Nutrient and mineral excesses, salinity, and salt toxicity
Salts are compounds that separate into positively and negatively charged molecules or elements in water or moist soil. These charged particles are called ions. Anions are negatively charged compounds or elements, such as nitrate, chloride, and sulfate. Cations are positively charged, such as ammonium, iron, calcium, magnesium, and sodium.
Plants obtain most nutrients from dissolved ions and tolerate many types of salts in low concentrations. However, foliage and roots can be injured by exposure to high concentrations of almost any type of salt, including those in certain fertilizers, low-quality irrigation water, ocean spray, road deicing salt, and soils.
Foliage exposed directly to excess sodium from irrigation sprinklers or sea spray discolors, beginning terminally and (in broadleaves) marginally. Leaves may drop prematurely. Foliar salt exposure typically produces a distinct pattern of damage, such as only on lower foliage wetted by sprinklers that apply low-quality irrigation water or on the windward side of plants facing an ocean breeze. Symptoms are more severe in sensitive plant species and as the salt concentrations increase. Buds, twigs, and entire branches may be killed, leading to witches’ broom growth the next season. Severely affected plants may die.
Root exposure to high sodium concentrations causes wilted foliage and stunted plant growth. This is because excessive salts in soil impede plants’ uptake of water and cause plant tissues to become dry and discolored. If salt is high, but not extremely high, plants may grow slowly but not show other obvious symptoms.
In broadleaves, excess salts carried with water into the plant will concentrate at leaf margins and tips, which turn yellow, then brown. Symptoms usually begin on the older foliage, which may die and drop prematurely. In evergreen broadleaves, foliage damage due to salts may be more pronounced on the south side of plants. Conifer needles turn yellow, then brown from the tip inward, then drop prematurely.
Soils high in exchangeable (readily available) sodium relative to calcium and magnesium are called sodic soils, and their soil pH usually exceeds 8.5. Sodic soils are sometimes evident because of a dark or white crust on the soil surface and slow water penetration. Sodium may damage roots through direct toxicity and kill sensitive plants. High levels of sodium can destroy the aggregate structure of fine- and medium-textured soils. This decreases porosity and prevents soil from holding sufficient air and water needed for plant growth.
Salt toxicity is most common in soils on the western side of the Central Valley of California, Northern California’s Livermore Valley, parts of the Mojave Desert, some coastal areas in Southern California, and filled soils around the San Francisco Bay shoreline. Toxicity also occurs from irrigation with water high in salts, such as from shallow wells, surface water that passes through arid areas, treated (recycled) municipal wastewater, and where salt is applied to deice pavement.
Frequent irrigation of poorly drained soils and excessive fertilization also can result in salt damage. Even moderately low salt concentrations of specific ions (e.g., boron) can damage plants. For a summary of conditions and locations where salt damage to plants is most likely to occur see this table.
Because symptoms similar to those of salt damage can be caused by water deficit and certain other maladies, test soil and irrigation water to definitely diagnose whether salinity is the cause of symptoms. If irrigating with domestic water, contact the water agency to obtain its test results for salinity and specific ions.
Testing for salinity. To determine whether excess sodium is present, have a plant diagnostic laboratory measure the soil’s sodium adsorption ratio (SAR), the ratio of sodium to calcium plus magnesium. Total salt (salinity) in soil is usually measured by a laboratory test of electrical conductivity of an extract (ECe) of a saturated paste (soggy soil) reported in units of millimhos per centimeter (mmhos/cm), which is the same as decisiemens per meter (dS/m), since 1 mmho/cm = 1 dS/m. Plants vary greatly in their tolerance for salinity. Depending on the species, sensitive plants may be damaged if soil ECe exceeds 2 to 4 mmhos/cm, while tolerant species are not damaged unless soil ECe exceeds about 8 to 10 mmhos/cm.
Salinity in irrigation water is measured as electrical conductivity (ECw), or more commonly as total dissolved solids (TDS) reported as milligrams per liter (mg/L) or parts per million (ppm). For landscape irrigation, water should generally be below about 1,000 mg/L TDS or below 1 mmho/cm ECw.
Salinity remediation. Saline soils cannot be remedied with chemical amendments or fertilizers. They can be remedied by leaching, but you may first need to improve soil drainage by adding subsurface drainpipes or tiles. To leach, apply water that is low in soluble salts to move salts deeper into soil. During each irrigation apply a greater volume of low-salt water than needed to wet the root zone. The extra water is called the leaching fraction. For example, water for a longer amount of time and, if warranted, increase the interval between irrigations to avoid overwatering and waterlogging the soil.
Sodic soils also need leaching, but first apply gypsum so calcium displaces sodium as water moves it through soil. If soil is high in free calcium (lime), sulfur may be applied before leaching. Before leaching sodic soil, have soil tested to determine whether it is beneficial to apply gypsum, sulfur, or other amendments, and if so, how much to apply.
Salinity prevention methods. To help prevent topsoil salinity apply mulch around plants to reduce evaporation of soil moisture. Evaporation concentrates minerals near the soil surface where most roots occur. Minimize fertilizer applications where salinity is a problem because fertilizers add salts to the soil. If applying fertilizers, use products with a low salt hazard (salt index). Obtain an analysis or test the salinity of animal manures, composts, and sewage sludge before deciding to apply them. Instead of using rock salt to deice pavement, consider using alternatives such as calcium magnesium acetate (CMA) or sand. When irrigating, provide enough low-salt water to move salts below the root zone.
Where salinity may be a problem, avoid planting species with a low salt tolerance. Grow salt-tolerant species. Especially when growing more salt-tolerant species, recycled water such as treated municipal wastewater can be acceptable for use in drip or flood irrigation or other ways that do not wet foliage. Foliage is more sensitive to salts than roots. Avoid using sprinklers if irrigating with salty water. If foliage is exposed to salts, rinse leaves with good-quality water if possible.
For more information on salinity testing and remediation and lists of plants’ salt tolerance consult Abiotic Disorders of Landscape Plants and Water Quality: Its Effects on Ornamental Plants .
Adapted from the publications above and Pests of Landscape Trees and Shrubs: An Integrated Pest Management Guide, University of California Statewide Integrated Pest Management Program (UC IPM).
Chlorotic and necrotic leaf margins on maidenhair tree due to salt toxicity.
Undersized hackberry leaves with marginal chlorosis and necrosis due to excess chloride.
Dieback of clematis leaf tips caused by salty irrigation water.
Necrotic (brown) frond tips in palms exposed to salty air from the ocean.
Salt toxicity in conifers presents first in the tips of needles then spreads downwards as in this pine.
Necrotic (brown) and dead (whitish) needle tips in coast redwood due to salinity.