Hydro pH Levels

Hydro pH Level Introduction
Understanding why pH levels are important in hydroponic gardening doesn’t need to be difficult or complicated. Nearly everyone has some familiarity with acids, bases and pH through everyday life. For example, swimming pools are periodically tested and adjusted in order to maintain an optimum pH level. Hydroponic gardening isn’t all that different. Like many technical topics, volumes of scientific literature have been written regarding the subjects outlined below. Fortunately for us, understanding a few basic principles is all that’s required for the purpose of enjoying a successful start in hydroponic gardening.

Acids are proton donors – Bases are proton acceptors
When mixed with pure water, chemicals that disassociate and release one or more hydrogen atoms (protons) from their chemical structure are considered acids. Bases operate on a sort of reverse principle; bases accept additional hydrogen atoms into their structure given the same conditions. Phosphoric acid is an example of one acid commonly used by hobbyist hydroponic gardeners. Found as an active ingredient in pH lowering products from companies like General Hydroponics, Grotek, Advanced Nutrients and many others, phosphoric acid is capable of disassociating one, two or even three hydrogen atoms. For the particular case below, only one hydrogen atom is illustrated as disassociating from the molecule of phosphoric acid.

Disassociation of phosphoric acid.

Introduction to the pH scale
Assume you wanted to compare the acidity of orange and lemon juice. We know both are acidic, although an ideal comparison would be in absolute terms and allow for a quantifiable statement like “This lemon juice is ten times as acidic as that orange juice!” Situations like this is where the pH scale comes into play. Most often, the pH scale is represented as a sort of ruler that stretches from 0 to 14. Pure water, under correct conditions, occupies the center of the scale at a pH of 7 and is said to be pH neutral. Anything with a pH below 7 is considered an acid while anything above is a base. Substances become more acidic as they approach a pH of 0 and more basic as they approach a pH of 14. Consequently, anything described as pH lower, pH down or so forth is going to be an acid. Conversely, any sort of pH raising product is a base. Remember, virtually anything that you mix into a hydroponic nutrient reservoir is capable of having an impact on the overall pH; although there are many scenarios where that impact will be immeasurably small. Below is an illustration of the pH scale with the pH of several hydroponic gardening products and household goods listed for comparison.

It’s worth noting that the term alkalinity is sometimes used interchangeably with basic.  Another important feature of the pH scale is that, like the Richter magnitude scale used for measuring the power of earthquakes, the pH scale is logarithmic. This is important! Something with a pH of 3 is ten times as acidic as a pH of 4 and one hundred times as acidic as a pH of 5. Remember, just a few points of change in the pH of a hydroponic nutrient solution can be, in fact, a very large swing. Don’t feel intimidated, it’s just an important fact to keep in mind.

pH levels alter the availability and absorption of nutrients
Plants are only able to absorb the nutrients required for growth if those nutrients are able to dissolve in water. Nutrients that persist in a solid state, i.e. salt crystals that refuse to dissolve, are effectively unusable by plants. This is true for both soil and hydroponic gardening. Water has a limited capacity to dissolve minerals and that capacity is dependent upon the pH of water. Certain nutrients are unable to dissolve effectively in a solution with a pH in excess of 7.5 and others begin to experience limited solubility at a pH below 6. When a substance in solution, such as hydroponic mineral salt, becomes insoluble and forms into a slurry-like powder resembling the contents of a snow globe, it’s called a precipitate.  To further emphasize the importance of pH, certain plant diseases also develop more readily when the pH of a hydroponic solution falls below 6. [10]

In addition, Dr. Lynette Morgan writes on the on the Growing Edge Website:

“As the pH increases above 7, plant uptake of some ions becomes less efficient so plants become deficient even if that ion is present in solution. The pH therefore has an effect on both nutrient solubility in the solution and the ability of the plant to uptake minerals.”

Dr. Morgan is emphasizing that even if particular nutrients remain available in solution, plants could still struggle to absorb them given excessively basic pH levels.

It may appear that managing the pH of a hydroponic nutrient solution sounds like a no-win situation. While pitfalls indeed exist, given a little experience and some attention to detail it’s possible to avoid virtually all the problems mentioned above.  It’s simply a matter of reaching a compromise between a pH that’s too high and one that’s too low.

Hydroponic nutrient solutions are maintained in a pH range of 5.5 – 6.5
Searching the web for information about hydroponic pH levels can prove very frustrating. There tends to be a large amount of inaccurate or even incorrect information about all matters of hydroponic gardening that freely circulates on the web. This should tell you a couple things. First is that plants are fairly resilient organisms that have evolved to tolerate pH fluctuations within a few points without harm. Second is that anecdotal evidence readily circulated and seldom verified within the hobbyist hydroponic online community. Below are hydroponic specific pH recommendations from several sources:

Research Universities:

Hobbyist Hydroponic Nutrient Suppliers:

Taken as a generalization from the above sources, it’s recommended to maintain a hydroponic nutrient solution within a pH range of 5.5-6.5.

References and additional reading:

Handbook of Chemistry and Physics. 85th Edition.; CRC Press. Boka Raton, FL., 2004.

TPS, pH in Hydroponics. http://www.tps.com.au/hydroponics/pheffect.htm (Accessed January 30, 2008)

Image of water droplet on garden nasturtium (Tropaeolum majus) by user Eulicidate from Wikimedia Commons.