Factors affecting salt accumulation in irrigated fairways and roughs in the arid Southwest

Many factors can be involved in salt accumulation, including irrigation systems and management, soil properties, and vegetative cover. Identifying the causes of salt accumulation is key to developing irrigation plans and appropriate management strategies.

By Seiichi Miyamoto, Ph.D.
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As the demand for low-salt water increases, mostly for urban developments, many golf courses in arid areas may have to use water with elevated salinity for irrigation. A survey conducted by GCSAA, for example, indicates that in the Desert Southwest of the United States, where water shortages are chronic, 37% of the courses surveyed in 2009 were using reclaimed effluent for irrigation (16). Salinity is by far the single most recognized constraint — at least in Texas (5) — on using reclaimed water for irrigation. However, many golf courses in the Southwest are also using groundwater with elevated salinity and sodicity. This paper examines the factors affecting salt accumulation in irrigated fairways and roughs in golf courses in arid areas of the Southwest.

Quality of water sources

Water resources used for irrigating golf courses in the arid Southwest are highly diverse in terms of dissolved salt content and composition (Table 1), although the traditional supply from the river systems has comparatively low salinity. Some water sources have a high proportion of sodium (that is, they are sodic), and others are rich in calicum and sulfate ions (that is, they are gypsic).

The water-quality data in Table 1 are for the water sources currently used, but the salt concentration of groundwater reserves can be much higher. An estimated three-quarters of the groundwater reserves in New Mexico, for example, is saline (8), and the majority of groundwater reserves in West Texas are highly saline, with some exceeding 5,000 ppm. Therefore, salt problems may become more than just an occasional nuisance — if they are not already — in many parts of the Southwest.

 Water-quality guidelines

The prevailing thinking among water supply engineers appears to be that the salinity of water used for irrigation dictates salt accumulation in soils and, therefore, salt damage to plants. This has led to the development of water-quality guidelines for irrigation (17). These guidelines also incorporate sodicity, which is commonly expressed by the sodium adsorption ratio (SAR). High sodicity is known to cause soil structural degradation and reduced soil permeability.

Foliar salt damage caused by sprinkling reclaimed water containing 350 ppm of sodium on Mulberry (left) and Arizona cypress (right) trees.
Photos by S. Miyamoto

Microscopic views of salt accumulations on leaves of trees (mulberry, left; cypress, right) sprinkled with reclaimed water (14).

The guidelines proposed by Wescott and Ayers (17) originated from the water-quality guidelines for agricultural crop irrigation and state that slight to moderate salt problems may result when water with 450 ppm to 2,000 ppm of dissolved salts is used for golf course irrigation. Several other guidelines have been proposed, and a commonly used baseline in the Southwest is 1,000 ppm dissolved salt contents and a sodium adsorption ratio of 6 or less.

Experience in the Southwest

Experience indicates that significant foliar damage can occur when broadleaf trees and shrubs are hit by irrigation sprinkler streams containing sodium or chloride concentrations as low as 100 ppm to 150 ppm (11). When the sodium concentration of irrigation water reaches 350 ppm, damage can be very extensive. Most landscaping plants in the Southwest, including woody species, can tolerate much higher levels of sodium or chloride in soil solutions (11), but some species, such as ponderosa pine, suffer greatly when irrigated even at a low sodium concentration of 100 ppm using under-canopy sprinklers (15). Although this article focuses on soil salinization, it is important to keep in mind that the salt hazard to plants comes through both roots and foliage.

We have also observed that salt accumulation in fairways and roughs is highly dependent on soil type and is spatially variable (4,13). More important, soil salinity readings (commonly expressed by the salinity of the soil saturation extract ECe) are poorly correlated with the salinity of irrigation water (Figure 1). (Electrical conductivity is a strange way to quantify salinity, but it is a good measure of ionized salt concentration in water or in the soil saturation extract [ECe], and it usually correlates better with plant performance than does the common unit of ppm (for further discussion, see (2)).

Each data point shown in Figure 1 is, in most cases, an average soil extraction extract (ECe) of 20 to 27 soil samples taken from a fairway on a golf course in West Texas or southern New Mexico. The turfgrass has been irrigated in accordance with the regional estimate or the real-time measurement of evapotranspiration.

The vertical bars in Figure 1 show the standard deviation of soil salinity. The data points without a vertical bar came from research plots in Arizona (GA1) (10) and Nevada (GN3 through GN5) (4). All other data are from southern New Mexico and West Texas. If salinity of irrigation water is such a dominant factor, soil salinity should increase with increasing salinity of irrigation water, as shown by the dashed line in Figure 1. These field data, however, show that soil salinity is highly variable and does not correlate with water salinity. This means that factors other than water salinity must be affecting salt accumulation.

Water salinity and salt balance

The depth of drainage required to avoid salt accumulation is widely considered to be
DD = DW (CW/CD) , (1)
where CW is salinity of irrigation water, CD is the salinity of drainage water, DW is the depth of irrigation and DD is the depth of drainage water. (Any water depth units such as inch/day or inches/month can be used for DW and DD.)

To maintain a desirable level of soil salinity, the drainage requirement increases with increasing quantity (DW) and salinity (CW) of irrigation water. This equation is a statement of mass conservation and applies to highly soluble salts. By definition, DD = DW – ET, where ET is evapotranspiration. Substituting this relationship into equation (1), and then rearranging it, we obtain the quantity of irrigation required to meet the drainage requirement:
DW = ET/ (1 – CW/CD) . (2)
In many instances, the leaching fraction, defined as DD/DW, is used in place of DW.

These salt balance equations have been widely cited. Examples of DW estimated for a typical evapotranspiration rate of 7 inches/month at various combinations of CW and CD are shown in Table 2. To use equation (2), it is necessary to link CD with the salt tolerance of the plants in question. In bermudagrass, growth may cease when ECe measured at the main root-zone depth exceeds about 8 decisiemens/meter in sandy soils (loamy sand to sandy loam) or 10 decisiemens/meter in clayey soils (silty clay loam to clay). Salinity of the soil saturation extract is usually about half of the soil solution salinity at field capacity. A rule of thumb is to keep CD in a range of 10 to 20 decisiemens/meter for salt-tolerant turf species and at lower salinity for less tolerant species. According to the estimate shown in Table 2, salt accumulation for salt-tolerant species can be managed, in theory, simply by adding water at 10% to 15% more than ET.

The reality is more complicated. The evapotranspiration of a given species is not a fixed constant and is influenced by the salinity (CW) as well as by the quantity (DW) of irrigation. Typically, evapotranspiration increases with increasing irrigation amounts and/or decreasing salinity (CW) (7). Evapotranspiration also varies within a golf course due to topographical effects. Therefore, actual leaching is likely to be different from the estimate based on a regional estimate of evapotranspiration for different grass species. Soil water content also increases with increasing irrigation, especially in clayey soils, which curtails the increases in soil solution salinity associated with soil water depletion. Equation (2) also ignores salt precipitation, which can be significant in gypsic water. Nonetheless, it serves as a conceptual model as well as the first approximation of salt accumulation as related to water salinity and water balance in a plant root zone under repeated consistent irrigation practices.

Irrigation systems and management

Irrigation uniformity and adequacy, which are interrelated, affect salt accumulation in soils. Salts can accumulate anywhere irrigation water has not been applied in a quantity sufficient to cause leaching, especially in clayey soils.

Irrigation systems

One of the common problems that causes salt accumulation is the design of the irrigation system itself. When the fairway has a single feeder line and the sprinkler heads are mounted in a single row, precipitation generally decreases as the distance from the feeder increases across the fairway. This results in a corresponding increase in salt accumulation as shown in Table 3.

When a fairway has two feeder lines with full-circle sprinklers, salts usually accumulate in the outer zones (Table 3), where reduced rates of water are applied. The rough at the golf course in Table 3 has been reduced to salt-frosted bare soil because of insufficient water application and associated soil salinization.

Some golf courses have used three-row systems with a triangular formation, which provides improved uniformity, and others have used large sprinkler heads and placed laterals across fairways and roughs to provide a high degree of uniformity throughout the golf course. However, such a system can cause foliar damage to trees and shrubs, as seen in the previous photos (Page 81). In such cases, the use of low-trajectory sprinklers is helpful in reducing foliar damage, but wetting the tree bark may cause problems for some species.

The presence of diverse plants with varying degrees of salt tolerance makes system design a complicated task. When water with elevated salinity is used, greater attention must be given to pressure regulation (to achieve the best uniformity) and to the flow control system (to accommodate spatial variation in irrigation requirements and minimize sprinkling of the tree canopy). Discharge control from individual sprinkler heads is certainly a desirable feature in golf courses with diverse topography and plant species.

There is a practical limit to attaining uniform water application, and low uniformity can be compensated for by increasing the amount of irrigation if the circumstances favor the option. Some studies have shown that increasing the amount of irrigation by 14% provided adequate growth of tall fescue irrigated with sprinklers rated as low as 0.65 on the Christiansen uniformity coefficient (9). This research was conducted in an experimental setup where soil properties were presumably uniform. In reality, most golf courses have spatial variation in soil properties, which can further complicate the real impact of water application uniformity on salt accumulation.

Irrigation management

Aside from salt damage on leaves, the effects of irrigation frequency on plant performance have been a matter of conjecture. One school of thought is that frequent and light irrigation is preferred because it provides the least fluctuation in soil moisture and salinity. This approach is also compatible with golfer preferences. A report from Nevada indicates that irrigation with highly saline water (6 decisiemens/meter) adversely affected bermudagrass grown in Calico sandy loam, but not in Gila silt loam, even when soil moisture levels in soil columns were kept relatively high (0.29 milliliter/cubic centimeter for Calico and 0.40 milliliter/cubic centimeter for Gila) using three irrigations per week (3). Low soil-water storage, which is common in sandy soils, does not provide a buffer against the increased salinity associated with soil-water depletion, especially when high-salinity water is used for irrigation.

An experiment conducted over a three-year period in California using Pachapa sandy loam shows that tall fescue growth was affected by salinity of irrigation water and by the leaching fraction, but not by the frequency of irrigation involving irrigation intervals of one, one to three, and four to six days, using saline water (4 decisiemens/meter) (7). However, the same study has also showed that infrequent irrigation improved salt leaching from the top soil.

Most experiments dealing with turfgrass are conducted with nearly perfect stands of thick grass cover that allow little evaporation from the soil surface. In salt-affected areas, sparse cover is common, and evaporation from the soil surface must be considered, especially during the spring. Figure 2 shows the effect of irrigation depth/frequency on water penetration and soil salinity when initially dry soils (Bluepoint loamy sand and Harkey silt loam) were irrigated with 2.2 decisiemens/meter of water at the rates specified in the figure. Water evaporation was measured daily through weighing, and the depth of the wetting front and the salinity of the wetted soils was measured after one month.

Note that the wetting front reached less than 6 inches in case A (Figure 2) in both soils. The wetting front penetration doubled and tripled in loamy sand with increasing irrigation (case B) or with a layer of mulch in loamy sand, but not necessarily in silt loam. Water evaporation (dotted lines in Figure 2) was about the same or slightly higher in silt loam, but the silt loam had twice the soil moisture content of loamy sand (0.28 versus 0.14 milliliter/cubic centimeter). Soil salinity increased with increasing irrigation frequency, but the layer of mulch kept soil salinity at a minimum. High-frequency light irrigation should be avoided for areas with bare soil or sparse turf cover.

Gypsum precipitation on putting greens.

Frequency of irrigation should also be kept at a minimum when gypsic water rich in calcium and sulfate ions is used for irrigation. Under light, very frequent irrigation —such as that used for putting greens — evaporation can cause gypsum particles to form directly at the soil surface. Likewise, irrigation should be deep enough to leach dissolved salts. Otherwise, salts that contain sodium can form directly on the turf, as shown in the photos on Page 86.

Leaching irrigation

Leaching irrigation, especially when carried out in winter when evapotranspiration is low, is effective for lowering soil salinity. Such a practice is helpful not only for leaching salts from stratified soil profiles, but also for reducing clogged pores caused by salt precipitation. The frequency of leaching irrigation can be monthly or quarterly, depending mainly on water quality, water availability, the season and soil types, and should be determined by soil salinity monitoring. The operation usually requires blocking fairways for at least a week or longer if the soil is clayey. Soil amendment application may be required before leaching irrigation.

Soil and topography

Historically, soil types have been regarded as a minor factor in salt accumulation, probably because the well-known salt balance equations are supposed to be applicable to any soils. However, widely scattered soil salinity readings, such as shown in Figure 1, raised a concern that salt accumulation could vary depending on soil type. We have already seen indications that salt leaching may be curtailed in fine-textured soils (Figure 2), mainly because of their greater water-holding capacity and slow infiltration rates. Exposed water evaporates rapidly and leaves salts at the soil surface.

In order to evaluate salt accumulation quantitatively, the following parameter called the salt concentration factor (SCF) was introduced (13):
SCF = ECe/ ECw . (3)
The salt concentration factor is simply a measure of the extent of increase in soil salinity divided by the salinity of irrigation water (ECw. The salt concentration factor can be slightly less than 1.0 when there is no salt accumulation in soils, and it increases with increasing salt accumulation. Under steady irrigation, the salt concentration factor is related inversely to the leaching fraction, which is defined as DD/DW or CW/CD. This parameter is readily measureable, whereas the leaching fraction is difficult to measure under field conditions. Working with diverse soil types in West Texas and southern New Mexico, we found that the salt concentration factor was similar among the same soil types and soil conditions irrigated with water of various salt levels.

Damage to creeping bentgrass caused by halite precipitation (left) and a microscopic view of the same damage.

Salinity of the soil saturation extract, ECe, relates directly to salt tolerance in many plant species, provided that the soil samples were collected from the major root zone. We can use the mean soil salinity for ECe or the mean plus the standard deviation. If the mean value is used for ECe, about half of the areas where the soil samples were collected are likely to have salinity exceeding the mean. If the mean plus the standard deviation is used for ECe, the estimated salt concentration factor would be more realistic.

Soil types and compaction effects

To evaluate the impact of soil texture on the salt concentration factor, the data from West Texas and southern New Mexico were used previously to develop three regression lines (13) (see the legend for Figure 3). With few exceptions, the golf courses surveyed had generally deep soils, extending 5 feet (1.5 meters) or more. In all cases, the sites have been irrigated using the regional estimate of evapotranspiration or the real-time evapotranspiration estimate. The salt concentration factor was computed using the mean soil salinity plus the standard deviation, and regression equations were developed against the saturation water content (SWC) in Figure 3. The saturation water content is a measure of soil textural class, as shown at the upper edge of the figure.

The best-fit lines indicate that the salt concentration factor increases exponentially with increasing saturation water content, much more so in parks than in golf course fairways. Soil compaction likely accounts for the difference. Salinity of soil samples collected from compacted areas was markedly higher than salinity of soil from uncompacted areas of the parks surveyed (Table 4). With increasing compaction (which is prevalent in park grounds), the salt concentration factor line shifted away from the best-fit line obtained at the golf course fairways (Figure 3).

Figure 3 contains some data from golf courses (G) and some from regional parks (P). The data points designated as P1a and P1b, for example, came from a large, severely compacted turf area in a regional park located in El Paso, Texas (Rio Grande Valley). The salt concentration factors of soil samples from golf courses G3a, G4b and G7a, also located in the valley, were much lower than those of the parks, even though the saturation water content was higher. These fairways had clayey soils and were irrigated with water with salinity ranging from 1.1 to 2.7 decisiemens/meter (Table 1).

An example of salinization of alluvial soil at a park in El Paso, Texas. Inset: A sample of salinized alluvial soil. The white substance in the root channel is precipitated salts.

Soil stratification and topography

Soil stratification is a representation of geological history, and is common in both alluvial and upland soils. It is known to reduce water infiltration and penetration, and thus salt leaching, especially when a poorly permeable calcic horizon (commonly known as “Caliche”) is present beneath. We observed a number of cases where the salt concentration factor of calcic soils exceeded the best-fit line for golf courses (as indicated by filled circles in parentheses in Figure 3). Recall that the best-fit line for golf courses was developed based on data from the deep soils of the Permian Basin and certain areas of the Rio Grande Basin. The data shown by filled circles in parentheses in Figure 3 came from upland areas containing the calcic horizon. Soil stratification likely reduces salt leaching.

It is rarely recognized that topography, especially the slope of the flow-limiting layers, affects salt accumulation. Typically, soil salinization occurs where topography is flat and lateral drainage is lacking. The data points marked G5b in Figure 3, for example, came from samples collected from hilltop fairways, whereas sample G5a, which had lower soil salinity, was collected from a sloped area. The profile configuration of the fairway was loamy sand (4 to 6 inches) over a poorly permeable calcic horizon.

Assessing soil salinization potential

One task that superintendents may face is to assess whether soil salinity readings at their golf course are within the norm for the salinity of the water used for a given soil type. We assume ECw to be 2 decisiemens/meter and ECe to be 5 decisiemens/meter. The salt concentration factor estimated by equation (3) would be 2.5. We also assume that the soil texture is clay loam with a saturation water content of 50 grams/100 grams. The plot of the data set in Figure 3 may show that the soil salinity reading is slightly higher than normal, but within the range of variability. If the soil texture is silt loam, however, the soil salinity reading is significantly higher than normal, and thus may require some investigation as to the cause.

Another situation may deal with projection of soil salinization potential when water of elevated salinity is to be used for irrigation. If the salinity of the water goes up for clay loam (to 3 decisiemens/meter, for example), the ECe, back-calculated from equation (3) exceeds 6 decisiemens/meter and may warrant detailed evaluation. Our experience in the Southwest is that salt accumulation is not a significant issue when the salt concentration factor is less than 2 to 3 when irrigated with low-salt calcic water (Table 1). When the salt concentration factor exceeds 3, soil salinization becomes an issue, especially when the soil is clayey, compacted or irrigated with sodic water. Obviously, this is a rule of thumb, and other factors may affect the actual extent of salinization.

Sodicity

There is no shortage of literature warning of the adverse effects of high sodicity on soil structure and permeability. Laboratory studies have convincingly shown that elevated levels of exchangeable sodium can encourage soil aggregate breakdown when the exchangeable sodium percentage is as low as 10% (1). An exchangeable sodium percentage of 5% can cause dispersion of clay particles and pore plugging with the dispersed particles when the salinity of the soil solution is less than 1 decisiemen/meter (6). Several earlier studies have also found that certain clay minerals swell at high exchangeable sodium percentage (>15% to 20%), and that the exchangeable sodium percentage is approximately equal to the sodium adsorption ratio of soil solutions at equilibrium.

A salinized hilltop fairway (G5b in Figure 3) (top) developed on Aridisols containing layers of indurated caliche (bottom).

The practical implication of these laboratory findings, however, is not clear. Our recent greenhouse study covering 27 irrigation events indicated that water infiltration rates can decrease significantly over time in young alluvial soils (Entisols) at an exchangeable sodium percentage as low as 6%, but not in upland soils rich in calcium carbonate (12). It appears that soil particles cemented with calcium carbonate are stable against the dispersive effect of exchangeable sodium. Calcic soils are the dominant soil of the Southwest.

The same study has also shown that the effect of sodicity on water infiltration decreases as irrigation depth per application decreases and becomes insignificant when the irrigation depth drops below 1 inch (2.5 centimeters) per application. This finding is consistent with an earlier study, indicating that the impact of exchangeable sodium percentage is pronounced in saturated flow, and is less so in unsaturated flow. It is also consistent with field observations that, until the soil is salinized, water infiltration into upland soils does not visibly decrease when the soils are irrigated with reclaimed water with a sodium adsorption ratio as high as 10. Soil salinization is accompanied by an increase in the exchangeable sodium percentage.

In spite of these findings, we also observed soil salinization in topdressed Camborthids (upland soils that have not experienced accumulation of calcium carbonate as a result of hydrologic alteration). These soils, although classified as Aridisols, are essentially dry alluvial sediments with no structural development, and upon wetting, behave like young alluvial soils. Soil particles of this type are readily dispersed when the exchangeable sodium percentage range is 6% to 9%, and soil permeability becomes very low, especially when rainfall or low-salt water is applied (12). Fortunately, many water-treatment options, including blending, can be used to lower the sodium adsorption ratio. It is also important to follow prudent soil management practices to maintain good soil structure and soil permeability.

Other factors

Many other factors encourage salt accumulation in soils, including thick thatch or a water-repellent soil layer, the presence of a large tree canopy, loss of turf cover, application of the wrong soil amendments or topdressing material, and the presence of excessive amounts of sulfate in irrigation water. Thick thatch absorbs irrigation water, thus limiting water penetration and salt leaching. The presence of a water-repellent layer also limits water penetration and salt leaching.

A large tree canopy sprayed with sprinklers can salinize soils at and near drip lines because the salinity of drips is elevated by water evaporation from the tree canopy. Trees essentially act as an evaporation tower. Once the trees are defoliated, salts crystallize on the surface of tree barks and, at times, translocate to the ground along the surface of the branches and tree trunk.

The loss of turf cover increases water evaporation and salt accumulation unless irrigation scheduling is adjusted. Application of the wrong soil amendments, especially compost made from dairy manure, can compound salt problems because these amendments are usually high in sodium and can also plug soil pores when compacted by tractor wheels. Careless uses of alluvial soils for topdressing upland sites can also create infiltration and salt problems, especially when sodicity of irrigation water is elevated. High concentrations of sulfate ions can increase the exchangeable sodium percentage or cause gypsum to form in soils when the calcium concentration is high. These conditions reduce subsurface drainage and cause salt accumulation unless periodic leaching irrigation is practiced. Correct identification of the cause(s) of salt accumulation is essential for improving salt management.

Acknowledgments

The work presented here was funded in part by a grant from the Bureau of Reclamation, U.S. Department of Interior. Data collection was assisted by Ignacio Martinez, and documentation of the results by student workers Jaime Garcia, Yvette Pereyra and Doriana Torres. The author extends sincere thanks to area golf courses for their cooperation during this study.

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S. Miyamoto (s-miyamoto@tamu.edu or sammiyamoto@yahoo.com) is a professor of soil science, specializing in salinity management at Texas A&M University AgriLife Research and Extension Center, El Paso, Texas.