 1). For example, saline soils must have electrical conductivity (EC) above 4 dS/m. Although the NRCS classification is useful for soil surveys, it has limitations for management decisions. For example, many crops are adversely affected at soluble salt values below the NRCS saline soil threshold. See Table 2 and the Glossary for definitions of units and terminology.

Similarly, the NRCS classifies the threshold for sodic soils as those with a sodium adsorption ratio (SAR) above 13 or exchangeable sodium percentage (ESP) above 15. Usually, the change from flocculated (aggregated) to dispersed soil structure occurs gradually as sodium level increases. Therefore, you are encouraged to think about salinity and sodicity as continuous variables that affect crop and soil productivity.

Saline soils

The predominant exchangeable cations in saline soils are calcium and magnesium. Saline soils commonly have visible salt deposits on the surface and are sometimes called “white alkali” soils. Most salts in soil solution have a positive effect on soil structure and water infiltration.

Therefore, water penetration is not a major concern with saline soils.

Salts in the root zone can reduce crop yield by making it difficult for roots to extract water from the soil. Salts cause water to move from areas of lower salt concentration (plant tissue) into the soil, where the salt concentration is higher. High salt concentration in the soil can cause plants to wilt even when soil moisture is adequate.

Sodic soils

Sodic soils are high in exchangeable sodium compared to calcium and magnesium. EC is less than 4 dS/m and often less than 2 dS/m.

Soil pH usually is greater than 8.5 and can be as high as 10 or even 11 in extreme cases.

High exchangeable sodium, high pH, and low calcium and magnesium combine to cause the soil to disperse, meaning that individual soil particles act independently. The dispersion of soil particles destroys soil structure and prevents water movement into and through the soil by clogging pore spaces (Figure 2).

Sodic soils often have a black color due to dispersion of organic matter and a greasy or oily-looking surface with little or no vegetative growth. These soils have been called “black alkali” or “slick spots.”

Saline-sodic soils

Saline-sodic soils are high in sodium and other salts. They typically have EC greater than 4 dS/m (mmhos/cm), SAR greater than 13, or ESP greater than 15. Soil pH can be above or below 8.5.

Saline-sodic soils generally have good soil structure and adequate water movement through the soil profile.

Soil testing is an important tool for managing salt-affected soils. A laboratory should be able to provide the analyses shown in Table 2.

See “Soil testing questions and answers” for additional information. Equations and detailed explanations of these analyses are given in the Glossary.

Beyond salt and sodium: Specific ion toxicities

High concentrations of sodium, chloride, boron or carbonates can reduce crop yield independent of saline or sodic soil conditions. Some research suggests that excessive sodium inhibits plant growth primarily via generalized salt effects, as any salt reduces water availability to plants, rather than via a specific toxicity mechanism.

Crops vary widely in their sensitivity to excess boron, sodium and chloride. For example, blackberries are sensitive to excess boron, showing injury at concentrations above 0.5 ppm (mg/liter) in a saturated paste extract. Tolerant crops such as asparagus can grow without injury at boron concentrations above 6 ppm. More crop tolerance listings are available in “For more information.”

Crops that are sensitive to excess sodium, chloride and boron are most susceptible to damage when irrigation water with high concentrations of these elements is applied to leaves via overhead sprinklers. Crop tolerance to chloride usually is greater when irrigation water is surface applied, avoiding contact with leaves. Drip irrigation is an easy way to accomplish this.

The potential for specific ion toxicities can be diagnosed by testing soil (Table 5) and by testing irrigation water (see Managing Irrigation Water Quality). Specific ion toxicities (B, Cl and Na) can be corrected the same way as a salt problem — by leaching salts from the soil using water with low salt concentrations.

Management of salt-affected soils

Table 6 provides information that is helpful in evaluating problems with salt- affected soils and in identifying appropriate management practices. Long-term data on how a soil has changed over time is essential to making well-informed decisions about irrigation water management, rates and types of soil amendments, and the probability of positive economic returns from managing salt-affected soils.

Once the necessary soil tests and field history have been collected and assessed, the next step is to identify economical options for reclamation. Salt-affected soil reclamation requires management and careful monitoring.

Where sodium and other salt problems are limited to one portion of a field, consider managing that part of the field as a separate management unit. See Monitoring Soil Units Using a Management Unit Approach, PNW 570, for suggestions on using a management unit approach.

Salt-affected soil problems do not develop overnight, nor are they solved quickly. It can take years for enough salt to accumulate to reduce crop growth and water infiltration. Reclamation can take just as long. Before undertaking a reclamation program, develop a plan with a knowledgeable consultant. Understanding the implications and costs of a plan is important. In some situations, it is not economical or even possible to reclaim a salt-affected soil.

Managing saline soils

Leaching

Saline soils irrigated with large quantities of water containing low to moderate levels of salts are reclaimed as the salts leach below the root zone. Initially, reclamation rate depends on the amount of water traveling through the profile and out of the root zone (the leaching fraction). Thus, it is important to ensure that there is adequate drainage in the soil to accommodate an adequate leaching fraction.

When salts come from a shallow water table, the water table must be lowered by providing drainage before reclamation can be accomplished. In some situations, lowering the water table might not be economical, and an alternate crop or land use might be a better choice.

Many saline soils are the result of irrigating with water containing moderate to high levels of salts. Although leaching will minimize salt accumulation, no amount of leaching will entirely correct the problem until an alternate irrigation source is secured to mix with or replace the poor-quality water.

Sandy soils require less management to leach salts because infiltration rates are high and more water can be applied over a shorter period of time. Clays require more management to move salts out of the soil because the infiltration rate is lower and water must be applied more slowly to avoid problems with ponding and runoff.

Other management practices

When good-quality water and adequate drainage are not available, the only option short of abandoning the field may be to select crops tolerant to saline soil conditions. Table 4 shows the expected yield reduction for several crops at various ECs. Use Table 4 to match the salinity level to suitable crops.

Plants are most sensitive to salinity at germination and become more salt-tolerant as they mature. Where germination is the primary concern, leaching all salts out of the root zone usually is neither feasible nor required. Moving salts away from the germinating seed may be all that is needed (Figure 4).

Soils can remain productive even where complete reclamation is not possible. These situations require careful management and continual monitoring to ensure acceptable productivity.

Crop growth and harvest will aid in salt removal, improve water penetration, and supply organic matter. Therefore, vegetation management is an important part of reclamation.

Plants have greater difficulty absorbing water under saline conditions. Therefore, saline soil must be maintained at a higher moisture level than nonsaline soil if the crop is to obtain adequate water. This often requires more frequent but lower volume irrigations. Drip irrigation can help to maintain high soil moisture in saline soils.

Soil amendments such as elemental sulfur, gypsum, other calcium materials and other soil amendments do not help reclaim saline soils, despite claims to the contrary. Instead, these materials add salts and compound the problem.

Manures, composts and other soil amendments should be analyzed to determine the types and quantity of salts present. Remember, anything soluble in water will add to the salt load and increase soil EC.

The only way to remediate saline soils is to remove salts from the root zone. This can be accomplished with good drainage and the application of high-quality irrigation water.

Management of sodic soils

Sodic soils usually are the most expensive to reclaim. In many situations, reclamation is not economical. The reclamation procedures discussed here can improve sodic soils, but many years or decades of good soil and crop management are required to fully remediate a sodic soil.

Drainage

Soils with sodicity problems must have drainage to facilitate sodium removal from the root zone. When a high water table is present, it must be lowered before reclamation can proceed. Drainage can be improved by altering the topography or by installing tile drains. Drainage can be improved in some cases by planting deep-rooted perennials such as alfalfa. It is crucial to maintain permeability. When irrigation canal seepage is the cause of high water, the canal water must be intercepted before it enters the field, or the canal must be sealed to reduce seepage.

Tillage and amendments

Tillage often is necessary to physically break up sodium-rich layers and mix amendments into the soil. Coarse organic materials that decompose slowly, such as straw, cornstalks, sawdust or wood shavings used for animal bedding, can help improve soil structure and infiltration when used with other reclamation practices.

Supplying calcium to improve water infiltration

Improving water infiltration rates in sodic soils requires increasing soil electrical conductivity to more than 4 dS/m (4 mmhos/cm) or reducing the exchangeable sodium percentage (ESP). The ESP required for improved water infiltration depends on soil texture and irrigation method. Soils with high amounts of sand can tolerate ESP up to 12 and still retain water infiltration and percolation properties. Soils that are sprinkler irrigated typically require a lower ESP for good water infiltration, compared to soils irrigated with surface irrigation systems.

Soluble calcium is required for sodic soil reclamation, as it will displace sodium and reduce the ESP and SAR (Figure 5). If possible, use irrigation water that is high in calcium and salinity during the initial phase of reclamation.

Injecting gypsum (calcium sulfate) into irrigation water increases salinity and calcium. As the sodium is replaced, water lower in calcium and salinity can be used.

Gypsum is the most common material used to supply calcium for sodic soil reclamation. The “gypsum requirement” is the amount of gypsum needed to reclaim the soil to a specified depth. General rates for gypsum application are shown in Table 7. One ton per acre is a minimum application rate. See “gypsum requirement” in the Glossary for how to calculate the gypsum requirement for a specific soil. Using table 7 requires a soil test value for exchangeable sodium.

Gypsum is used because it is calcium-rich, dissolves at high pH, and does not contain elements or compounds that might interfere with reclamation. The sulfate in gypsum is not likely to be a problem for crops, even though it is applied in quantities in excess of plant requirements.

Calcium nitrate or calcium chloride minerals can be used to reclaim sodic soils, but they can be more costly and are likely to produce other negative effects on plant growth or to the environment. Nitrate is considered a groundwater contaminant and is not a good choice. Calcium chloride applied at the typical rates needed for remediation will likely cause chloride toxicity.

Limestone is another commonly available soil amendment that contains calcium. However, it is not used for reclaiming sodic soils because it is not soluble at the high pH levels common in these soils.

Elemental sulfur (S) can be used for sodic soil reclamation in cases where free lime already exists in the soil. The addition of sulfur does not directly add calcium to the soil. However, elemental sulfur oxidizes to form sulfuric acid, which dissolves lime (calcium carbonate), which often exists in arid and semiarid zone soils. The dissolution of indigenous lime provides the calcium necessary to reclaim a sodic soil. When adequate moisture and temperatures are present, oxidation of elemental sulfur will be completed within one or two growing seasons. For more information on soil acidification techniques, see Acidifying Soil for Crop Production: Inland Pacific Northwest, PNW 599.

Soil amendments such as elemental S should be incorporated to increase the rate of reaction and to speed reclamation. When elemental S is left on the soil surface, or when the soil is dry or cold, microbial conversion of elemental S to sulfuric acid is delayed.

Table 7 shows the amount of elemental sulfur needed for reclamation based on sodium concentration and reclamation depth. Where available and economical, addition of acids directly to the soil accomplishes the same effect as elemental S, but specialized equipment is required due to safety concerns. Before using elemental sulfur or acids, verify that the soil contains sufficient lime to be dissolved.

Irrigation water management

Some irrigation waters, typically those pumped from deep wells, contain high concentrations of bicarbonate and have a high SAR (high concentration of sodium relative to calcium + magnesium). Application of these waters can create sodic soils over time. Both the EC and the SAR of the irrigation water determine the effect of water application on soil structure and the potential for water infiltration problems.

Irrigation water that is high in bicarbonate or carbonate can react with calcium in the soil solution to form calcium carbonate. This process removes calcium from the soil solution. As calcium in the soil solution is reduced, soil SAR and sodium hazard increase. Precipitation of calcium carbonate in pore spaces also reduces water infiltration and percolation in soils that are not tilled.

The most effective means of avoiding this problem is to acidify the irrigation water prior to application. When an acid such as sulfuric acid is added to irrigation water, it reacts with bicarbonates to form water and carbon dioxide.

For more information on monitoring and managing irrigation water quality, see Managing Irrigation Water Quality for Crop Production in the Pacific Northwest, PNW 597.

Management of saline-sodic soils

Saline-sodic soils must be treated as sodic soils first. These soils require calcium to correct a sodium problem, followed by leaching to remove salts (Table 9). It is important to know how much of a sodium problem exists before applying clean water to leach salts.

High EC irrigation water and soil helps maintain soil structure, increase water infiltration and prevent sodium from dispersing soil particles. However, except for its positive effect on soil structure, high EC (salt) irrigation water is not beneficial for crop production.

Saline-sodic soils often are caused by factors beyond the landowner’s control. Many of these soils are the result of natural events, and no amount of reclamation will return them to a satisfactory level of productivity. Under these conditions, use tolerant vegetation such as some grasses to maintain soil cover.

1:1 or 1:2 soil solution: 1 part water is added to 1 or 2 parts soil (weight:weight) in order to measure pH or electrical conductivity.

Acidic soil: Soil with a pH below 7.

Alkaline soil: Soil with a pH above 7.

Anion: A negatively charged ion such as chloride (Cl^-), sulfate (SO₄^2-), carbonate (CO₃^2-) or bicarbonate (HCO₃^-).

Cation: A positively charged ion such as calcium (Ca²⁺), magnesium (Mg²⁺), sodium (Na⁺), potassium (K⁺), or ammonium (NH_4⁺).

Cation exchange capacity (CEC): A measure of the net negative charge of a soil. Measured as the total quantity of cations that can be exchanged on a unit of soil material, expressed as milliequivalents per 100 grams of soil (meq/100 g), millimoles of charge per kilogram of soil (mmol [+ charge]/kg) or centimoles of charge per kilogram of soil (cmol [+ charge]/kg).

Dispersion: Breaking up of “clumps” of soil particles or aggregates into individual soil particles (sand, silt, and clay). Soil aggregates form larger, more continuous soil pores than do individual soil particles. The larger pores provide better water and air movement.

Drainage: Unimpeded downward movement of water beyond the root zone.

Electrical conductivity (EC): The ease with which electrical current passes through water. EC is proportional to the salt concentration in the water. Consequently, total salt concentration in a soil or irrigation water can be estimated by measuring EC. The higher the EC, the greater the salt concentration.

Elemental sulfur (S⁰): A yellow, inert crystalline mineral that is finely ground. In soil, elemental S is oxidized to sulfuric acid via microbial activity. The rate of elemental S oxidation in soil is most rapid in warm, moist soils. Complete oxidation of elemental S to sulfate often takes one to several years.

Evapotranspiration (ET): Combined water use by plants and water evaporated from the soil surface in a given time period. ET usually is expressed as inches or millimeters of water per day.

Exchangeable sodium percentage (ESP): Percentage of the cation exchange capacity that is filled by sodium. It is calculated as:

ESP = [Na⁺, meq/100 g soil / CEC, meq/100g soil] x 100

To convert SAR to ESP, use the following equation:

ESP = (1.475 x SAR) / [1+ (0.0147 x SAR)]

Flocculation: The joining together of smaller individual particles of soil, especially clay, into larger units or flocs.

Gypsum: CaSO₄•2H₂O, the common name for calcium sulfate. Applied as a source of calcium to reclaim sodic and saline-sodic soils.

Gypsum requirement (GR): The approximate amount of gypsum needed per acre to lower the ESP of the soil to a desired level. It is calculated as:

GR = (present ESP – desired ESP) x CEC x 0.021.

The factor of 0.021 assumes CEC is in meq/100 g or cmol (+charge)/kg units. If CEC is in mmols (+ charge)/kg, the factor is 0.0021. These factors assume 90% reclamation efficiency and a desirable SAR in the irrigation water.

Infiltration: Entry of water into soil.

Leaching: The downward movement of soluble ions in the soil profile.

Leaching fraction: The fraction of infiltrated irrigation water that percolates below the root zone.

Leaching requirement: The leaching fraction necessary to keep soil salinity, chloride or sodium (whichever is the most limiting factor) from exceeding a tolerance level for the crop rotation. Leaching requirement refers to long-term average conditions.

meq/L: Milliequivalents per liter.

Perched water table: A shallow water table formed above a soil layer impermeable to water.

pH: A measure of the acidity or basicity of a material or solution. Below 7 is acidic, above 7 is basic, and 7 is neutral.

ppm: Parts per million. Also expressed as mg/kg in a solid matrix or mg/liter in solution.

Saturated soil paste: A reference-state mixture of soil and water used for measuring EC, SAR and pH. At saturation, the soil glistens slightly as it reflects light, flows slowly when the container is tipped, and slides freely and cleanly from a spatula.

SAR = [Na⁺] / the square root of 0.5 ([Ca²⁺] + [Mg²⁺])

Extension and outreach publications

• Drip Irrigation Salinity Management for Row Crops, Publication 8847. 2011. University of California.
• Monitoring Soil Nutrients Using a Management Unit Approach, PNW 570. 2003.
• Managing Irrigation Water Quality for Crop Production, PNW 597. 2007.
• Acidifying Soil for Crop Production: Inland Pacific Northwest, PNW 599.
• Soil Structure: The Roles of Sodium and Salts, Publication 1414. University of Arizona Cooperative Extension.
• Monitoring Soil Nutrients Using a Management Unit Approach, PNW 570. 2003.

Handbooks

• Ayers, R.S. and D.W. Wescot. 1994. Water Quality for Agriculture (FAO Irrigation and Drainage Paper 29). 1994.
• Hanson, B. 2006. Agricultural Salinity and Drainage. Publication 3375.
• Carrow, R N., and R.R. Duncan. 1998. Salt-affected turfgrass sites: assessment and management. Chelsea, Mich.: Ann Arbor Press. ISBN: 9781575040912.
• Wallender, W.W. and K.K. Tanji. 2011. Agricultural salinity assessment and management (No. Ed. 2). American Society of Civil Engineers.

Website

• North American Proficiency Testing Program. Quality control/quality assurance program for soil, plant and water samples.