The soil solution is normally a weaker solution than the plant cell contents. In these circumstances plants readily take up water through their roots by osmosis. As more salt, such as soluble fertilizer, is added to the soil solution, salt concentrations are increased and less water, on balance, is taken up by roots. When salt concentrations are balanced as much water passes out of the roots as into them. When salt concentrations are greater in the soil the roots are plasmolyzed. The root hairs, then the roots, are ‘scorched’, i.e. irreversibly damaged, and the plant dries up.
Symptoms of high salt concentration above ground are related to the water stress created. Plants wilt more often and go brown at the leaf margin. Prolonged exposure to these conditions produces hard, brittle plants, often with a blue tinge. Eventually severe cases become desiccated.
Salt concentration levels are measured indirectly using the fact that the solution becomes a better conductor of electricity as salt concentration is increased. The conductivity of soil solution is measured with a conductivity meter (see ionic compounds).
Salt concentration problems are most common where fertilizer salts accumulate, as in climates with no rainfall period to leach the soil and in protected culture. Periods when rainfall exceeds evaporation, as in the British Isles during winter, ensure that salts are washed out of the ground. Any plant can be damaged by applications of excess fertilizer. Some plants, such as tomatoes and celery, are more tolerant than others, but seedlings are very sensitive. Young roots can be scorched by the close proximity of fertilizer granules in the seedbed (see band placement).
In protected culture large quantities of fertilizer are used and residues can accumulate, particularly if application is not well adjusted to plant use. Sensitive plants, such as lettuce, are particularly at risk when following heavily-fed, more tolerant plants, such as tomatoes or celery. Salt concentration levels should be carefully monitored and feeding adjusted accordingly, applying water alone if necessary. Soils can be flooded with water between plantings to leach excess salts. Large quantities of water are needed, but should be applied so that the soil surface is not damaged. Every effort should be made to minimize the effect on the environment and quantities of water needed to flush out the excess salts by reducing the nutrient levels as the crop comes to an end.
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