You MUST know the alkalinity units of measurement!!!

By Dr. Claudio Pasian

Department of Horticulture and Crop Science

The Ohio State University

Columbus, 43210

 

If “only 25” is my response to your question “What is the distance?”, your reaction will be “Twenty five what?  Inches? Feet? Yards?”  Knowing the units of measurement I use in my response is the only way you can  know if “only 25”  is a lot or next to nothing.

It is the same with the results of your water or growing mix chemical analyses.   Except for pH, all other numbers must be followed by a unit of measurement.  For example, electrical conductivity (EC) is measured in millisiemens per centimeter (mS ∙ cm-1).  Occasionally, you will see EC expressed as millimhos per centimeter which is an old unit now replaced with mS ∙ cm-1.  All other numbers in your analysis report also will be expressed on a proper unit of measurement.  For example: parts per million (ppm), milligrams per liter (mg∙ l-1), percent of dry tissue (%), etc.  The interpretation of the results depends on the units of measurement.

One of the parameters measured when you conduct a water test that needs special attention is alkalinity.  Water alkalinity, the measure of all the bases present in the water, can be expressed in different ways by different laboratories:

·         Milli-equivalents per liter (meq∙L-1) of calcium carbonate (CaCO3)

·         Parts per million (ppm) of calcium carbonate (CaCO3)

·         Parts per million (ppm) of calcium bi-carbonate (Ca(HCO3)2)

Knowing the alkalinity level and the units it is expressed is essential to calculate how much acid needs to be applied in order to reduce alkalinity to levels that do not affect growing mix pH and nutrient availability. 

Let me share with you a case I had to deal with recently. A new greenhouse grower called me and told me that he had a problem with high alkalinity in his irrigation water.  He was trying to reduce it to a manageable level.  He shared with me that he had sought help from a friend who also is a greenhouse grower. This friend had done some calculations using the ALKCALC alkalinity calculator found in the University of New Hampshire Cooperative Extension website (http://extension.unh.edu/Agric/AGGHFL/alk_calc.cfm).  However, my caller indicated his uneasiness because he felt the amounts of sulfuric acid to be added according to the calculations were too high.  I then asked him to send me all the information via e-mail so that I could take a close look at all the numbers.

It did not take me long to figure out what the problem was: The right numbers were entered into ALKCAC but the wrong units were selected:  instead of 367 ppm CaCO3, 367 meq∙L-1 were entered.  The latter would be a very, very high level of alkalinity requiring large amounts of acid for neutralization.  No wonder the grower was surprised by the results he received from his colleague.  I re-calculated using the proper numbers and units of measurement and forwarded the results to him via e-mail.

So, the take home message is that you have to become familiar with the units used to express water alkalinity and you have to use them correctly when doing the calculations.

By the way, when using ALKCALC, be careful when selecting what type of acid you will use.  For example, you can choose from four different concentrations of sulfuric acid.

If you need help in determining how much acid you need to apply, please feel free to contact me at pasian.1@osu.edu.