Landscape irrigation with salty water
Managers and superintendents of golf courses, parks, school grounds and other large landscape installations face increasing public and budgetary demands to use reclaimed sewage water or salty non-potable water for irrigation. Such pressures are especially strong and politically popular in water-short areas of the arid Southwest. However, some grounds managers make the decision to switch from potable to salty non-potable water for irrigation without an adequate understanding of the various constraints or the potential for soil degradation.
Most grounds-care professionals are aware of the potential for problems stemming from the use of salty water. However, many factors other than water quality affect the degree of hazard, making such appraisal difficult. Here, we'll outline some key factors that affect appraisal of potential salt problems from using salty water for irrigating landscapes in the arid Southwest. Much of the information we present here is the result of a project supported by the U.S. Bureau of Reclamation and a cooperative agreement between El Paso Water Utilities and the Texas A&M University Agricultural Research Center at El Paso.
Effects of excess salt Salt affects growth, vigor and appearance of landscape plants in at least two ways. The first is damage caused by foliar contact with salts, and the second is the damage induced by exposure of roots to salty soil solutions.
Foliar-induced salt damage to leaves is common in shrubs, trees and flowers irrigated with sprinklers and can occur on broadleaf plants at dissolved-salt contents (principally sodium and chlorine) as low as 500 ppm, especially with frequent use of mist-forming sprinklers.
Concentrations above 1,000 ppm severely affect many shrubs and trees when directly sprinkled on their foliage. Turf species, however, are relatively tolerant of this form of salt damage (excepting Dichondra and possibly some salt-sensitive bluegrass species). The extent of salt-induced foliar damage varies widely according to plant species, frequency and timing of irrigation, sprinkler type and weather conditions. Fortunately, managers can readily correct such problems with modification of their irrigation schedules or systems.
Root-induced salt damage also can occur in most plants. With some exceptions, plant material prevalent in the Southwest tolerates greater salinity than the existing levels in reclaimed sewage water or salty non-potable water (see Table 1, opposite page). Therefore, root-induced salt problems should, in theory, only occur in situations where dissolved salt from irrigation water accumulates or in soil that is inherently saline. In the Southwest, the first scenario prevails, as irrigation water in this region contains appreciable amounts of salts, typically 500 to 1,500 ppm. Irrigation with such water sources brings in 3 to 12 tons of salt per acre per year at a typical annual irrigation rate of 4 feet.
The salts deposited through irrigation must be leached for sustained growth of landscape plants. Otherwise, soil salinity rapidly will increase with repeated irrigations. Table 2 (below right) shows the drainage (leaching fraction) necessary to keep soil salinity at or below the target level. The term leaching fraction (LF) refers to the fraction of water that drains relative to the amount you apply. Higher salinity in irrigation water demands a greater LF, as do plants with low salinity tolerance. In essence, an appraisal of potential salt problems is a consideration of whether you can maintain an adequate LF on a sustainable basis.
Salts may affect plant growth in other, indirect ways. These include sodium effects, which reduce water penetration and also can increase waterlogging or water stress. Other indirect effects include difficulties with nutrient uptake and potential increases in salt-tolerant weeds. However, these effects are beyond the scope of this article.
Experiences of golf courses Many golf courses in the Southwest and Southern California already use reclaimed sewage water or salty non-potable water for irrigation. El Paso Water Utilities and the consulting firm of CH2M Hill (Albuquerque, N.M.) recently surveyed 13 golf courses using water sources with differing levels of salinity (see Table 4, page 24, for summary of results).
The two golf courses using reclaimed municipal sewage water with dissolved salts of less than about 500 ppm reported no problems.
Three out of seven courses in the survey that used water with 500 to 1,000 ppm dissolved salts reported some, but not widespread, salt problems. These include Coronado (El Paso, Texas) and two courses in Southern California (Tustin Ranch and Harding Park). Salt problems at the latter two courses are confined at the moment to greens with salt-sensitive bentgrass varieties. At Coronado, sporadic salt problems are occurring on fairways and roughs on soils containing an undisturbed layer of hardened caliche. Soil salinity in such areas increased to 6 to 13 dS m-1 in the saturation extract, which is high enough to cause salt damage in many species. Four other Southern California golf courses included in this survey (but not listed in Table 4) reported no salt problem. However, these golf courses use low-salt water for greens, and some of them do not have a long enough history to assess the real salt hazard.
The three golf courses we surveyed that use water with more than 1,000 ppm each reported salt problems.
1. At a Los Angeles course, these were limited to bentgrass greens and to fairways on low-permeability clay soils.
2. Salt problems at a course near El Paso were mostly confined to some high ground where construction activity removed the topsoil over a caliche layer during construction. In addition, many trees that received direct sprinkling with salty water suffered extensive salt-induced foliar damage (see photo, page 24). However, a high level of salt accumulation also occurred in one area where coarse sand underlaid 7 inches of loamy soil (see photo, opposite page). This type of artificial, layered soil profile makes water penetration and salt leaching difficult unless you adjust irrigation depths.
3. Salt problems at another West Texas course have been widespread, and the course has required major renovation. Salts accumulated at the ground surface, including greens, even though the greens are sand-based and the fairways are on deep loamy sand. Trees and shrubs at this course also sustained salt-induced foliar damage.
Soil-salinity surveys Appraisal of the potential for salt problems based on water quality is useful, especially for appraising salt-induced foliar damage. However, it leaves many unanswered questions about long-term soil-salinization potential, which is influenced by soil quality, especially soil permeability.
Parks and schools. With this in mind, we surveyed the soil-salinity status of city parks and schools on seven different soil series in the El Paso area. We had previously sampled most of these same sites in 1978, providing a long-term basis for comparison (see Table 3, at left). We took soil samples in the spring of 1997 from what we considered to be a representative area of each site and analyzed them for salinity of the saturation extract. All the grounds we surveyed had common bermudagrass and have been irrigated for 20 years or more using water with 600 to 800 ppm of dissolved salts and a sodium adsorption ratio (SAR) of 5.5 to 6.5.
The survey showed that high levels of salt accumulation occurred in the Harkey and Glendale series. These soils have texture ranging from silt loam to silty clay loam. The permeability of silty clay loam or silty clay is inherently low and seems to have gotten worse at these sites due to soil compaction and the precipitation of calcium salts in soil pores. The condition of the turf in these areas is not good, and salt leaching seems to have ceased for all practical purposes.
This situation is similar to using salty water to refill a pan as its water evaporates: Water leaves the pan via evaporation, but the dissolved salt does not. Each time you refill, you add more salt and the water becomes saltier. Thus, the use of water with higher salinity is not advisable at sites such as these unless you can find an economical way of improving soil permeability.
Additionally, we found some silt-loam areas that had a water table within 30 inches of the surface, even during winter when no irrigation was occurring. This situation prevents leaching and therefore precludes the use of salty irrigation water without a practical way to cope with the high water table.
We also saw high levels of salt accumulation in limited areas of the Delnorte and Hueco series where a hardened caliche layer remained undisturbed (similar to some areas of the Coronado course located on Delnorte soil). The salt readings there already are high enough to affect growth of intolerant plant species and will rise further with continued use of higher-salinity water for irrigation.
Soil salinity generally increases in proportion to irrigation-water salinity. However, we found no significant salt accumulation in Bluepoint and Canutio soil series, which are deep, well-drained sandy or gravelly soils. These findings indicate that soil texture and the presence of a caliche layer play a major role in salt accumulation by inhibiting leaching.
Golf courses. In addition, we surveyed the soil-salinity status at two golf courses: one that has been irrigated for 35 years with salty water (1,200 to 1,600 ppm) and another along transects with slopes ranging from 15 to 30 percent.
Soil-salinity readings from the former were quite variable, but the variability was higher on higher ground. This may have been due to the effects of grading as well as non-uniform irrigation patterns. Also, the surface of this soil type has small mounds of wind-blown loamy sand and sandy loam that might have filled low spots during grading. The coefficient of variability in salinity readings at high and low grounds was 35 and 14 percent respectively, while the mean salinity did not differ a great deal. This means that complex topography increases variability and can lead to development of salt spots, as we also observed at the other course we surveyed. If you want to determine a meaningful average salinity of low and high grounds, you'll need to take three to ten samples from each.
Suggestions for your situation Salts can adversely affect growth, vigor and appearance of landscape plants through foliar contact via irrigation water, root exposure to salty soil solutions and through indirect effects. A survey of golf courses in the Southwest and Southern California indicates sporadic salt problems on greens and fairways, with poor drainage or poor water infiltration when irrigation-water salinity exceeds about 500 ppm. Above 1,000 ppm, salt problems are common and include both foliar- and root-induced damages. Foliar-induced damage can be quite extensive when water with salinity exceeding 1,000 ppm is sprinkled directly on trees and shrubs.
Long-term salt accumulation is highly soil-dependent, and in soils with texture finer than loam and soils with a caliche layer it can reach 20 to 40 dS m-1 in a matter of 20 years, even when you use relatively low-salt water (600 to 800 ppm) for irrigation. This means that assessing potential salt problems based on water-quality testing alone is too simplistic. You should check soil salinity and leachability of salts before using salty, non-potable water for irrigation, using a soil map and topographical features as a framework for sampling. Remember that complex topography and steep slopes tend to increase soil-salinity variability. Thus, you'll need 3 to 10 soil samples per area consisting of the same soil type and topography for a credible assessment of soil salinity. In soils with poor salt leaching, the grounds manager must weigh the benefit of using salty non-potable water against the cost of amending the soil to make it more permeable, drainable and manageable. Once you make the decision to use salty water, you should conduct periodic soil-salinity monitoring to evaluate salinization trends and make management adjustments.
Dr. S. Miyamoto is professor of soil and water science at the Texas A&M University Agricultural Research Center (El Paso, Texas); Ricardo Galceran is manager of biosolids and water re-use for El Paso Water Utilities; and Richard Garcia is manager of parks operations, Parks and Recreation Department, City of El Paso.
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