High Soil Salts Injury to Vegetable Crops
Vegetable Disease Information Note 10 (VDIN-0010)
Charles W Averre, Extension Plant Pathologist
J. Paul Lilly, Extension Soil Scientist

Actively growing vegetable crops require a continuous supply of balanced nutrients in the soil. These are usually provided by the application of fertilizers which include soluble salts. However, if the concentration of any soluble salt in the soil including those from fertilizers, become too high, the roots and later the plant tops are injured. In NC the majority of soluble salt problems originate from the improper use of fertilizers.

Symptoms of "salt" or "fertilizer" injury may be variable but usually include: slow and spotty seed germination, sudden wilting, stunted growth, marginal burn on leaves (especially lower, older leaves), leaf yellowing, leaf fall, dead roots, restricted root development, and sudden or gradual death of plants. While the major effect of high soil salts are to the roots, the tops of plants may show "salt injury" while the roots are apparently unaffected. In this case, the soluble salts enter the roots and are moved through the plant vessels to the leaves where the water evaporates and gradually concentrate the salts to toxic levels. When soil soluble salts are excessively high, the roots are unable to absorb water and the plant wilts. The problem is apt to be more pronounced after fertilizing, or during droughts or semi-drought conditions. Allowing the soil to become too dry for even a few hours can result in "salt injury". Plants may recover from "salt injury" provided the highs salts in the soil is reduced. This may occur naturally after a rain or alleviated by irrigation.

How to Avoid High Salt Injury

Have soil tested prior to planting by Agronomic Division (Reedy Creek Road, Raleigh NC 27607) and fertilize according to the recommendations including placement.

Use the correct amount of fertilizer of the proper analysis. If the recommendation calls for 200 lb of an 8-8-8, do not get mixed up and use 200 lb of 12-12-12 or 20-20-20 or a 0-0-60. High analysis fertilizer have less salt problems per unit of plant food than low analysis fertilizers. The salt index of sodium nitrates is 100 while the salt index of ammonium nitrate is 104. However, because ammonium nitrate contains more than twice as much nitrogen as sodium nitrate, the salt index per unit of plant food is half as much (sodium contributes to salt index but is not a plant nutrient). Try to avoid the use of high-salt fertilizers such as "soda" (sodium nitrate), in greenhouses.

Place the fertilizer correctly. There is less danger of salt injury when fertilizers are broadcast and then disked in. Banded fertilizer applications require care. Do not seed or plant on top of the banded fertilizer or closer than 4 inches to it. "Sidedress" the plants so that the fertilizer is dispersed over the root zone, not concentrated close to the stem or concentrated in a narrow band over the roots.

The excess "soil salts" must be leached (washed) from the root zone. This can be done by overhead irrigation with three to four inches of water. For slight "salt" problems, maintain good, uniform soil moisture and avoid drought conditions by irrigation to supplement rainfall; this will gradually eliminate the problem as plants assimilate nutrients portions of the salts.

In some cases, banded fertilizers can be dispersed by shallow cultivation.

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Recommendations of specific chemicals are based upon information on the manufacturer's label and performance in a limited number of trials. Because environmental conditions and methods of application by growers may vary widely, performance of the chemical will not always conform to the safety and pest control standards indicated by experimental data. All recommendations for pesticide use were legal at the time of publication, but the status of registration and use patterns are subject to change by actions of state and federal regulatory agencies.