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Salinity, sodicity, and high pH in soils can impact plant growth and yield potential. These issues are common in the western growing regions of the U.S., such as the Great Plains, and are primarily caused by the weathering or breakdown of soil parent material or by the use of poor quality irrigation water.
Soil salinity is the content of soluble salts in the soil, which can readily dissolve in the soil water solution and be taken up by plants. These ions include sodium (Na+), potassium (K+), magnesium (Mg2+), calcium (Ca2+), chloride (Cl-), sulfate (SO42-), carbonate (CO32-), bicarbonate (HCO3-), and nitrate (NO3-).
Soil sodicity is the concentration of exchangeable sodium ions in the soil. Sodic soils have high levels of sodium and low levels of other salts. Sodic soils can have structural issues because the sodium ions weaken soil aggregates, resulting in a collapse of the soil structure. This is especially common in sodic soils with high clay or silt content. Sodic soils often have a high pH, often greater than 8.4.
Some soils may have both high saline and high sodium levels; these are termed saline-sodic soils. When salts build up in the soil, the following issues can occur in plants:
Soil salinity and sodicity issues can occur anywhere, but are most common in areas with limited rainfall and high evapotranspiration, such as in arid and semi-arid growing regions, where the solutes are less likely to be leached through the soil profile and collect in the root zone. Irrigation water that contains high levels of solutes and high water tables that carry the solutes to the surface can also contribute to soil salinity and sodicity problems.
Soil pH is a measure of the acidity (low pH) or alkalinity (high pH) of the soil. The generally accepted pH value for corn production is between the range of 6.0 to 6.5. High pH problems typically begin at pH 7.8 or higher and are often accompanied by saline soils, sodic soils, or saline-sodic soils.
Problems with soil pH (too high or too low) can cause the following issues in plants:
To the naked eye, it may be difficult to determine the nature of a soil problem, but the following symptoms may indicate a problem:
A soil test is the best way to accurately diagnose problems with the soil. Most basic soil tests report the pH and electrical conductivity (EC), which measures salinity. General soil tests should be conducted every 4 years, but additional testing should be conducted if a problem is suspected. Soil cores should be taken from a 6- to 12-inch depth at several locations of the field, concentrating on the areas suspected to have issues. Keep track of where each sample was collected and keep them separate. For high pH soils, the sodium absorption ratio (SAR) or exchangeable sodium percentage (ESP) should also be calculated, as the two conditions are often linked. The SAR and ESP are two different measurements of the concentration of sodium ions in the soil. Irrigation water samples should also be tested for salt levels where applicable.
Management of these soil issues requires different approaches, so each situation is addressed separately. Management approaches will also vary by region and soil type.
Management options for saline soils:
Management options for sodic soils:
Note that the practice of leaching the salts from the soil may also remove nutrients from fertilizer applications and pesticides, and will reduce irrigation efficiency. Consider the soil fertility level (fall may be the best time as nutrients have already been utilized by the plant), drainage, the quality of the irrigation water at the time of the leaching event, the availability of the irrigation water, and the type of irrigation system (to ensure the irrigation capacity is sufficient to apply enough water in a short enough period of time to cause leaching).
Management options for high pH soils:
When it comes to high pH soils, it may be more difficult to lower the pH than to manage the availability of soil nutrients. This is because high pH is often caused by the parent material of the soil, which will continue to break down over time and buffer any attempts to acidify the soil. Soils that contain free carbonates are the most difficult to alter in terms of pH.
For more details on the diagnosis and management of these issues, including the types of fertilizers to apply, application rates, and calculations for the amount of leaching water needed, see the publications listed in the sources.