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When we measure pH, we are measuring the concentration of hydrogen (H+) ions in the irrigation water or soil (nutrient) solution. As a guideline, water for irrigation purposes is usually best if its pH lies between 5.0 - 7.0. The measure of alkalinity gives us the water's ability to neutralize acidity. The level of bicarbonates, carbonates, and hydroxides in water determine its alkalinity. The desirable alkalinity range for irrigation water is 0 to 100 ppm carbonates, with 30 - 60 ppm optimum for most plants.

Whenever you test the water you'll be using for irrigation, you should always include the measure of both pH and alkalinity. A pH test on its own, does not give any indication of alkalinity. Water with high levels of bicarbonates or carbonates (high alkalinity) always has a pH value >7, but water with a high pH doesn't necessarily have high alkalinity. This is a very important concept because high alkalinity exerts more significant effect on plant nutrition and the fertility of growing media than pH.

High pH irrigation water generally causes no problems as long as the alkalinity is low. Since high pH water with low alkalinity has little ability to neutralize acidity, it's effect on media pH will be minimal. Cause for more concern are situations where you must utilize water having both high pH and high alkalinity for irrigation, which will cause the pH of the growing medium to increase substantially as media ages. If your water analysis is known to be alkaline, you may need to significantly reduce the addition of compounds containing Ca or Mg because of the dilute solution of limestone in the water. The smaller the container - the more serious the issue because small volumes of soil offer less buffering to pH change. In high alkaline situations, you can often expect/experience Ca and Mg deficiencies along with micronutrient deficiencies (both real and antagonistic).

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