All water used for irrigation contains some dissolved salts. When using irrigation water it is important to know the effect of water quality on salt content (salinity) of the soil. It's also wise to know the:
Remember, it is difficult to isolate any one of these factors because they are all interrelated.
|Irrigation Suitability Test Includes:|
Sodium Absorption Ratio
|Individual Dissolved Minerals:|
|Total N-P-K Test: |
(for dilute fertilizer solutions)
(for all above water analysis packages)
Hardness (Mg & Ca)
Interpreting a Laboratory Report
To aid in understanding a report it is important to define technical terms used in the interpretation:
Elements found in smaller amounts are: Boron (B) and Nitrate-Nitrogen (NO3-N).
Two reporting units are used for expressing concentrations: parts per million (ppm) and milliequivalents per liter (meq/l).
Milliequivalents per liter (me/l) is the most meaningful method of reporting the major constituents of water. This is a measure of the chemical equivalence of an ion. . A milliequivalent is one-thousandth of an equivalent or 0.084 grams per liter. To convert to milliequivalents per liter divide the ppm by the equivalent weight of an ion. For example, a laboratory reports 54 ppm of sulfate. The equivalent weight of sulfate is 48. Divide 54 by 48 and find the water has 1.12 me/l of sulfate.
Salts are made up of a combination of cations (sodium, calcium, magnesium, etc.) and anions (chlorides, sulfates, bicarbonates, etc.). Total salt content is usually reported as the electrical conductivity (EC). Chemically pure water does not conduct electricity but water with salts dissolved in it does. The more salt in the water, the better conductor it becomes. The ability of a water sample to conduct electricity is used to determine its salt content. EC is reported as millimhos per centimeter (mmhos/cm).
The total salt content or Total Dissolved Solids (TDS) is usually as ppm. This value is calculated from the EC value; (EC in mmhos/cm is multiplied by 640), this equals total dissolved solids in ppm. Example: EC of 1.6, (l.6 x 640) = 1024 ppm total dissolved solids. The me/l of total salts can also be estimated by multiplying the EC (mmhos/cm) by 10. Example: EC 2.62 mmhos/cm x 10 = 26.2 me/l of total salts.
The pH value expresses the acidity or alkalinity of water. A pH reading of less than 7.0 is acidic and above 7.0 is alkaline. Most well waters range from pH 7.0 to pH 8.5. Some stream waters may be as acidic as pH 6.5. The pH measurement is not important if the major ions are reported and is often omitted from laboratory reports.
Soil Properties and Water Quality
Calcium (Ca) is found to some extent in all natural waters. A soil predominantly saturated with calcium is friable and easily worked, usually permits water to penetrate easily and does not puddle or run together when wet. For this reason calcium in the form of gypsum is often applied to soils to improve the physical properties. Generally, an irrigation water containing predominantly calcium is desirable.
Magnesium (Mg) is also usually found in measurable amounts. Magnesium behaves much like calcium in the soil.
Hardness is measured by calculation; it is the addition of the measured calcium and magnesium. High hardness is associated with ìwhiteî stains and plugging of sprinkler heads.
Sodium (Na) salts are all very soluble and as a result are found in all natural waters. A soil with a large amount of sodium associated with the clay fraction has poor physical properties for plant growth. When wet it runs together, becomes sticky and is nearly impervious to water. When it dries, hard clods form, making it difficult to till. Continued use of waters with a high proportion of sodium may bring about these severe changes in an otherwise good soil. Eventually these soils may become alkali, usually spots high in exchangeable sodium. High sodium conditions can be reversed by applying soluble calcium (usually gypsum) directly to the soil or injected into the irrigation water. Calcium replaces the sodium on the soil particles so that it may be leached below the root zone.
Potassium (K) is usually found in only small amounts in natural waters. It behaves much like sodium in the soil. Very rarely will potassium be high in natural water sources.
Bicarbonate (HCO3) is common in natural waters. It is not usually found in nature except in solution. Sodium and potassium bicarbonates can exist as solid salts: for example baking soda (sodium bicarbonate). Calcium and magnesium bicarbonates exist only in solution. As the moisture in the soil is reduced by evaporation, calcium bicarbonate decomposes, carbon dioxide (CO2) goes off as a gas and water (H20) is formed leaving insoluble lime (CaC03) behind:
Chloride (Cl) is found in all natural waters. In high concentrations it is toxic to some plants. All common chlorides are soluble and contribute to the total salt content (salinity) of soils. The chloride content must be determined to properly evaluate irrigation waters.
Sulfate (SO4) is abundant in most water sources and in most forms readily soluble. Sulfate has no characteristic action on the soil except to contribute to the total salt content and is an excellent source of sulfate fertilizer. As an excepted rule; as the EC value increase so does the sulfate value.
Nitrate (NO3) is not commonly found in large amounts in natural waters. Even small amounts can affect its use as irrigation water by supplying plants with more than the desired amount of this plant nutrient. High nitrates in water may indicate contamination from fertilizers or sewage. Nitrates, however, have no effect on the physical properties of soil except to contribute slightly to its salinity. In addition, high nitrate will stimulate algae growth in irrigation holding ponds.
Boron (B) occurs in water in one or another anion form. The usual range in natural waters is from 0.01 ppm to 10 ppm. Concentrations greater than this are known but are most often from hot springs or brines. Boron has no measurable effect on the physical properties of soil but can be toxic to sensitive plants at quantities of greater than 2.0 ppm. Boron is not as readily removed from the soil as chloride or nitrate but most of it can be removed by successive leaching.
Classifying Irrigation Water
Classification of irrigation water is difficult because of the interaction of the water with the soils drainage properties. A poor quality water source could be acceptable on well drained soils and cause a salt build up on marginal drained soils. Keep in mind that even low salt irrigation water can cause major problems on poor drained soils. The following ratings are an attempted to communicate to the user when a water source can be contributing to a salt problem.
A saline soil is a soil containing soluble salts in such quantities that they interfere with the growth of crops. A saline soil does not have high sodium.
A sodic soil contains enough sodium attached to the clay minerals to interfere with the structure of the soil (a well developed soil structure, usually has well developed drainage). If a sodic soil is relatively free of soluble salts it is called a non-saline sodic soil. If, in addition to being alkali, it has sufficient soluble salts to restrict plants it is called saline-sodic soil.
Salinity Hazard - One of the hazards of irrigation is the possible accumulation of soluble salts in the root zone. Some plants can tolerate more salts than others but all plants have a maximum tolerance. With reasonably good irrigation practices the salt content of the saturation extract of soil is 1.5 to 3 times salt content of the irrigation water. Where ample water is used to remove excess salt from the root zone the salt level in the saturation extract is about 1.5 times that of the irrigation water. Where water is used more sparingly, there may be 3 times as much salt.
An acre-foot of water (the amount of water covering one acre, one foot deep) weighs approximately 2,720,000 pounds; therefore, 1 ppm of a salt in an acre-foot of water weighs 2.72 pounds. This means that one acre-foot of water containing only 735 ppm (EC = 1.15 mmhos/l) carries one ton of salt! Many managers apply several feet of irrigation water per year to produce a crop or they apply more than 2 tons of salt/acre every year! This high lights the need for proper irrigation management.
With ordinary irrigation methods there is some leaching, hence the accumulation of salts in the soil water is reduced but not eliminated. Before a critical assessment of the salinity hazard of any irrigation water is made, it is necessary to know how much salt the crop can tolerate and how much leaching is needed to maintain the desired salt level in the soil water.
Sodium or Permeability Hazard. In most cases permeability of water becomes a hazard before sodium affects plant growth. As the proportion of sodium attached to clay minerals increases, the soil tend to disperse or "run together" bringing about reduced rates of water penetration. The sodium adsorption ratio (SAR) indicates the relative activity of sodium ions as they react with clay. From the SAR the proportion of sodium on the clay fraction of the soil can be estimated when irrigation water has been used for along period of time with reasonable irrigation practice.
The sodium (Na), Calcium (Ca) and Magnesium (Mg) are expressed in milliequivalents per liter (me/l).
A refinement of the SAR called the "Adjusted SAR" (SARa) has recently been developed. SARa includes the added effects of precipitation of calcium in soils as related to CO3 + HC03 concentrations.
If the SARa is less than 6.0 there should be no problems with either sodium or permeability. In the range of 6.0 to 9.0 there are increasing problems. If the SARa is greater than 9.0, severe problems can be expected.
Increasing the calcium content of the water by adding gypsum or some other soluble calcium salt.
Reducing the HCO3 in water by adding sulfuric acid, sulfur dioxide (SO2) or some other acidifying amendment.
Bicarbonate in irrigation water is particularly troublesome. The SARa is a good index of the sodium and permeability hazard if the water passes through the soil and reaches equilibrium with it. Unfortunately, in the field this is not always the case. Water with relatively high bicarbonate and low calcium and magnesium content would have a high SARa and hence an exceedingly slow permeability.
In many cases, plant growth will not permit water to remain on the crop long enough to allow for deep leaching. As evaporation and plant removal occurs, the soil water becomes more concentrated and excess bicarbonate will remove calcium and magnesium from the soil forming insoluble carbonates. Sodium thereby replaces the calcium and magnesium on the soil and further aggravates the permeability problem. Without adequate leaching the soil will not only become saline but will also become sodic.
On the opposite side, irrigation water with a very low salt content may also present a water penetration problem. This very low soluble salt water will pick up the calcium from the clay minerals and leach it away. The addition of some salt, such as gypsum, would be helpful. There is evidence that all irrigation waters should contain a minimum of 20 ppm of calcium (1.0 me/l) to prevent dispersion of the soil. The EC of the water should be over 0.5 mmhos/cm.
Bicarbonate ions in water can also be a problem with overhead sprinkler irrigation. A white deposit of calcium carbonate forms on the leaves and fruit. This can render some fruits and ornamentals unmarketable because they are unattractive. This coat of "whitewash" is not known to have any other adverse effect on plant growth. Levels below 1.5 me/l of HC03 should cause no problem. Increasing problems occur between 1.5 and 8.5 me/l and severe problems are found above 8.5 me/l. At this high level of HC03 most waters are unfit for irrigation for other reasons.
Summary - The following table summarizes the limits which were discussed in the text.