Good yield of crops with high quality can be produced by using good quality water for irrigation. The quality of irrigation water can be defined by determining its characteristics, which are greatly variable in different water sources. Even there is greater variation in the quality of water in different regions. Generally, these differences are based on climate and geology. The differences can also be great in the quality of water depending on its sources such as groundwater aquifers, above-ground sources of water (ponds and lakes), with the variable geology and based on the chemical and physical treatments of that specific water bodies. The chemical constituents of the water used for irrigation purposes have direct influences on the growth and development of plants either through the direct deficiency, or toxicity and by the indirect way by the alteration of the availability of essential plant nutrients.
For evaluation of the quality of water that is to be used for irrigation purposes, the important characteristics of water about plant growth should be identified. The water should only be recommended for the irrigation purpose if the concentration or level of these significant factors is within the tolerance range. In the first step for the determination of the quality of irrigation water, the testing of water should be done in a reputable laboratory. The interpretation of results should be on the knowledge basis, which allows the users to get a good idea about correctly identifying the problems associated with the irrigation water and helps greatly for the selection of fertilizers and techniques of irrigation for avoiding the damage to the crops.
Water Quality is Affected by Various Factors
Many factors should be considered for the determination of the quality of water for irrigating the crops and other plants. Most commonly, there is an analysis of individual water for testing the water quality.
pH
pH is an important parameter for the measurement of the basicity or acidity of water. pH is properly defined as the negative log of the concentration of hydrogen ions, so, in the acidic water the concentration of hydrogen ions is high and in the basic water, the concentration of hydroxyl ions is higher than the hydrogen ions. For most of the plants, the optimum level of pH is between 5.6-6.2. Mainly, the pH of water greatly affects the growth and development of plants by controlling the availability of the nutrients. At the low pH, the concentration of manganese and iron is more bioavailable which may lead to the toxicity of these elements, and thereby causing the deficiencies of magnesium, and calcium.
Table: 1 Desirable components and nutrient levels of irrigation water
| Measurement of Water Quality | Desirable Range |
| pH | 5.8-6 |
| Alkalinity | 0.75-2.5 meq/L CaCO3 |
| Soluble salts (EC) | < 1.5 mmhos/cm |
| Hardness | 100-150 mg CaCO3/l |
| Calcium | 40-100 ppm |
| Magnesium | 30-50 ppm |
| Sodium | <50 ppm |
| Sulfate | < 50 ppm |
| Chloride | <100-150 ppm |
| Boron | <0.5 ppm |
| Fluoride | <0.75 ppm |
Alkalinity, Presence of Carbonates and Bicarbonates
Mainly, the effect of irrigation water is not on the pH of the growing medium, but pH affects the alkalinity of the growing medium. Alkalinity is the measurement of the concentration of the compounds that are soluble in water and are strongly capable of the neutralisation of the acids. Alkalinity is directly related to the pH, as the highly alkaline water has the greater buffering capacity or the capacity for the neutralisation of the acids that are being added.
The reporting of alkalinity is in milligrams per liter, or parts per million (ppm), of the equivalent’s calcium carbonates, (mg/l of CaCO3).
Remember, here,
One meq/l CaCO3 = 50 mg/l CaCO3 (Note: 1 mg/l = 1 ppm).
The main chemical constituents that cause the development of alkalinity in the water are dissolved compounds of carbonates and bicarbonates which are dissolved from the groundwater sources or aquifers. Dolomite and limestone are the main sources for the development of alkalinity in the irrigation water. With time, the pH of the medium is increased due to the continuous addition of carbonates and bicarbonates by the neutralisation of hydrogen ions in the solution of the medium. On the other side, the fact is that there is zero alkalinity in the rainwater. Currently, there are no established levels for the optimum and toxicity of alkalinity. If the concentration of carbonates or bicarbonates is reported to be 1 meq/l then it is significant enough to raise the pH of the medium solution with time. Typically, the recommended range is 1-1.6 meq/l for some of the plugs and 2.6-3.6 for meq/l for the majority of plants.
In some cases, the alkalinity in the irrigation water can be low enough to have no effects on the buffering capacity of water against the minor or major changes in pH. When this is happening, the pH of the media solution can be significantly declined when the fertilizers consisting of acid residues are utilized for providing nutrients to the plants.
In the table 2, recommendations are given for the upper limits of alkalinity levels for various kinds of production systems. Generally, if the rooting volume is large, then the allowable alkalinity is also higher. This happens due to the reason as the media is having the limited ability for supplying the hydrogen ions for neutralisation of the alkalinity of the water to be applied to the crops.
Solubility of Salts
In the irrigation water, different kinds of soluble salts are present which can be measured by the determination of electrical conductivity EC of water in terms of mmhos/cm that is equivalent to the milli Siemens per cm. Electrical conductivity can also be defined as salinity or specific conductivity. The EC of irrigation water should be first measured for the determination of total dissolved solids or total soluble salts before applying the fertilizers to the crops. By controlling the amount of leaching, the soluble salts in the media can be controlled at the optimum amount. For the irrigation water, the value of EC should be less than 1.5, for the addition of normal fertilizers but for the plugs, the values of less than 1 are highly recommended.
Due to the presence of an excess amount of soluble salts the normal functioning of the roots is impaired which can considerably reduce the uptake of water and often leads to the deficiencies of essential plant nutrients.
Table 2: Recommended Level of Alkalinity Upper Limits in the Irrigation Water
| Container | Minimum Alkalinity (meq/l) | Maximum Alkalinity (meq/l) |
| Plugs or seedlings | 0.75 | 1.3 |
| Small pots/ shallow flats | 0.75 | 1.7 |
| 4-5 inches pots/ deep flats | 0.75 | 2.1 |
| 6 inches pots/ long term crops | 0.75 | 2.6 |
Hardness
The amount of magnesium and calcium can be indicated by the measurement of the hardness of the water. Generally, the amount of hardness is measured in the mg/l or ppm. In the irrigation water, the amount of these elements is highly variable. For example, if the limestone is the major component of the groundwater aquifer then it may have greater than 100 ppm of calcium, whereas the water from the sandstone or granite aquifer may have less than 10 ppm of calcium. When the hardness of water is in the range of 100-150 mg CaCO3 per litre, then it is categorised as the best one for irrigating the crops. Although, some plants can easily tolerate the higher level of these elements and in some cases, the toxicity is not of major concern. However, due to the excessive hardness the foliar deposits of magnesium or calcium in the system of overhead irrigation. Soft water that is generally having less than 50 ppm of CaCO3/l may require the additional application of magnesium or calcium over time to achieve the god growth and development of plants.
Major Nutrients for Plant Growth
Magnesium and Calcium
These are essential elements for the growth and development of plants and on the weight basis, they are generally reported in the ppm. Magnesium in the range of 30-50 and calcium in the range of 40-100 ppm is most desirable for the irrigation purpose.
Sodium
The municipal and well water sources often contain higher levels of sodium. The higher amount of sodium in the water and root zones significantly reduces the uptake of calcium and resultantly excessive leaching of magnesium and calcium in the medium is reported. Possibly, due to the absorption of sodium by the foliar means, the leaf is often burned. The amount of sodium less than 50 ppm in the irrigation water is considered good for irrigating the crops. Along with the evaluation of magnesium and calcium levels in the irrigation water, the amount of sodium should also be evaluated before its application to the crops. For the determination of effects of sodium, the sodium absorption ratio SAR is measured. If the value of SAR is < 2 and the concentration of sodium is less than 40 ppm, then the availability of magnesium and calcium is not limited due to the presence of sodium.
Phosphate and Potassium
These are essential nutrients for the growth and development of plants but their dissolved concentration in the irrigation water is extremely low. Generally, they are measured in ppm units and if they are present in the higher amount, they may be indicating the level of pollution in the irrigation water from some contaminants or fertilizer sources.
Sulfates
For the normal growth of plants, sulfur is the most essential element and commonly there is no incorporation of this element in the fertilizers. In the irrigation water, it is measured as an indication of the deficiency problems. If the concentration of sulfur in the irrigation water is less than 50 ppm, then there is a need to add supplemental sulfate to ensure good growth of plants.
Chlorides
In the municipal and well water sources, a higher amount of chlorides is present in the water. Sometimes chlorides are excessive, and, in some cases, their deficiency is also reported. In the sensitive plants, the higher level of chlorides often results in the burning of leaves under the overhead irrigation systems. If the concentration of chlorides is < 100 ppm, then no damage is caused by excessive foliar absorption. However, if the concentration is < 150 ppm then there is no potential hazard of toxicity from the root uptake.
Ammonium and Nitrate
The testing of these nutrients is prioritized for an indication of contamination in the water sources. If these elements are present in a significant amount, such as > 5ppm then they should be considered for addition to the fertility programs.
Micronutrients and Trace Minerals
The most significant micronutrients are boron, iron, manganese, zinc, and copper. They can be present in less or even in toxic quantities. Due to the excessive presence of manganese and iron, the unsighted residues can be observed on the foliage in the system of overhead irrigation. Sometimes, there is sensitivity to higher concentrations of boron, in the irrigation water, and the optimum level of boron should be < 0.5 ppm. The high level of fluorides is also harmful to the foliage of plants. In the irrigation water, the concentration of fluorides should be less than 0.75 ppm. Significant problems may arise due to fluoride-treated municipal supplies.
The Good Concept of On-Site Water Testing
pH and electrical conductivity EC are two main characteristics of water which should be periodically tested at the growing facility. It is significantly beneficial for the growers for the indication of the quality of irrigation water and to look for the possible measures and checks for the treatment of water. Different types of pH meters are available ranging from inexpensive pens to sophisticated units. It is a strong recommendation to buy the pH meters that can be easily calibrated by using the standard calibration methods and solutions. The calibration ensures that the reading obtained by the meter is standard and correct. As compare to the pH meters, the EC meters are generally more expensive. But they are extremely useful for measuring the quality of irrigation water and gives a good indication about the requirements of fertilizers for the plant growth.
Issues Related to Correction of Water Quality
Mainly, there are 3 categories associated with the problems of water quality and it is possible to correct them by the physical and chemical treatment systems. By the addition of acids, the problem of alkalinity can be easily neutralized. Total soluble salts or total dissolved solids are measured together as ECw and individually as ppm. Different purification systems are being used for the removal of these soluble salts. If the amount of total dissolved solids is not much high, then they can also be removed individually from the water. Before opting for any investment in a system for the treatment of water it is essential to investigate that if there is any possibility to opt for the alternative system or not. Sometimes, the water from different sources is also mixed to avoid the problem of extra costs. In table 3, the methods for purification of water and their applications are summarized.
Adjustment of pH/ Neutralization of Alkalinity
The ability for the correction of pH problems in water and the maintenance of proper pH of the medium by using the cycling of the crop is greatly dependent on the alkalinity in the water that is to be used for irrigation purpose. For decreasing the pH of water, the amount of acid required is more in the highly alkaline water. One of the most economical ways for the elimination of alkalinity is to reduce the pH of water by neutralisation reactions with the acids. For the calculation of the required amount of acids, it is essential to know the alkalinity and starting pH of the irrigation water. One can easily target the pH level or ending point of the alkalinity during the process of treatment. Then a grower can easily monitor the processes of acidification in the irrigation water with the help of pH meter.
In table below, the values given show the optimum level of the requirements of acids for reaching the endpoint pH of about 5.8 and for the neutralization of 80% alkalinity in the irrigation water. As in this aspect, the beginning point of pH is not considered so there is strong requirement about the adjustment of the amount of acid. For example, the amount of sulfuric acid can be estimated for the neutralization of 80% of alkalinity. One can easily determine the approximate amount of the requirement of acid for the neutralization of alkalinity by decreasing the pH of the irrigation water by using the syringe of baby medicine, beaker, pH pen, and phosphoric acid. It is recommended to put at least one quart of the irrigation water in the plastic or glass beaker and then a drop of acid should be added by using the baby syringe. The amount of added acid should be recorded. After the addition of acid, the pH reading should be taken by stirring the water. When the level of targeted pH is reached, then the number of fluid ounces added to the water should be noted. By using these numbers, the pH of 1000 gallons of water can be easily calculated.
| Acid Type | Amount of Acid Required | Nutrients by Acids |
| Nitric acid (67%) | 6.6 | 1.6 ppm N |
| Phosphoric acid (75%) | 8.1 | 2.9 ppm P |
| Sulfuric acid (35%) | 11.0 | 1.1 ppm S |
There are some merits for targeting the endpoint alkalinity other than the pH. By setting the target of around 2 meq/l the resultant pH of the water will be in the range of 6-6.2. This method is employed for the seasonal variations in the endpoint alkalinity that is generally occurring in the wells and is creating potential problems for the growth of plants. The amount of alkalinity for the neutralization to the desired level can also be determined. For example, if the alkalinity in the water is 225 ppm, and one is interested to reduce it up to 100 ppm then it is a must neutralize the alkalinity of about 125 ppm. Generally, an injector is used for adding the acid to the irrigation water.
A computer-based spreadsheet is also available for the calculation of the amount of acid for neutralization of alkalinity in the irrigation water based on pH, soluble salts, and alkalinity. It allows the user or grower the easy targeting of the alkalinity level or pH to be optimized by the acidic treatments. This program is efficient enough for the calculation of the requirement of acids, pH and endpoint alkalinity. It also gives a good understanding of the number of nutrients by the alternatives, and the costs of alternative acidic treatments. If anyone is interested in this computer-based program, then he or she can contact the agricultural office for the extension purpose in the ornamental horticulture and landscape design department at the University of Tennessee.
Even for the acidified water, the alkalinity should be retested after the passage of one day and then after the 2-3 weeks to ensure that the alkalinity and pH are in an acceptable range.
Types of Acid
The most common acids used for reducing the problem of alkalinity in the irrigation water are phosphoric acid (75%, and 85%), sulfuric acid (35% and 93%), and nitric acid, (61.4% and 67%). All of these acids are generally dangerous but phosphoric acid is most safe among them. The use of phosphoric acid is only suitable when the requirement for the neutralization of alkalinity is 1-2 meq/l. The requirement for acid for the neutralization of alkalinity is higher than the levels of alkalinity and is far greater than the requirement of plants. Nitric acid can also be used for a reduction in alkalinity and for supplying nitrogen to plants at the same time.
During application, maximum safety should be ensured for the handling of acids. Due to splashing small droplets are generated which can accidentally enter the eyes. Additionally, it is essential to wear long pants and sleeves goggles and shoes for handling the acids. Here it is an important thing to note that only a small amount of acid should be added to the water in a slow manner.
Purification of Water for Removing Total Soluble Salts
Deionization and reverse osmosis are the most commonly used methods for the removal of soluble salts from water. Electrodialysis and distillation are purification processes for water. These processes are highly efficient to produce good quality water, but the associated cost is very high.
Reverse Osmosis
This system is most cost-effective and is widely used for crop production in the containers. It can remove 95-99% of total soluble salts from the water at the cost of about 0.02$ per gallon. This system works on the mechanism of osmosis in which solvent is passed through the semipermeable membranes which are specialized for separating the two solutions having different concentrations of salts. Solvents can easily pass through the semipermeable membrane, but the entry of solutes is restricted. By the application of pressure on the solution, the movement of water having the higher salt contents is ensured through the semipermeable membranes and the salt contents are left behind. Resultantly, there is an accumulation of pure water on the other side of the semipermeable membrane. Sometimes, this process is also called as hyperfiltration. Generally, a RO unit that is capable to deliver the 6000 gallons of water in one day is compact enough consisting of several tubes of varying diameter containing the membranes.
The requirement of pressure for the forced movement of water to the other side of the membrane is 150-400 psi. Polyamide (hollow fiber) and cellulose acetate (composites of the thin film) are the most commonly used membranes. For the replacement and maintenance of membranes, additional cost is required in the system of reverse osmosis. Low-cost membranes with less efficiency are also available but they require lower energy and less operating pressure 100-300 psi.
The degree of the removal of salts and the amount of purified water that is delivered in the specific time is dependent on the membrane type, the pressure applied by the system, amount of total soluble salts in the water and temperature. The efficiency of the system is greatly dependent on the cleanliness and integrity of the membranes. Chlorine is well known for causing degradation in the membranes and most common clogging is often observed due to sediments. Due to this reason, the pretreatment of water is preferred before its purification by the RO system. If the pH is greater than 7 then the level of chlorine, calcium carbonates, and pH is adjusted. Although the removal of total dissolved salts can be removed up to 95-99%, the efficiency of removal of individual salts is variable. Generally, magnesium, sulfate, and calcium are efficiently removed than lithium, sodium, borate, chloride, nitrite, and potassium. However, a disadvantage is associated with the utilization of the reverse osmosis systems that there is the production of brine wastewater. For the disposal of the brine, it is essential to follow the regulations set by the government.
Deionization
The soluble salts in the water are present in the charged form either as anions or as cations. Some good examples of cations are calcium (Ca++), sodium (Na+), iron (Fe2+), magnesium (Mg++), and potassium (k+). Whereas, some common examples of anions found in the irrigation water are fluoride (F–), bicarbonates (HCO3–), sulfate (SO4–), and chloride (Cl–).
In the process of deionization, the cations and anions from the water are removed using exchange resins. Usually, they are solid beads and are covered with positive or negative fixed charges. The fixed negative charge is present on the cation exchange resins, which is then neutralized by the hydrogen ions. By the passage of irrigation water over the resin the cations found in the irrigation water are replaced with the hydrogen ions and then are held over the surface of the resin. In a similar way, fixed positive charges are present on the anion exchange resins which are then neutralized due to the presence of hydroxyl ions. By the passage of water on this resin, the anions present in the water are replaced by the hydroxyl ions and then are held on the surface of the resin. The released hydroxyl and hydrogen ions then combine and form the molecules of water. In the units of deionization, both anions and cations are present that lead to the complete removal of all kinds of salts.
The process of deionization is very effective and is the best one to produce higher quality irrigation water for crop production. With the increase in the number of salts, the cost of deionization is also increased. If the salt contents are higher then there is frequent need to replace or regenerate the resin. As compared to the purified water by the RO system the cost of water purification by the process of deionization is 5-6 times higher. The RO system can be utilized as an initial step for the purification of water and finally, the high-quality irrigation water can be achieved by the process of deionization. In this way, the final costs can be significantly reduced as compared to the costs of deionization only.
Removal of Individual Salts
Manganese and Iron
In the water, the manganese and iron are oxidized to the greatly insoluble forms and cause the production of brown and black strain on the plant’s foliage which is covered by the overhead irrigation system. For the irrigation systems at the microlevels, the concentration of iron of < 0.3 ppm is required. Many ways can be used for the removal of these elements from the water. If there is the availability of enough space then the least expensive approaches can be used for the pumping of water to the tank or pond where insoluble compounds of manganese and iron can undergo the precipitation and settling.
Most often the water is pumped in, in the form of a spray to favor the rapid oxidation of manganese and iron to the insoluble forms. For this purpose, enough time should be given to ensure the settling of precipitates. Oxidation filters are also used for oxidizing the manganese and iron to the insoluble forms by using chlorine, air, or potassium permanganate. The filters ensure the removal of sediments that should be periodically cleaned by the backflushing. Sometimes, sand is also used as a filter. The chemical oxidant filters must be renewed after usage. The oxidation of manganese and its settling in water both are slower. To remove the manganese more efficiently the coagulation by the chemical means before the filtering and sedimentation may be required. If bacteria are also present along with the manganese and iron, then there is no need to use the oxidizing filters. The bacteria cause the quick blinding of oxidizing filters.
In the aquifers of Tennessee manganese and iron bacteria are most commonly encountered, so, sand filtration and chlorination should be utilized in the presence of bacteria. There is another approach for the elimination of precipitation problems is to keep the manganese and iron in the most soluble forms. The chelates of polyphosphates which are added to the water for attaching the soluble manganese and iron and they are managed for becoming oxidized. The chelated manganese and iron complexes that pass through the irrigation systems are not precipitated on the parts of plants.
Generally, the chelation complexes best work if the concentrations of manganese and iron in the water are low in the range of 1-2 mg/l. Furthermore, manganese and iron in the irrigation water are oxidized due to their exposure to the air and thus they cannot be chelated. The water in which there is the addition of chelates cannot be heated as the heating causes the breaking of polyphosphates and then causes the release of manganese and iron.
Magnesium and Calcium
It may be essential to remove the magnesium and calcium from the hard water for the elimination of deposition of salts on the foliage of plants that are covered by the overhead irrigation system. It can also be achieved by the softening of water that is the replacement of magnesium and calcium with the potassium. Usually, in the water softening, sodium is widely used than the potassium. Higher levels of sodium are very harmful to the plants and in the softening units, the use of sodium should be avoided. In the way the potassium is utilized for the growth and development of plants and the amount of total salt contents in the water is also kept unchanged. If the water is very hard, then due to the overfertilization the toxicity of potassium may occur. In the softening units, the potassium chloride must be recharged.
Fluoride
The fluoride contents can be easily removed from the water by adsorption using the activated carbon or activated alumina. In the first step, there is an adjustment of pH to 5.5. There can be the regeneration of activated alumina with the strong base sodium hydroxide and then it can be reused. Before the start of treatment, there is no need to adjust the pH of carbon with the activated carbon unit. After reaching the adsorption capacity the carbon in the unit is replaced. When the pH is raised above 6, then the fluoride is not soluble so the maintenance of the pH of the media solution should be maintained for reducing the toxicity-related issues.
Boron
In the many irrigation water sources, boron is naturally occurring in the anionic borate form. The anionic exchange resins which are like the deionization systems should be used but at a considerable level of expenses. For increasing the efficiency of boron removal in the reverse osmosis system, the pH of the water should be adjusted to the slightly alkaline conditions. The composite membranes consisting of thin films have more tolerance to the high pH and their use should be preferred.