  |
 |
 |
 |
Montana Home
About Us
News
Programs
Technical Resources
Partnerships
Contact Us |
   |
|
|
Irrigation Water TestingGround and surface water to be used for irrigation should be sampled before application and analyzed to determine its suitability for tree and shrub irrigation. Additionally, water quality should be tested over the course of the irrigation season. Water quality testing should include the following parameters: salinity (Electrical Conductivity [ECw]) or Total Dissolved Salts [TDS], sodicity (Sodium Absorption Ratio [SAR] and perhaps Soluble Sodium Percent [SSPw]), specific ion toxicity (particularly sodium [Na + ], chlorides [Cl-] and boron [B]), pH, and alkalinity (carbonates (CO3--) and bi-carbonates (HCO3-) as measured by Residual Sodium Carbonate [RSCw]). Table 1 lists common tests for salt-affected water and unit measures. (1)Adapted from University of California Committee of Consultants 1974, and
Ayers and Westcot, 1994 (modified). Degrees of restriction severity are somewhat arbitrary as changes occur gradually. All use restrictions in Table 1 are guideline values; the user should allow 10 to 20 percent variation above or below a guideline value when making interpretations. The restrictions on use in Table 1 are based on several assumptions including; soil texture ranging from sandy-loam to clay-loam with good internal drainage; low rainfall; no shallow water table; normal irrigation methods are used relative to the delivery, frequency, amount, and leaching fraction; water use by the crop; and salt distribution within the soil profile (see Water Quality for Agriculture (in References) for more information). In Table 1, the degree of restriction on use “none” indicates full production capability without special management practices. “Slight to moderate” restrictions indicate that species choice may be limited and/or specialized management will probably be necessary to achieve full production. “Severe” restriction on use indicates an increasing need for specialized management, although it does not necessarily indicate a complete lack of suitability for use. Sprinkler versus Surface Applied Irrigation Water. Water quality use restrictions may be more limiting when irrigation water is applied by sprinkler systems (see Table 1) rather than by surface irrigation methods. Acceptable concentrations of specific ions in surface applied irrigation water may cause plant injury when applied directly to foliage. Frequent, low volume applications of salty water can, however, result in salt accumulation in the root zone, especially on heavy-textured (clay) soils. Potentially damaging ions include Na+, Cl-, and HCO3- and CO3--. It is important to note that tests results may be reported in various units of measure. Appendix 1 lists unit conversion factors that may be useful when interpreting water quality. Total salinity measurements are based on the following relationships: [1 dS/m = 0.1 S/m = 1000 µS/cm =1 mmhos/cm = 1000 µmhos/cm] (Camberato 2001) Water Na+ hazard is expressed as either the SARw (no units) or as percentage Na+, so it is not included in Appendix 1. 1. Irrigation Water Salinity. Salinity is an indicator of the total amount of soluble ions (salt) in solution in a given water sample. High soil water salinity can cause a “physiological drought” for plants by creating osmotic potentials so high that roots cannot extract soil water even though the soil appears moist or even saturated. Salinity tests indicate the total salt level in the water sample, but will not identify which salts or ions comprise that salinity. Salinity is be measured indirectly by an Electrical Conductivity (ECw) test using an electric current to determine the amount of salts in solution. ECw is reported in various units, including deciSiemens per meter (dS/m), Siemens per meter (S/m), microSiemens per centimeter (µS/cm), millimhos per centimeter (mmhos/cm), or micromhos per centimeter (µmhos/cm). Salinity is also measured directly via a Total Dissolved Solids (TDSw) test, and are reported in milligrams/l (mg/l) or parts per million (ppm). As a result, it is easy to mistakenly compare different units of measure when referencing tables of standards or limits. Only compare identical or equal units of measure. You can convert ECw readings (in mmhos/cm or dS/m) to TDS (mg/l or ppm) by multiplying the ECw value by 640. This conversion is an approximation; the conversion factor actually varies between 550 and 800 depending on the ECw value (see Table 1 for potential water use restrictions as a function of irrigation water salinity). The tolerable level of irrigation water salinity also varies with soil texture, with plants growing on coarse-textured or “light” soils (sands/loamy sands) tolerating more water salinity than plants growing on fine- textured or “heavy” soils (clay/sandy clay/silty clay) (see the Montana Irrigation Manual or Irrigation Water Quality for Montana (1983) MT 198373A [in References] for more information). Note: Using water with an ECw as low as 1 dS/m can result in unacceptably high soil salinity levels with long-term, repeated use. As water evaporates or is used by plants, the salts from the water, as well as the soil, precipitate in the profile and can become concentrated over time if not leached. Consider tree and shrub salinity tolerance, as well as anticipated management practices, when designing irrigation systems and schedules with marginal quality water. As irrigation water quality values approach “severe”, it is suggested that small scale trials be conducted to determine plant response before initiating attempts at large scale production. 2. Irrigation Water Sodicity (Sodium). The concentration of Na+ in the source water is an important factor in the evaluation of irrigation water suitability because Na+ strongly influences water infiltration and soil aeration. High irrigation water Na+ tends to reduce soil aeration, a critical tree and shrub survival parameter, by causing clays and organic matter to disperse. This dispersal reduces soil structure and clogs soil pores, thereby reducing soil aeration and water infiltration. This condition is exacerbated on soils low in Ca ++ and magnesium Mg ++ . The Sodium Adsorption Ratio (SARw) indicates the amount of Na + in a water (or soil) sample relative to Ca ++ and Mg ++ . a. Sodium Adsorption Ratio (SARw) – SARw calculates the ratio of Na + in solution relative to Ca ++ and Mg ++ ions and is expressed by the equation:
where Na+, Ca++, and Mg++ are expressed in milliequivalents per liter (meq/l) or millimoles per liter (mmol/l). Equivalent weights are calculated by taking the atomic weight of an ion and dividing it by the charge value of that ion. Appendix 2 lists specific ions, their charge (valence), atomic weight, and equivalent weight. Similarly, the concentration of an ion is milligrams per liter (mg/l) or parts per million (ppm) can be converted to milliequivalents per liter (meq/l) by dividing the mg/l or ppm value by the equivalent weights (the atomic weight of the ion divided by its charge). See Appendix 3 for multiplication factors for converting between meq/l and ppm for the major salt ions. It should be noted that when irrigation water contains a significant amount of bicarbonate, the Adjusted Sodium Adsorption Ratio (SARadj) is used. See Saline and Sodic Soils in Montana (1982) 2B1272 (in References) or the Montana Irrigation Manual for more information. It is important to note the relationship between SARw and ECw as an indicator of irrigation water quality through their effect on water infiltration. Although it seems counter intuitive, for a given SARw, water infiltration tends to be better as the ECw increases (see Table 1). Although infiltration may be acceptable when SAR and ECw values are both high, high salinity may inhibit plant growth. Furthermore, it is relatively rare in Montana to have reduced infiltration when both the SARw and ECw are low. Chart 1 depicts the interaction of irrigation water salinity and Na+ hazards on potential usefulness for woody plant irrigation. As a rule of thumb, if the SAR is >10 times the ECw, poor infiltration is likely to occur (see Appendix 4 for explanations of the classification system). To use the chart, locate your water ECw or TDSw on the horizontal (X) axis. Then find your water SARw on the vertical (Y) axis. Draw imaginary lines perpendicular from each axis value. The point where the two lines meet identifies the use category. Suitability depends on irrigation water management, soil type, and plant species selection. Chart 1. Potential Use Restrictions Based on the Relationship between Water SARw and ECw as They Affect Soil Infiltration and Potential Salinity Buildup over Time When Surface Applied. (1)
(1) Based
on USDA Agriculture Handbook 60, (modified). b. Soluble Sodium Percentage (SSP) – SSP is another measure of irrigation water Na+ hazard. SSP is the ratio of Na+ in epm (equivalents per million) in water to the total cation epm multiplied by 100. Irrigation water with an SSP greater than 60% may result in Na+ accumulation and possibly a deterioration of soil structure, infiltration, and aeration. 3. Toxicity from Specific Ions in Irrigation Water. Certain ions alone, either because of their direct toxicity to plants or through their effect on water and soil chemistry, can be indicators of irrigation water quality. Substances sometimes found in concentrations in water that are toxic to trees and shrubs include Na+, Cl-, and B. Use the values in Table 1 as approximate guidelines for determining potential use limitations. 4. Irrigation Water Carbonate Level. Bicarbonates (HCO3-) and carbonates (CO3--) in high pH irrigation water can worsen the soil Na + hazard by causing Ca ++ and Mg ++ to precipitate, thereby becoming unavailable to counteract the negative effects of Na + . Irrigation water carbonates (both HCO3- and CO3--) is measured by two methods. The first method directly measures the total carbonate level, and is used when irrigation water Ca ++ and Mg ++ are low. Problems with calcium carbonate (CaCO3) precipitation begin at approximately 1.5 meq/l and are considered severe above 8.5 meq/l. When water concentrations of Ca ++ and Mg ++ are high, the second method, Residual Sodium Carbonate equation (RSC), is used. High levels of Ca ++ and Mg ++ can offset the negative effects of high carbonates on water infiltration (see Table 1). Over time, the repeated use of irrigation water with a high RSC value can lead to soil alkalinity or create a sodic soil if the water contains an appreciable amount of Na+ (SAR > ~4). If RSC values are high (> ~2) while SARw values are low (< ~4), it is unlikely that infiltration problems will occur, although soil pH is still likely to rise to a detrimental level. As a general rule, RSC values less than or equal to 1.25 meq/l are safe for irrigation; 1.25 to 2.5 are marginal; and greater than 2.5 are unsuitable. 5. Irrigation Water pH. Irrigation water pH, in conjunction with soil pH, has its primary effect on plant survival and growth via the availability of essential plant nutrients, although it also influences many physical and biological properties of soil. As alkaline irrigation water raises the soil pH, the availability of certain micronutrients, particularly iron (Fe) and manganese (Mn), is reduced. Inter-veinal chlorosis is a common symptom of Fe deficiency and many species of woody plants are susceptible to low plant-available Fe including Amur maple (Acer ginnala), members of the mountain ash genus (Sorbus sp.), quaking aspen (Populus tremuloides), members of the spirea genus (Spiraea), and other species found growing naturally on lower pH (acidic) soils. Irrigation water pH within the 6.5 to 8.4 range is considered acceptable for most plants. Reducing high soil pH caused by application of high pH water is typically accomplished by applying acidifying fertilizers to the soil, often in conjunction with supplemental chelated Fe and Mn if inter-veinal chlorosis is noted. < Back to Determining the Suitability of Salt-Affected Water and Soil For Tree and Shrub Plantings Last Modified: 08/21/2008 |
|
|
|
Back to Top