|Purple gold: a contemporary view of recycled water irrigation|
By Dr. M. Ali Harivandi
Recycled waters usually contain a wide variety of other elements in small concentrations. Some of these elements are toxic to turfgrasses and other plants if they accumulate in the soil to sufficient levels. The most common toxicities are due to accumulations of sodium, chloride, and boron. Plant roots absorb sodium and transport it to leaves, where it can accumulate and cause injury. Symptoms of sodium toxicity resemble those of salt burn on leaves. Sodium toxicity is often of more concern on plants other than turfgrasses, primarily because accumulated sodium is removed every time grass is mowed.
Chloride (Cl), in addition to contributing to the total soluble salt content of irrigation water, is another ion that may be directly toxic to landscape plants. Although not particularly toxic to turfgrasses, it affects many trees, shrubs, and ground covers. In sensitive plants, chloride toxicity causes leaf margin scorch in minor cases and total leaf kill and abscission in severe situations. Fortunately, chloride salts are quite soluble and thus may be leached from well-drained soils with good subsurface drainage.
Recycled water may also contain boron (B), a micronutrient essential for plant growth in very small quantities. Injury from excess B is most obvious as necrosis on the margins of older leaves. Turfgrasses are more tolerant of boron than any other plants grown in the landscape. Table 1 provides general guidelines for assessing the effect of sodium, chloride, and boron in irrigation water.
pH, a measure of acidity, is valued on a scale of 0 to 14. Water pH is easily determined and provides useful information about water’s chemical properties. Although seldom a problem in itself, a very high or low pH indicates that water needs evaluation for other constituents. On the pH scale, pH 7 represents neutral (i.e., water with a pH of 7 is neither acidic nor alkaline). Moving from pH 7 to pH 0, water is increasingly acidic; moving from pH 7 to pH 14, water is increasingly basic (or “alkaline”). The desirable soil pH for most turfgrasses is 5.5 to 7.0; the pH of most irrigation water, however, ranges from 6.5 to 8.4. Depending on the soil on which grass is grown, an irrigation water pH range of 6.5-7 is desirable. Recycled water with a pH outside the desirable range must be evaluated for other chemical constituents.
The bicarbonate (HCO3) and, to a lesser degree, carbonate (CO3) content of recycled irrigation water also deserves careful evaluation. Recycled waters are especially prone to excessive levels of bicarbonate. High bicarbonate levels in irrigation water increase soil pH and may affect soil permeability; combining with calcium and/or magnesium, bicarbonate precipitates as calcium and/or magnesium carbonate, both of which increase the SAR of the soil solution. As noted previously, high SARs can lead to reduced soil permeability. To determine the negative impact of the bicarbonate content of recycled water, it is not reported as meq/L (milli-equivalent per litre) of HCO3, but as Residual Sodium Carbonate (RSC). RSC is calculated from the equation shown below, in which concentrations of all ions are expressed in meq/L.
RSC = (HCO3 + CO3) - (Ca + Mg)
Generally, recycled water with an RSC value of 1.25 meq/L or lower is safe for irrigation, water with an RSC between 1.25 and 2.5 meq/L is marginal, and water with an RSC of 2.5 meq/L and above is probably not suitable for irrigation.
Recycled water can also be high in nutrients, whose economic value may be an important consideration. Nitrogen, phosphorus, and potassium, all of which are essential to turfgrass growth, are the primary nutrients present in most recycled waters. Even if the quantities of nutrients in a given recycled water are small, they are efficiently used by turfgrass because they are applied frequently and regularly. In most cases, turf obtains all the phosphorus and potassium and a large part of the nitrogen it needs from recycled water.
Sufficient micronutrients are also supplied by most recycled Green Section waters. Water chemical analysis must therefore be thoroughly evaluated to determine the kind and amount of each nutrient applied through irrigation; the turf’s fertility program can then be adjusted accordingly. Most agricultural testing laboratories will provide the nutritional contents of recycled water upon request.
Suspended solids in recycled water may include inorganic particles such as clay, silt, and other soil constituents, as well as organic matter such as plant material, algae, bacteria, etc. These materials do not dissolve in water and thus can be removed only by filtration. The suspended solids in tertiary treated (advanced) recycled water are negligible and not a cause for concern. However, if secondary treated recycled water is used for irrigation, suspended solids should be monitored.
In addition to plugging irrigation equipment, solids can fill air spaces between sand particles, reducing infiltration and drainage, and increasing compaction. Since these effects vary considerably with type of solid, irrigation system, and soil, it is difficult to standardize suspended solid values for irrigation water. Overall, the complexity and variability of irrigation waters and systems make effective filtration the most sensible approach to controlling hazards posed by suspended solids in any water.
Recycled water quality varies significantly among sewage treatment plants as well as on a seasonal basis, and it must be analyzed individually and regularly. There are very few recycled water sources that are absolutely unsuitable for turfgrass irrigation. Furthermore, the nature and magnitude of potential problems with a specific water will depend on its interaction with climate and soil chemistry and physics.
Soil physical characteristics and drainage both play important roles in determining a rootzone’s ability to handle salinity. For example, water with an ECw of 1.5 dS/m may be successfully used on grass grown on sandy soil with good drainage and high natural leaching, but may prove injurious within a very short time if it is used to irrigate the same grass grown on a clay soil or soil that has limited drainage because of salt buildup in the rootzone. Consequently, soil characteristics must be evaluated along with water quality to determine if irrigation-induced problems are likely.
Fine-textured soils (clays) are more likely to accumulate salts than coarse-textured soils (sands). Also, layering in the rootzone that interferes with drainage (and therefore salt leaching) can lead to water-induced plant injury despite irrigating with seemingly acceptable recycled water. In other words, lack of drainage leads to salt buildup. Soils already saline or sodic are obviously more likely to contribute to salinity injury due to recycled water irrigation, regardless of their drainage characteristics. Application of excessive fertilizer can also contribute to the salt load and may create salinity problems where the salt load from recycled water alone may not be high enough to cause damage.
If water salinity, sodium, and other chemical components are potential problems, management is key to agronomic success. Following is a list of management practices that can be used to address potential recycled water irrigation challenges.
Select salt-tolerant turfgrass species. If salinity problems are expected with recycled water irrigation, salt-tolerant grass species should be considered for planting. Salt tolerance of turfgrasses is usually expressed in relation to the salt content of the soil. Table 2 provides a general guide to the salt tolerance of individual turfgrasses, based on ECe values (electrical conductivity of soil water extract). Grasses are listed in columns indicating the highest levels of salt at which they perform adequately. As indicated, soils with an ECe below 3 dS/m are considered satisfactory for growing most turfgrasses; soils with an ECe between 3 and 10 dS/m can support a few moderately salt-tolerant turfgrass species, while soils with an ECe higher than 10 dS/m will support only very salt-tolerant grasses.
Apply extra water to leach excess salts below the turfgrass rootzone. Extra irrigation water needed to leach salts below the turfgrass rootzone, thus preventing salt buildup to toxic levels, is referred to as the leaching requirement or fraction.
A leaching requirement is based on the recycled water’s salt content and the salt tolerance levels of the grass (expressed in ECe) at the site. For example, if a turfgrass species with salt tolerance of not more than 2.5 dS/m is irrigated with a recycled water with an electrical conductivity of 1.2 dS/m, 10 per cent more water than is dictated by evapotranspiration (ET) alone must be applied to leach salts out of the rootzone.
Any changes in a system’s input, such as rainfall, can affect the amount of water that must be applied for leaching. As the Leaching Requirement increases (and therefore more salt leaching occurs), salt accumulation in the rootzone decreases. As a result, highly saline recycled water may be used successfully for irrigation in high rainfall areas, while the same water may cause severe salinity damage to turfgrasses in arid and semi-arid locations.
Provide drainage. Clearly, successful leaching requires adequate drainage. In all cases where recycled water is used for irrigation, good drainage is essential. Drainage can be natural or can be improved by installing tile drains. An example of a site where drainage must be improved: a golf course with greens built on modified native soils (i.e., push-up greens) converting to recycled water for irrigation. The course can either rebuild greens on a sand-based rootzone mix or install an effective drainage system to provide for salt leaching. The objective is to keep percolated saline water below the turfgrass rootzone.
Modify management practices. Certain management practices may alleviate the deleterious effects of salinity. On golf greens, especially, reducing or removing accumulated surface organic matter (thatch) is crucial under recycled water irrigation. Thatch and mat layers stop the flow of water (and salts) through the soil and impede leaching of salts. On golf greens with a uniform rootzone profile, drainage is often adequate for salt leaching. However, if a given golf green rootzone profile indicates excessive organic matter (thatch) accumulation or, worse, the existence of a layering problem within the soil profile, then every effort must be employed to remove thatch or eliminate layering prior to the initiation of recycled water irrigation. Aeration (particularly useful on golf course greens and sports fields) punches through impermeable layers, facilitating faster and better water movement through the soil profile. Aerators remove soil cores at regular intervals. Cores should be removed from the soil surface of golf greens and similar specialty turf, and holes should be topdressed with sand. Often, just spreading sand over the aerated surface fails to fill the holes. Sweeping, brushing, or blowing sand into the holes left by aeration ensures optimum sand application. Holes should be filled all the way to the soil surface to provide channels for water percolation through the layers of sand/organic matter.
Modify the rootzone mixture. Where turfgrasses are grown on soils with minimal natural drainage (e.g., heavy clay soils, soils with a hard pan or clay pan) and recycled irrigation water is high in salts or sodium, total modification of the rootzone mixture may be necessary. Sand-based golf greens or sports fields generally drain well and can tolerate recycled waters that may be too saline for irrigation on heavy clay or compacted soils. Blend irrigation waters. Frequently, poor-quality water can be used for irrigation if better-quality water is available for blending. The two waters can be pumped into a reservoir to mix them before irrigation. Although the resulting salinity will vary according to the type of salts present and climatic conditions, water quality should improve in proportion to the mixing ratio. For example, when equal volumes of two waters, one with an ECw of 1 dS/m and the other with an ECw of 5 dS/m are mixed, the salinity of the blend should be approximately 3 dS/m. Deleterious effects of sodium/bicarbonate may be reduced by blending poor-quality water with lesssodic (waters with elevated sodium/bicarbonate contents), better-quality water. Although the resulting sodicity will vary according to the amount of sodium/bicarbonate present in the two waters, water quality will improve in proportion to the mixing ratio.
Use amendments. Applying soil and water amendments, such as gypsum (calcium sulfate), calcium chloride, sulfur, and sulfuric or N-phuric acids, can aid in reducing the negative effects of sodium and bicarbonate. They may also help with improving water/soil pH and partially help with salinity control. These amendments increase the soil supply of calcium, either directly, as in the case of gypsum and calcium chloride, or indirectly, as in the case of sulfur and sulfuric or N-phuric acids. Sulfur and sulfur-containing fertilizers applied to soils naturally high in calcium may make calcium more soluble. Once available, calcium can then replace sodium on clay particles, preventing excess sodium accumulation. Subsequent leaching will flush sodium salts out of the rootzone. The amount of sulfur amendment required depends on a soil’s sodium content, SAR of the irrigation water, the quantity of water applied, soil texture, and type of amendment.
The impact of bicarbonate on pH may also be reduced by applying an acidifying fertilizer, such as ammonium sulfate, as part of a regular fertilization program, or by acidification of the irrigation water. In some cases, water with high residual sodium carbonate may require acidification with sulfuric, N-phuric acid (a type of urea-sulfuric acid), or phosphoric acids, or by use of a sulfur burner (which produces sulfurous acid). Amendments may be applied directly to the turfgrass/soil or injected into the irrigation system. Acidification of water by acid injection requires unique measurements and equipment. A turfgrass manager must work closely with a consulting laboratory to determine whether acidification is required and, if it is, how it may best be accomplished. The same care is required for use of a sulfur burner. In general, however, the sulfurous acid produced by a burner is not as caustic as sulfuric acid.
There are many advantages to treating recycled water with direct injection of amendments into the irrigation system, among which are:
•More effective than surface application (gradual and frequent application)
•No disruption in site use
•No dust problems
•Reduced burn potential of sulfur
•Reduced risk of “overdose”
•Reduced pH and salinity fluctuations
There are also potential disadvantages to direct injection:
•Material more expensive
•Equipment and maintenance expensive
•Danger of handling acids
•Irrigation efficiency and uniformity must be optimal
•Segregation of areas are not possible (e.g., greens vs. fairways in golf courses)
As human population grows and fresh water becomes increasingly scarce, recycled water is a viable alternative to costly, limited potable water for irrigating turfgrass sites. Recycled water is often better tolerated by turfgrasses than by other landscape plants; simultaneously, turfgrass venues (golf courses, parks, cemeteries, green belts, campus grounds, sports fields, and sod production farms), with their large expanses and trained maintenance staffs, are particularly well-suited to incorporate recycled water in their irrigation programs. Urban population growth ensures an expansion of turfgrass sites for a variety of recreational and functional uses, and this means that irrigation with Purple Gold will be a permanent part of our urban landscape schemes.
Reprinted with permission by the USGA Green Section Record, Vol. 49 (45), Dec. 16, 2011.
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