The presence of excessive soluble salts is perhaps the most limiting factor in the production of greenhouse crops. Generally speaking salt accumulations result from the use of poor quality irrigation water, over fertilization or growing media with an inherently high salt content. Although soluble salts can inhibit plant growth, when managed properly their effects may be reduced.
Salt Injury to Plants
Plant injury resulting from excessive soluble salts may first occur as a mild chlorosis of the foliage, later progressing to a necrosis of leaf tips and margins. This type of injury is largely attributed to the mobility of soluble salts within the plant. As these salts are rapidly translocated throughout the plant, they accumulate at the leaf tips and margins. Once the salts reach a toxic level they cause the characteristic “burn” associated with excessive salts.
Roots may also be injured by the presence of soluble salts. This often predisposes the plant to a wide range of root diseases (i.e., phythium, fusarium, etc.). Extreme injury may also interfere with water uptake and result in excessive wilting of the plant. It is extremely important to inspect the root systems of plants on a regular basis in order to monitor the effects of soluble salts.
Irrigation water is a major contributor of soluble salts to the growing medium. These occur primarily as salts of Na, Ca and Mg, although others may be present.
Soluble salts in irrigation water are measured in terms of electrical conductivity (EC). The higher the salt content the greater the EC. In general EC values exceeding 2.0 millimhos/ cc are considered detrimental to plant growth. Water quality should be monitored on a frequent basis in order to avoid potential problems from soluble salts.
Fertilizers are forms of salts and therefore contribute to the total soluble salt content of the growing medium. Depending on the inherent salt content of the irrigation water used, fertility levels must be adjusted to avoid salt accumulations.
Fertilizers are often classified by the amount of total salts they contain. This “salt index” can be used to determine the amount of salts contributed to the growing medium. Table 1 presents the salt index of a number of commonly used fertilizers.
Sodium nitrate was arbitrarily set at 100. The lower the index value the smaller the contribution the fertilizer makes to the level of soluble salts.
Growing media can be formulated from a variety of components. These include peat, perlite, vermiculite, pine bark and others. Generally speaking these materials do not contain excessive quantities of soluble salts. However it is important to monitor the quality of media components carefully.
In some cases it is necessary to thoroughly leach a medium before using it. This is particularly important for seed germination and other forms of propagation. Leaching may be accomplished by running water through individual pots or trays prior to planting or by leaching the entire volume of bulk medium.
For a quantitative evaluation of this process the electrical conductivity of the leachate may be evaluated. When the EC is less than 2.0 millimhos the medium is free of excessive salts.
Managing Soluble Salts
Managing soluble salts involves an integrated approach to production. This includes the type of growing medium used, irrigation frequency, water quality, fertility regime and plant tolerance.
Growing media should contain a substantial quantity of large pores to facilitate good drainage. Media with these characteristics are easily leached and reduce the potential for the accumulation of soluble salts. When irrigating this medium it is important to apply enough water to allow sufficient quantities to leach through the container. Approximately 15-20% more water than the container can hold should be applied at each irrigation if the salt hazard is high. Water pressure must be adjusted to avoid overflow.
Since the concentration of soluble salts in plant tissues increases as moisture levels decrease, it is important to monitor the water content of the growing medium. In the presence of excessive soluble salts, growing media should not be allowed to dry out. Maintaining adequate moisture levels can be difficult in porous growing media and requires careful attention.
Providing adequate fertility is important in maintaining optimum plant growth. However if fertility levels are too high injury from soluble salts may occur. Determining the amount of nutrients to use must be based on the quality of irrigation water as well as the fertilizer’s salt index. Generally most fertility regimes used for the production of potted greenhouse crops are between 150 and 350 ppm (N). Higher levels of fertility create a much greater potential for injury from soluble salts.
Perhaps the most effective means of managing soluble salts is to avoid producing salt sensitive plants. Each plant species has a distinct response to salt accumulations and growers often can select those with tolerance. Among the plants with a known susceptibility to soluble salts are chlorophytum, African violets, calceolaria, chrysanthemums, geraniums and petunias.