Feature

Understanding Salt in the Soil

By Arthur Siller, UMass Extension Soil Health Educator

What kind of salt is in the soil?

It can be a little confusing to understand exactly what is meant by salt in soil. There are a number of words that come up and that you might see on soil tests, Extension factsheets, and horticultural guides. This confusion stems from the fact that we use the same word – salt – to talk about both 1) any solid material that dissolves in water as positively and negatively charged bits and 2) one specific material – sodium chloride or rock/table salt. This makes it important to understand the terminology.

Salinity, or soluble salt content, refers broadly to the amount of any type of salt present in the soil. The salts could be dissolved in the soil water, attached to soil particles on cation exchange sites, or as crystals in particularly saline dirt. When the salts are split into their charged halves, the halves are often called ions.

Sodicity refers to the amount of sodium ions in the soil. This can occur independently of salinity, but it’s definitely possible for soil to have both high salinity and sodicity.

What does salt do in soil?

Salts affect both soil physical properties and soil water dynamics. A high concentration of salts in general can draw water out of plant tissues... killing them. This desiccation can also happen directly on aboveground leaves and stems in direct contact with water-soluble fertilizer or fresh manure. Sodium in the soil also causes soil aggregates to fall apart more easily, leading to reduced water infiltration and percolation. Excess sodium in the absence of other salts also leads to high pH (>8.5).

How much salt is too much?

For salinity, it mostly depends on the variety of plant. Some plants are more tolerant of salinity. In many cases, water management can reduce salts but if there is a chronic salt problem, choose tolerant species and varieties. Because more salts in the soil water make it carry electricity more easily, salinity is measured in the laboratory and reported as electrical conductivity, with above 0.6 dS/m considered saline by the UMass Soil Testing Lab. Remember than many plants can tolerate mild to moderate salinity.

More than 15% sodicity is considered problematic. This is when more than 15% of the small, positively-charged molecules (cation salts) are sodium atoms. The amount of sodium that it takes to make dirt sodic depends on the overall amount of space in the soil for cations. This cation exchange capacity (CEC) is higher in fine-textured soil or in dirt with higher organic matter levels so these soils can contain more total sodium before becoming sodic.

How does dirt get salty?

Salinity in Massachusetts is uncommon in soil outside of protected environments (high tunnels, potted plants) because we get enough rain to keep excess salts from accumulating in the root zone. Temporary high salinity can also occur after manure and fertilizer application (especially in dry weather) or from ice melting salt (especially in spring).

Sodicity occurs from the use of rock salt (sodium chloride) on roads and sidewalks in the winter. It is primarily a problem if salted snow is collected near plants or if rock salt is accidentally spread on plants outside of pavement.

What can be done about salty soil?

As with many things, prevention is ideal. Be careful spreading rock salt, use composted manure when possible, and keep fertilizer from direct contact with plants in dry conditions.

In sodic dirt, gypsum should be added before flushing the soil because it helps to loosen the sodium that is attached to clay and organic matter.

You may notice that these methods require good water drainage. If drainage is poor, it is very difficult to remove either sodium or salts in general; extra attention should be paid to prevention and salt tolerant plants need to be used if salt accumulation is already a problem.

Further Reading

The Impact of Salts on Plants and How to Reduce Plant Injury from Winter Salt Applications https://ag.umass.edu/landscape/fact-sheets/impact-of-saltson-plants-how-to-reduce-plant-injury-from-winter-salt

Impact of Road Salt on Adjacent Vegetation https://plant-pest-advisory.rutgers.edu/impact-of-road-salt-on-adjacent-vegetation/

UMass Hort Notes 2024 Vol. 35:9