Irrigation Water for Greenhouses
Water pH
Water pH is a measure of the relative concentration of hydrogen ions (H⁺) to hydroxide ions (OH¯). The greater the number of H⁺ ions in relation to OH¯ the more acidic the solution becomes. The greater the ratio of OH¯ ions to H⁺, the more basic the solution becomes. The pH can vary on a scale from 0 to 14 with a pH of 7 being neutral, less than 7 considered acid and above 7 called basic or alkaline. The main effect of water pH on plant growth is through control of nutrient availability, especially micronutrients (Figure 18.2).
The Importance of pH
The pH of irrigation water is not generally as critical a measurement as the pH of the substrate. The recommended range of irrigation water pH and media solution pH for container production depends on the crop being grown. The generally accepted pH range is 5.2 to 6.8 for irrigation water and 5.2 to 6.3 for the substrate solution. Although hydrogen ions are not considered one of the nutrient elements, pH does influence the solubility and thus the availability of many other nutrients (especially micronutrients).
pH Meters
Electrode-type pH meters, which are the most accurate for measuring pH, are also the most expensive and require the most maintenance. Placing the pH probe directly into the solution until the reading stabilizes gives a quick reading. Many of the better pH meters have temperature compensation, but the temperature dependence is small (usually within 0.1 to 0.3 units) in the temperature range of irrigation water used by most growers.
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