Amelioration of Salt Damage to Cotoneaster By Gypsum

Materials and Methods

Healthy, vigorous rooted cotoneaster cuttings were transplanted into 15cm plastic pots containing a soil:peat:perlite (1:1:1) medium or a gypsum-amended medium of the same composition. Three gypsum sources were used: United States Gypsum Granular; Sof’n-Soil Lawn and Garden (granular); and Sof’n-Soil Lawn and Garden (fine). These sources were incorporated or surface applied at rates of 20 and 40 lb/100 sq. ft. (17.6 g/pot=20 lb rate or 35.3 g/pot=40 lb rate). The surface treatment was applied after transplanting, while the incorporated gypsum was mixed with the media and the plants potted thereafter. Control consisted of the medium without gypsum.

Plants were pruned to a uniform height and placed in the greenhouse under 15-hour photoperiods at 24° day/20°C night temperatures. Plants were fertilized for 8 weeks with Hoagland’s No. 1 solution (4) at a rate of 200 ml/container on alternate days. Leachates were collected weekly from each container and pH and electrical conductivity were determined. When no significant pH or conductivity changes were evident, the leachates were collected at 2 week intervals thereafter. Only initial and final values are presented.

Salt treatments were initiated in the ninth week. Each container was supplied with 200 ml of 0.15N NaCI on alternate days and with a Hoagland’s or deionized water treatment the other days. The experiment was terminated after 4 weeks when the majority of the control plants showed 70 to 80 percent or greater necrosis. Plants were evaluated using a rating index (See Table 1 for criteria). Plants were arranged in a completely randomized design with 5 single plant replicates per treatment.

Results and Discussion

Gypsum ameliorated the salt damage on cotoneaster in all treatments except the Sof’n-Soil (fine) surface applications (Table 1). Control plants were approximately 70 to 80 percent necrotic while plants grown with the 3 incorporated gypsum sources at the 2 rates were less than 40 percent necrotic. A high rate of soil salt application was used in this study to produce toxicity symptoms. Under normal conditions container-grown plants would not be exposed to these quantities and the ameliorative effect of gypsum would possibly be greater.

The incorporated gypsum proved more effective than surface applications. The use of gypsum on established plants in tree lawns and containers, however, would probably necessitate surface applications. Three gypsum applications methods used in Maine experiments were most effective in the following order: surface, subsurface, and subsurface combined with mechanical soil aeration (9). Drill hole application would work but the cost might preclude this method. In new plantings the gypsum could be easily mixed with the backfill.

Both rates appeared to be effective. The 20 and 40 lb./100 sq. ft. translate into 4.36 and 8.72 tons/acre, respectively. Rates of gypsum application required to achieve soil salts reduction varies from 4 to 15 tons/acre (9). Plice (7) reported that 4 to 6 tons/acre reclaimed strongly saline soils, and that 12 tons/acre offered the advantage of reducing salts more quickly. Gypsum treatments are effective over a long time period and annual or biennial treatments are not necessary.

The electrical conductivities (measure of total ions in solution) were significantly lower in all incorporated gypsum treatments than in the controls. Surface applications of Sof’n-Soil (granular) at 40 lb and Sof’n-Soil (fine) at both rates also resulted in lower conductivities. The plants grew better in the soils with lower salt content. This is to be expected in view of the reported effects of high soil salts on plant growth (8). Although tissue analysis was not completed, it is logical to assume that levels of Na and Cl were lower in plants grown with gypsum. Ayoub (1) reported a 30 to 85 percent reduction in leaf Na from plants grown in saline soils treated with gypsum. Jacobs (6) showed that gypsum applications resulted in greater soil Cl reductions than those of controls (non-gypsum). Both Na and Cl ions have been implicated in plant salt damage (2,5,10) and if they are effectively reduced in plant tissue then increased vigor should result.

The pH changes due to gypsum treatments were negligible although there was 0.5 to 0.8 unit change from the initial values to the final. This was due largely to the Hoagland’s solution which is 100% nitrate nitrogen and pH rises with NO₃ fertility.

Summary and Conclusions

Gypsum appears to have potential for alleviating soil-salt damage to landscape plants. A broader spectrum of plants must be screened and a more definitive range of gypsum concentrations should be examined. Soil and plant tissue analyses are needed to corroborate the observed beneficial effects. All sources of gypsum, when incorporated, were effective. The Sof’n-Soil (fine) is difficult to work with. The granular forms are recommended.