pH and alkalinity, what's the difference?

Simply put, pH is a measure of how acidic or basic a solution is. Positively charged molecules such as H+ will make solutions acidic (lower pH) and the negatively charged OH- molecules will make a solution more basic (raise pH).  The pH scale runs from 1-14 with the value 1 being most acidic, 7 is neutral, and 14 most basic.  This scale is logarithmic, meaning a change in one whole unit is 10 times more or less acidic. For example, pH 5 is ten times more acidic than pH 6. Sometimes people will refer to basic solutions (high pH) as alkaline and this is where folks can get confused with the term ALAKINITY.

Alkalinity is the ability of water to neutralize acids due to the dissolved alkalis (bicarbonates) in the water.  Alkalinity is often reported in part per million (PPM) of calcium carbonate (CaCO3). As alkalinity increases in the water, more acid will be needed to neutralize the alkaline substances.   The primary source of alkalinity in ground water aquifers, rivers, ponds and lakes are limestone deposits that have reacted with water over time.  Throughout the seasons, water levels within aquifers can change.  High water levels can dilute alkalinity levels just as low water levels can concentrate them.  Sample your water 2 or 3 times during the year as you notice conditions change.  If your samples show significant change, then you know that regular sampling is necessary.

Water quality considerations in the greenhouse and high tunnel:

When alkalinity is high, it's likely that pH is above optimum as well and the alkalinity level of your water may need to be adjusted to manage your pH.    Think of alkalinity as "liquid limestone".  Each time you water, you increase the pH of the soil/media a little bit. Over time, the soil /media pH increases significantly and ties up nutrients. For example, we commonly see iron and manganese deficiencies in tomatoes growing in media with pH over 7.   This effect happens faster in containers.

Alkalinity regulates the buffering capacity of the water and affects how much acid is required to change the pH.   Many growers use, fertilizer injectors like a Dosatron or Chemilizer to add acid  to water to reduce alkalinity. The acid combines with the bicarbonates to form carbon dioxide and water. ENYCHP specialists can help you design an acid injection program to adjust your pH. The ideal alkalinity level for irrigation water is generally around 100 PPM CaCO3.

Alakinity and pH affect the performance of Pesticides and Sanitizers:

Another important reason to pay attention to water pH and alkalinity is that it plays a significant role in the efficacy of many pesticides used. This is true for both organic and synthetic pesticides.  In a pH over 7, pesticides can break down in a process called hydrolysis.   The higher the pH the faster the reaction. Adding a buffering agent such as LI700, is an easy way to change the pH of the water for mixing pesticides that require a lower pH.   To determine how much buffer should be used, applicators need to know the pH of the water and the volume needed to treat the crop/area.

Here is an example taken from a PyGanic label.

"GENERAL USE DIRECTIONS

IMPORTANT: It is recommended that the final spray mix be buffered to a pH of 5.5-7.0.

Outside of this range pyrethrins can degrade and the product will lose effectiveness."

Word of caution, fixed copper pesticides in a low pH become soluble and can cause phytotoxicity.  The pesticide label will often state optimal pH ranges. If you are unsure contact a technical representative of the product manufacturer.  Applicators should test their water pH prior to a spray application using a pH litmus strip or a pH meter.

Products other than pesticides are sensitive to pH. According to the label, the sanitizer Sanidate 5.0 works best when diluted in water with a neutral pH (close to 7), but sodium hypochlorite (bleach)works best when water pH is lowered to 6.0-6.5.

Water alkalinity and pH adjustments are easy to make and significantly improve the performance of pesticides and sanitizers. If you need help or have questions on the process, ENYCHP specialists can help. We have pH meters and titration kits to determine alkalinity  or can guide you to labs that can test water quality parameters.