Greenhouse and Nursery - Water Alkality and Growing Media

High alkalinity in irrigation water used in greenhouses and nurseries can lead to media related pH problems. The following is some information on the topic from the University of Maryland.
The pH of a soil-less growing medium is primarily influenced by the alkalinity of the irrigation water.

What is Alkalinity?

• A measure of the water’s ability to neutralize acid.
• Composed of:
- Bicarbonates - HCO3- - Primary Component (Ca, Mg, or Na)
- Carbonates - CO3= (only at pH > 8)
- Hydroxyls – OH- (small influence)

Hard Water

• Hardness refers to cations of calcium and magnesium
• Alkalinity is a measure of bicarbonate and carbonate anions.
• High alkalinity is not necessarily hard water.
• A water high in sodium bicarbonate has high alkalinity, but is not hard.
• Water softeners are not the answer!!

Effect of Alkalinity on Growing Medium pH

• Waters with high alkalinity will cause the pH of the growing medium to rise over time.
• These effects are most dramatic in small containers, e.g. plug trays.


Reprinted from the March 14, 2008 edition of the Greenhouse TPM/IPM Weekly Report from the University of Maryland Cooperative Extension, Central Maryland Research and Education Center http://www.ipmnet.umd.edu/08Mar14G.pdf