SALT AND SOIL
SALT DAMAGE TO PLANTS
Contact: Dr. Leonard Perry, Extensio
University of Vermont
“Salt is good, but if salt has lost its taste, how shall its saltiness be restored? It is of no use either for the soil or for the manure pile. It is thrown away. He who has ears to hear, let him hear.”
Most people are only too aware of the
damage and corrosive effects of salt on automobiles. On heavily traveled highways from 40 to 80 tons of salt per lane mile per year may be applied. Landowners along these roads also are aware of the damage to plants that such salt can cause.
Deicing salt is usually refined rock salt consisting of about 98.5 percent sodium chloride, 1.2 percent calcium sulfate, 0.1 percent magnesium chloride, and 0.2 percent rock. Calcium chloride is reported to be less toxic to plants but is seldom used because it is much more expensive than rock salt and more difficult to handle.
When sprayed onto plants from passing cars and plows, salt may enter plant cells or the spaces between the cells directly. One result of this "salt application" is that buds and small twigs of some plant species lose cold hardiness and are more likely to be killed by freezing.
Salt accumulation in the soil also may cause plant injury. This frequently occurs when salt-laden snow is plowed off streets and sidewalks onto adjacent lawns.
Anyone who has tried to get table salt out of a wet shaker knows how readily salt absorbs water. Rock salt exhibits the same property in the soil and absorbs much of the water that would normally be available to roots. Thus, even though soil moisture is plentiful, high amounts of salt can result in a drought-like environment for plants.
When salt dissolves in water, sodium and chloride ions separate and may then harm the plants. Chloride ions are readily absorbed by the roots, transported to the leaves, and accumulate there to toxic levels. It is these toxic levels that cause the characteristic marginal leaf scorch.
Measures to prevent or lessen injury from salt include using calcium chloride, where feasible, or using sand or cinders. Late season applications (after March 1) are most detrimental and should be avoided if possible since this is the time plants are coming out of dormancy and are most susceptible to injury.
The salts are harmful when they reach a concentration too high for the optimum of plant growth and yield. It is important to know what this means, since a plant which is not growing at its optimum often has no visible symptoms other than it is growing and producing less than it should. Excess concentrations of salts dissolved in soil water are harmful to the plant in two ways. One way is by osmotic influences and the other by specific ion toxicities. We all learned in high school the process of osmosis, where a solvent (in this case water) flows through a semi-permeable membrane into an area of higher solute concentration. Since the cells in plant roots generally have a higher concentration of solutes (solutes being the sugars and organic compounds that the cells carry) than the surrounding soil water, water flows through the semi-permeable cell walls and into the root cells, and the plant takes up water. However, as the salt concentration soil rises, this difference is reduced, and water does flow not as freely into the cells. The plant can compensate by synthesizing organic compounds such as sugars and organic acids, or accumulating salts to raise the concentration between inside and outside. This costs energy, with the result being a plant which is growing and producing at less than the optimum because of this diversion of resources. Another way salts in the soil water can affect the growth of plants is by specific ion toxicities. Ions are atoms or groups of atoms with a net positive or negative charge. Ions of soil minerals which make up salts, such as chlorine, boron, and sodium, are absorbed by the plant roots and accumulate in the plant stem and leaves over time. These accumulated ions can become toxic to plants, and ions such as boron can be toxic to plants even at low concentrations. Symptoms of ion toxicity can vary by crop tolerance and stage of growth, but oftentimes ion toxicity will manifest itself as “leaf burning” at the leaf edges, especially on the older leaves.