One of the challenges to growing a successful garden in the desert is the proper management of salt.
Since salt accumulation is unique to desert environments, those new to desert gardening may not recognize the effects that salt accumulations can have on plants. When you see wilting plants, leaves with tips and edges turning brown, and salt-induced iron deficiencies know that you probably are experiencing the buildup of salt in and around your garden and landscape plants. Fortunately, salts usually can be successfully managed with a little planning and attention to watering habits.
Salts are chemical compounds of common, everyday elements that are broken down from rock formations. Soluble in water, they are easily dissolved as rain or irrigation water moves through the soil profile. In high rainfall areas, the large volume of water moving through the soil carries these compounds down through and past the root systems of plants so that accumulations never reach toxic levels. This is why salts are rarely a problem in the less arid areas of the country.
In the desert, however, rainfall is rarely sufficient to dissolve and leach salts below the root systems of plants. Many native plants are adapted to these conditions and do not seem to be bothered by salt in their environment. Those native plants that are susceptible to salts simply do not grow where salt accumulations in the soil have reached toxic levels. When susceptible, non-native plants are brought into the desert and planted into a salty environment, trouble occurs.
Salt injury can occur at any time during the year but seems to be more frequent during the hot summer months, when plants are under peak stress and using water at a rapid pace. The increased metabolism of plants during the summer speeds the accumulation of salts and hastens the appearance of symptoms in plant tissues.
Salts, dissolved in water, are generally absorbed by plants through the roots. Once inside, they move up through the water-conducting tubes to the leaves. When the water within the leaves evaporates and exits through the process of transpiration, the salts are left behind. When sufficient salt has been deposited to reach levels toxic to that particular plant, symptoms appear.
The most common symptom of salt injury in plants is the dying back of tips and edges of leaves. As salts accumulate, the plant tissue in these areas may turn yellow first, but this is not always the case. The browning of these tissues is caused by the actual death of cells and tissues. As the condition worsens, the dead, brown areas may get larger until the entire leaf dies and falls from the plant.
Sometimes salt accumulates in the soil around roots to the point that it prevents the absorption of water by the plant. It is easy to tell when this happens by looking at the leaves and soft stems. The soil will be moist around the plant, but the plant itself will show water stress symptoms, including hot, wilting, drooping leaves and stems. Often times, these symptoms will be accompanied by the tip and margin burn symptoms described earlier.
Another symptom of salt problems is iron deficiency. Because salts have a basic or alkaline pH, they can raise the soil pH to the point where iron and other nutrients are insoluble in the soil. High soil pH values make it difficult, if not impossible, for the plant to absorb and use these essential nutrients.
Leaves of tender plants show iron deficiency symptoms when they start to turn a pale to brilliant yellow color, but the veins of the leaves stay green. This is a sign of iron deficiency. When this happens, cut back on the watering frequency while increasing the length of time the water is being applied to the soil and treat the plant with a chelated iron fertilizer product. The leaves should turn green within a week to 10 days.
The solution to salt problems is usually fairly simple. In most cases, salt accumulations are caused by frequent, shallow irrigations. First, know and understand that the roots of most plants go deep into the soil and that when we are irrigating, no matter whether it is with a hose or a drip system, that the entire root system must be moistened to move the salts down and out of the root zone. Remember also that a salt layer can form out laterally from the tree. Most people have seen a white ring around their irrigated plants. That is the salt layer. Make sure that you are irrigating out to the edge of the plant and not just next to its stem or trunk.
There is a handy rule of thumb to help estimate the depth needed to properly irrigate plants. We call it the “rule of 12's.” The rule of 12's states simply that bedding plants, turf, vegetables, flowers and other shallow rooted plants need to be irrigated to a depth of 12 inches. The soil around shrubs needs to be irrigated to 24 inches and trees should be moistened to a depth of 36 inches.
I like to use a soil probe to see how far the water has sunk into the ground. The probe slides easily into wet soil but stops abruptly when it reaches the dry layer. I place my fingers on the barrel of the probe at the point where the probe emerges from the soil. Then, when I pull out the probe, I can easily see the depth of water penetration.
Second, during irrigations, turn down the volume of water coming from the hose to a trickle and slowly fill the basin around the plant. The extra water from the longer irrigations will sink deeper into the soil and move the salts down and away from the plant. There is an added benefit to doing this. Water stored deep in the ground will allow lower roots to help support the plant’s water needs and lengthen out the time necessary between irrigations. For those using drip irrigation systems, increase the duration of the irrigation set to provide the volume of water needed to leach the salts.
For leaves that already have dead tips or margins, there isn’t much that can be done to alleviate the damage, because the tissue is already dead; but new leaves that grow after treatment should not show new damage if the problem has been solved.
Sometimes the soil does not readily accept water because of a hard layer, like caliche, or because of a chemical imbalance, like an overabundance of sodium. Physical barriers can be fixed by digging or drilling holes down through the compacted layer to a more permeable soil layer which will accept the salty water and move it away from the root zone. Backfill the holes with sand to help keep the hole from caving in and reconsolidating.
If sodium salt is a problem, water may stand for hours and sometime even days before it sinks in or evaporates away. The rule of thumb for managing sodium is this: If it takes more than one hour for water around a plant to sink in, treat for sodium.
Gypsum, available by the bag at most nurseries, is an ideal way to deal with sodium problems. Sprinkled on the surface of the ground and raked gently in before a deep irrigation, gypsum will replace the sodium in the soil with calcium. The deep irrigation then leaches the sodium out of the root zone.
Soil sulfur can also be used to eliminate sodium but it will only work if there is calcium or free lime already in the soil. Sulfur, moistened with water, becomes sulfuric acid which combines with the calcium to form gypsum. The process then proceeds as described above.
The coming warm weather months are a difficult time for plants, even those that are adapted to the extreme heat of the desert. Salt damage is one of the more common problems that gardeners must face during this time. By correctly managing irrigation water, most salt problems can be prevented.
If you have questions, you can reach one of the Master Gardeners at the Cooperative Extension office, 820 E. Cottonwood Lane, Building C, in Casa Grande. The telephone is (520) 836-5221, extension 204. The author’s email address is gibsonrd@ag.arizona.edu.
The University of Arizona is an equal opportunity, affirmative action institution. The University does not discriminate on the basis of race, color, religion, sex, national origin, age, disability, veteran status, or sexual orientation in its programs and activities.
Rick Gibson
Extension Agent, Agriculture
University of Arizona Cooperative Extension
820 E. Cottonwood Lane, Building C
Casa Grande, Arizona 85122
Voice: (520) 836-5221
Fax: (520) 836-1750
email: gibsonrd@ag.arizona.edu
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