Salt and plant health

USING SODIUM CHLORIDE TO CONTROL PLANT DISEASES
W. H. Elmer
The Connecticut Agricultural Experiment Station
123 Huntington St.
P. O. Box 1106
New Haven, CT 06504 USA

Long before scientists understood the role of sodium or chloride in crop production and plant disease management, farmers routinely applied sodium chloride to salt-tolerant crops to boost vigor and yields. Interestingly, a steady flow of studies over the past half century conclude that when sodium chloride is applied in quantities equal to macronutrients, certain crops fare better. These studies validate the opinions of a number of agriculturalists that touted the value of sodium chloride in crop management.

The first mention of using NaCl in crop management was a recommendation in the early 1800’s that salt be used as a top-dressing on barley to prevent lodging (a condition that may have been precipitated by a root disease) (Tottingham 1919). In the late 1800’s, a spring dressing of rock salt (NaCl) was considered to be “an excellent thing to apply to asparagus” to promote growth and suppress weeds (Walker 1905) and as later research has shown, to lessen damage from Fusarium crown and root rot (Elmer 1992). Chloride salts were also routinely applied to other plants like beets, celery, and Swiss chard. If NaCl applications were not critical for crop growth, a significant consensus has emerged over the past two centuries that they are certainly beneficial for farming.

Studies of salt application were often done despite the prevailing opinion that NaCl applications were harmful. The fictitious account about the Romans salting Carthage to destroy plant life led to misguided beliefs about the use of salt in agriculture. Studies on salt-sensitive crops like tomatoes and strawberries and roadside salt damage along highways reinforced an attitude that salt had no place in agriculture. In fact, plant physiologists have not identified an essential role for sodium in plant health and simply assumed that chloride, known to be essential for photosynthesis, was so highly available in soils that salt supplements were unnecessary.

This confusion about NaCl use arose from conflicting reports as to its effect on certain crops. The benefits were observed only intermittently or not at all. Those studies led to the observation that the benefits of NaCl were most evident when plants were under stress by disease or drought. These observations suggested that NaCl fertilization may improve defense mechanisms against stress factors and may explain the lack of response when disease or stress factors are absent.

While sodium is not essential in plant growth, some isolated studies have found that sodium can substitute for the role of potassium when potassium levels are low. Most salt-tolerant plants have evolved the ability to exclude sodium from their cells or compartmentalize it in vacuoles.

Chloride is a different matter. Plant roots readily absorb chloride. Although the amount of chloride required by plants for photosynthesis is sufficed by extremely small concentrations, high rates of chloride have notably positive effects on soil/root relations, such as inhibiting the conversion of nitrate to ammonia, enhancing manganese availability, and increasing beneficial microorganisms. Chloride affects physiological processes, such as osmoregulation and organic and amino acid synthesis, which also have direct effects on nutrient cycling and root exudation. Inasmuch as all these factors directly or indirectly influence the plant’s ability to withstand stress and resist disease, sodium chloride may function through many mechanisms that are not mutually exclusive from each other. The majority of reports demonstrating disease suppression with NaCl fertilization have been made on monocots such as asparagus, barley, coconut, and date palm. However, dicots like beets and celery also have shown considerable benefit from NaCl. A discussion on how NaCl affects different diseases of various crops is discussed below.

Asparagus

Long before herbicides were available, growers annually dressed their asparagus fields with rock salt (sodium chloride) to suppress weeds and promote growth. The practice was discontinued in the 1940’s when synthetic herbicides were made. In the decades that followed, the number of reports of Fusarium crown and root rot increased in the US. Pathogenic species of the fungus Fusarium cause the disease. When NaCl was applied to fields affected by the disease, spear yield was increased by 15-30% and disease symptoms were reduced. For example, in these declining fields, yield averages 2,000-2,500 lb/A. Sodium chloride (applied as rock salt) at 500 lbs/A costs $7.50 to $30.00 depending on supplier. Assuming fresh asparagus would market at $2.00 /lb, A 15-30% increase per acre would generate between $700 to $2,143 /A in additional revenue. Spear number was usually not affected indicating that growers were getting larger spears. The chloride salts NaCl, KCl, CaCl2, MgCl2, and NH4Cl all have some ameliorating effect on disease, but NaCl was superior. Sodium carbonate or sodium nitrate (NaNO3) had little effect. The preference for NaCl over the other chloride sources may be because asparagus restricts Na uptake. This may allow the plant to absorb large concentrations of Cl in the absence of a metabolically active cation like K+ or NH4+ (Elmer 1992).

Barley

Of all the edible grains, barley has the highest tolerance for NaCl. The influence of NaCl on barley was first observed in the mid 1800’s where chloride applied as NaCl was found to stiffen the straw and prevent lodging. It is not clear from these reports if root disease was present, but root-rotting organisms are certainly a possibility. Long-term field tests were conducted with chloride on barley in Saskatchewan, Canada (Tinline et al., 1993), which found that both NaCl and KCl reduced common root rot. Other groups similarly found that NaCl or KCl did not differ in their effects and reported that 50 kg NaCl/ha reduced common root rot of barley in 2 of 6 experiments.

Beets

Beets presumably evolved from the wild maritime habitats, so it is not surprising to find them ranked as moderately tolerant to salinity. Several researchers interested in the role of potassium and sodium fertilizers first examined the beneficial role of NaCl on growth of sugar beets, table beets, and Swiss chard (Adams 1961). A study on the role of chloride salts on Rhizoctonia root rot caused by Rhizoctonia solani found that NaCl, KCl, CaCl2, and MgCl2 all equally suppressed disease and promoted growth (Elmer 1997).

Coconut and Date Palm

The knowledge that coconuts and date palms respond to NaCl may not be surprising given the likely evolution of these plants in saline soils and their proximity to the shoreline habitats. The role of NaCl on growth and overall health of coconut was investigated long before its role in disease was discovered. Later researchers realized that NaCl applications could help restore infertile soils in the Philippines back to productive plantings. In the mid 1970’s, researchers found applications of NaCl would suppress leaf spot disease caused by Bipolaris incurvata on coconut seedlings and that this treatment worked as well as fungicides (Magat et al., 1977). Given the relative cost of NaCl versus chemical fungicides, the savings were significant.

Corn

For over half a century, researchers have examined the role of chloride on corn in the suppression of stalk rot caused by Gibberella zeae and Gibberella fujikuori. As with most crops, most researchers focused on KCl believing that the potassium ion was more important, but studies eventually demonstrated that chloride was the disease-suppressing ion. When the actual role of sodium was investigated, they found that compared to controls, NaCl reduced smut by 23% as compared to an 11% reduction with Na2SO4 (Kostandi and Soilman, 1998).

Wheat

The effects of chloride salts on wheat diseases have probably been studied more thoroughly than on any other crop. A 1978 study found that applications of NaCl or KCl reduced the severity of stripe (yellow) rust of wheat caused by Puccinia striiformis (Russell 1978). An application of NaCl at 1130 kg/ha reduced disease incidence by an average of 63%. The effect was observed on six cultivars of wheat.

How does it work?

There have been many attempts to decipher the role of NaCl on disease. Sodium chloride is not fungicidal in soils since most soilborne pathogens grow better in culture as NaCl concentrations are increased to 0.5 to 1.0%. Moreover, densities of pathogens in soil remain relatively unchanged following NaCl applications. Alternatively, researchers suggest NaCl acts through the soil environment, host physiology, and/or microbial community. In acids soil, NaCl inhibits conversion of NH4-N to NO3-N presumably due to its inhibitory effect on species of Nitrosomonas bacteria. Maintaining nitrogen as NH4 can lower soil pH, change microbial populations, and alter host nutrition. In addition, acid soils treated with NaCl show an immediate release of soluble manganese ions. Manganese has been implicated in disease suppression probably through its effect on increasing host resistance. Since NaCl can also suppress disease and increase manganese levels in alkaline soils as well as in acid soils, it is obvious that mechanisms other than nitrification and chemical reduction of manganese must be operating.

As mentioned above, sodium is not known to benefit many physiological systems in plants. Chloride, on the other hand, is essential for photosynthesis and is the only inorganic anion that is not structurally bound to metabolites. One of its major roles is to serve as a charge-balancing ion to the vast number of cations present in plant cells. When a cell absorbs chloride, it accumulates in the cell vacuole and lowers the cell water potential below that of the medium surrounding the cell. Water then flows into the cell and increases hydrostatic cell pressure so it maintains a pressure that exceeds the force exerted by the plasmalemma. The cells remain turgid and are able to grow even when drought conditions prevail. This was first investigated in England in the 1970’s when applications of NaCl increased the water capacity of the sugar beet leaves and improved growth of the plant during periods of soil moisture deficits. Similar reports of NaCl suppressing disease while reducing osmotic potentials have been made on pearl millet affected by downy mildew, wheat affected by take-all disease, and asparagus affected by Fusarium. Other reports have shown similar effects using KCl to suppress disease.

Changes in osmotic potential affect the water cycling of plants and the exudation of carbon substrates. These substrates serve as a food base for microbes that live on and around the root. In 1980, the possible role that beneficial Pseudomonas species might play in disease suppression on chloride-treated wheat plants was recognized. Studies with KCl on celery confirmed that root exudates were being altered. When asparagus plants were treated with NaCl, an increase in the beneficial Pseudomonas species was noted (Elmer 2003). Thus, treating salt-tolerant plants with NaCl causes a root-mediated effect on the microbial community.

The major influence of NaCl fertilization on plant disease appears to be reduction of cell osmotic potential, increased manganese uptake, and enhancement of beneficial microbes via altered root exudation. Soil pH may have a governing effect on whether manganese uptake is mediated chemically or microbiologically. In acid soils (<6.6), NaCl suppresses nitrification, whereas in neutral to alkaline soils, NaCl may enhance manganese availability by altering the nutritional composition of the root exudates that, in turn, favors microbes that possess the manganese reduction trait. These mechanisms would seemingly have far-reaching effects on both foliar and root diseases.

The final resolution of NaCl’s effect on plant disease, however, may require the use of genetic manipulations of chloride-sensitive plants. For example, NaCl-tolerant and -sensitive lines of Arabidopsis are available. Specific genes that affect sodium and chloride accumulations and partitioning in the plants could be used in studies designed to test water potential effects in the absence of manganese fluctuations and vice versa. Furthermore, transfer of particular genes that confer tolerance to NaCl into salt-sensitive plants may allow salt applications to be useful in managing these crops when stress prevails.

A better understanding of rates and timing of NaCl supplementation for plants at critical periods of development or pathogenesis will help agriculturalists better target NaCl nutrition, thus reducing demand on alternative control strategies, such as fungicides and fumigants. Many asparagus, beet, and coconut growers around the world have already adopted NaCl into their management programs.

Key References

Adams, S. N. 1961. The effect of sodium and potassium on sugar beet on the Lincolnshire limestone soils. J. Agric. Sci., Cambridge 56:283-286.

Elmer, W. H. 1992. Suppression of Fusarium crown and root rot of asparagus with sodium chloride. Phytopathology. 82:97-104.

Elmer, W. H. 1997. Influence of chloride and nitrogen form on Rhizoctonia root and crown rot of table beets. Plant Dis. 81:635-640.

Elmer, W. H. 2000. Use of NaCl to suppress root diseases of asparagus, beets, and cyclamen. Pages 234-237, In Proc. of Salt 2000 Symposium, May 2000, The Hague, The Netherlands.

Elmer, W. H. 2003. Local and systemic effects of NaCl on asparagus root composition, rhizosphere bacteria, and development of Fusarium crown and root rot. Phytopathology 93:186-195

Fixen, P. E. 1993. Crop responses to chloride. Adv. Agron. 50, 107-150.

Hammer, P. M., and Beene, E. J. 1941. Effects of applying common salt to a muck soil on yield composition and quality of certain vegetable crops and on the composition of the soil producing them. Agron. J. 33:952-979.

Kostandi, S. F., and Soilman, M. F. 1998. Effect of saline environments on yield and smut disease severity of different corn genotypes (Zea mays. L). Phytopath. Z. 146:185-189.

Magat, S. S., Margate, R. Z., and Prudente, R. L. 1977. Utilization of common salt (sodium chloride) as a fertilizer and for the control of leaf spot disease of coconut seedlings. Phil. J. Cocon. Stud. 13, 2:39-45.

Maas, E. V. 1986. Physiological responses to chloride. Pages 4-20, In: Special Bulletin on Chloride and Crop Production T. L. Jackson, ed., No. 2, Potash & Phosphate Institute, Atlanta, Georgia.

Milford, G. F. J., Cormack, W. F., and Durrant, M. J. 1977. Effects of sodium chloride on water status and growth of sugar beet. J. Exp. Bot 28:1380-1388.

Russell, G. E. 1978. Some effects of applied potassium and sodium chloride on yellow rust in winter wheat. Ann Appl. Biol. 90:163-168.

Tinline, R. D., Ukrainetz, H., and Spurr, D. T., 1993. Effect of fertilizers and of liming acid soils on common root rot in whear and chloride on the disease in wheat and barley. Can. J. Plant Path. 15:65-73

Tottingham, W. E. 1919. A preliminary study of the influence of chlorides on the growth of certain agricultural plants. J. Am. Soc. Agron. 11:1-32.

Walker, E. 1905. Asparagus and salt. Arkansas Agric. Exp. Stn. Bull. 86:31-36.