6/16/2021 - By Lynn Griffith

Understanding the quality of irrigation water and how it affects nutrition and plant growth is important for both growers and allied vendors. This article will hopefully explain the basics of water quality in easy to understand terms. Take a look at this blog for more information on water quality and plant health.

Units

Most laboratories report analytical results for irrigation water in parts per million or milligrams per liter (mg/L), which are essentially the same units. Some labs report the mineral contents in milliequivalents per liter (meq/L), which is perfectly legitimate. However, I find that most growers cannot explain to you what a milliequivalent is. Parts per million are units most growers can relate to.

pH

pH stands for ‘potential of hydrogen’. When you measure the pH of a media or water sample, you are actually measuring the quantity of hydrogen ions in the material. Most irrigation waters range between 4.5 and 8.5, though I have seen waters as high as 9.0. Understand that the pH scale is a logarithmic scale like the Richter scale is for earthquakes. A pH of 8 is 10 times higher than a pH of 7, and 100 times higher than a pH of 6. Growers should adjust the acidity of their fertilizers and media to keep the pH in the desired range for the crop.

Electrical Conductivity (EC)

Absolutely pure water conducts virtually no electricity. As you begin to dissolve salts in water, the salts tend to ionize and the water will now conduct electricity. The amount of electricity conducted is directly proportional to the amount of salts in the water. Most labs report EC in either millimhos per centimeter (mmhos/cm) or decisiemens per meter (dS/m). Both of these units are the same.

You learned in grade school that an ohm is a unit of resistance. A mho (ohm spelled backwards) is a unit of conductance. What the labs are really telling you is how much electricity a solution conducts, using electrical units. An Electrical Conductivity reading, in these units times 640, tells you Total Dissolved Solids, or the total quantity of salts in the water. If you add up all of the numbers in a complete irrigation suitability analysis, that value should be very close to the total dissolved solids number.

Alkalinity

Alkalinity measures the buffer capacity of the water, which is its ability to resist pH change. Alkalinity is made up of carbonates (C0₃), bicarbonates (HCO₃) and hydroxyls (OH). Carbonates generally don’t exist in waters with pH less than 8.0. Therefore, in most waters, bicarbonates are the main source of alkalinity. Nurseries that pump irrigation water out of limestone aquifers generally have high bicarbonates.

Bicarbonates can be directly toxic to roots, and they can interfere with trace element availability. Understand that you can have 10 different waters, all with a pH of say 8.0, but they can all have different alkalinity values. High pH and alkalinity are not the same thing.

Acid Injection

Some growers with high pH water inject small quantities of acid into the water to neutralize the alkalinity. Many like to lower their water pH to 6.0-6.2. As you begin to drip acid into alkaline water, the carbonates and bicarbonates are ultimately released as CO₂ into the atmosphere. Waters with high bicarbonates require more acid to lower pH than waters with low bicarbonates. Usually, if you inject acid to a pH of 6.0-6.2, HCO₃ levels are no longer a problem. Growers with high bicarbonates may benefit from more acidic fertilizer sources.

Hardness

Hardness refers simply to the quantities of calcium and magnesium in a water source. Waters typically contain more calcium than magnesium. When water comes in contact with limestone, which is primarily calcium carbonate, some of that limestone gets dissolved in the water. The calcium and magnesium dissolved in hard water are generally available to plants as nutrients. Hard water may cause white stains on foliage. A water softener simply exchanges the calcium and magnesium with sodium. Softened water is generally not desirable as an irrigation source.

Sodium and Chloride

Most all waters contain some degree of sodium and chloride, which we refer to as ‘salt.’ Some sources disagree, but in general we call sodium and chloride high above 70 ppm and very high above 300 ppm. Salt in irrigation water can significantly increase EC in media, as well as create potential toxicities of both sodium and chloride in plants. The degree of salt tolerance depends on plant species.

Sulfate

Some irrigation waters are high in sulfates. These can be directly dissolved sulfate ions, or sometimes from dissolved hydrogen sulfide (rotten egg smell) in the water. Injecting hydrogen peroxide helps remove the hydrogen sulfide smell. Dissolved sulfates do act as nutrient sources for plants. If irrigation water is very low in sulfur, supplemental sulfur nutrition may be required. High sulfates in irrigation waters generally don’t cause significant plant problems.

Boron

Some parts of the country have elevated boron in their irrigation water. Sea water contains 4.6 PPM boron. Growers dealing with salt intrusion may also be dealing with boron intrusion. Boron is the only micronutrient that leaches significantly from media. Boron levels above 0.3 ppm in water can begin to cause problems with boron toxicity.

Fluoride

Some plants with long, tapered leaves, or plants in the lily family, can be sensitive to fluoride toxicity. Fluoride prevents the closure of the stomates, and can cause tip burn in older leaves. Sensitive varieties are best grown with water containing 0.25 ppm fluoride or less. Many municipalities fluoridate their water to 1 ppm, which is four times too high for fluoride-sensitive plants.

Rain Water vs. Irrigation Water

Rain is generally superior to irrigation water for four reasons.

• First, rain water is usually quite pure. It’s “hungry water”, which is more efficient at dissolving minerals into the soil solution than mineral-laden irrigation water.
• Second, rainwater tends to be acidic, which can sometimes be beneficial and sometimes not.
• Third, raindrops strike soil at 20 mph. Irrigation drops strike soil at 5 mph. You therefore get better penetration from raindrops.
• Fourth, rain water is usually saturated with air, which serves to oxygenate the root zone.

Now, with these terms and concepts in mind, I hope this helps you to get a better reading on the state of your irrigation waters quality, nutrient levels, and how you can better balance it out for your specific plants.