Irrigation Water for Greenhouses
Water Alkalinity
Alkalinity is the concentration of soluble compounds in the water that can neutralize acids. The major chemicals responsible for alkalinity in water are bicarbonate ions (HCO3¯) from dissolved salts such as calcium bicarbonate (Ca(HCO3)2), sodium bicarbonate (NaHCO3), and magnesium bicarbonate (Mg(HCO3)2); and to a lesser extent carbonate (CO32¯) from dissolved salts such as calcium carbonate (CaCO3) and magnesium carbonate (MgCO3). Hydroxide ions (OH¯) are a minor contributor in most cases. Ammonia, borates, organic bases, phosphates, and silicates can also be minor contributors to alkalinity. The dissolved carbonates and bicarbonates increase the media pH over time by neutralizing hydrogen (H⁺) ions in the media solution. The higher the alkalinity, the greater the amounts of acid that will be required to produce a desired pH drop.
Why Is Water Alkalinity Important?
Why is the alkalinity of greenhouse irrigation water so important to a grower? Because the alkalinity of the irrigation water impacts the growing medium pH over time, which in turn impacts the availability of nutrients. When the alkalinity is high, the pH of the growing medium will rise quickly when water is applied through top irrigation or through subirrigation with no leaching potential.
Alkalinity versus pH
The effects of both pH and alkalinity are important to the nutrition and root health of your crops. pH and alkalinity are related but separate measurements. pH measures the hydrogen ion concentration, and alkalinity indicates a solution's ability to resist or neutralize the effects of acids. Alkalinity acts as a buffer to acidic materials. The higher the alkalinity, the greater the amounts of acid that will be required to produce a desired pH drop.
Units of Measurement of Alkalinity
The units that quantify alkalinity are another possible source of confusion for growers (Table 18.3). Alkalinity is reported as milligrams per liter (or as parts per million, PPM) of calcium carbonate equivalents (mg/LCaCO3) or milliequivalents per liter (meq/L) of calcium carbonate equivalents (meq/L CaCO3). One meq/L CaCO3 = 50 mg/L CaCO3 (Note: 1 mg/L = 1 PPM). The term meq/L is a chemistry term that is not only dependent on a material’s concentration but also on its molecular weight and charge. Since bicarbonates and carbonates are the major components of water alkalinity, some laboratories assume that Total Carbonates (TC = carbonates + bicarbonates) equals alkalinity. Some laboratories assume that all alkalinity is derived solely from bicarbonates (HCO3¯) and will report alkalinity as bicarbonates using PPM (mg/L) or meq/L.
General Guidelines of Upper and Lower Alkalinity Limits
The recommended upper limit for alkalinity in irrigation water varies depending on the buffering capacity and volume of the substrate, the crop time, and the pH sensitivity of the crop. The larger the substrate volume, the more buffering capacity is present, so the pH will change much more gradually in large containers than in small containers or individual cells in a plug tray.
Measuring Alkalinity
There are a few different options available to measure alkalinity, and they vary in ease, accuracy, and price. The simplest method is to use alkalinity test strip kits and dip the test strip in a sample of water and wait for the coloration of the strip to indicate the approximate alkalinity concentration.
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