Know the difference between water pH and alkalinity

Many growers are confused by high pH and high alkalinity in relation to water quality and growing medium pH, said Univ. of Mass. horticulture professor Doug Cox. He said generally water with a high pH (7-8) and low alkalinity (less than 100 parts per million calcium carbonate) doesn’t cause problems. This type of water has very little effect on pH relative to fertilizer type and other cultural factors. Cox said with this type of water acid injection is not generally needed to maintain the proper growing medium pH range when water has a low alkalinity regardless of pH level.
Cox said a combination of high pH and high alkalinity (greater than 100 ppm calcium carbonate) is a problem. Irrigating with high alkaline water can lead to an increased growing medium pH, which could adversely affect some plant species and plant stages (i.e., certain plug species). In some situations acid injection may be necessary. A combination of high pH and high alkalinity is of particular concern with seedling plug trays. Trace element deficiencies and imbalances of calcium and magnesium can result from irrigating with high alkalinity water.