Monday, September 5, 2011

pH in Aquaponics

     Compared to aquaculture and hydroponics on their own, aquaponics requires very little in the way of water quality monitoring.  A simple home system can thrive without expensive meters or chemical test kits that will tell you how much dissolved oxygen or nitrite is in the water.  There are a couple of water quality factors that should be checked on a regular, if not daily, basis, though.  PH is one of these water quality factors.  A simple pH meter can be purchased for less than 50 US dollars and will be more than adequate for home purposes.  They are widely available online.
     PH is a measure of the hydrogen ion concentration in water and effects all three of the main aquaponic components (fish, bacteria, and plants).  The pH scale ranges from 0 to 14.  Levels below 7 are acidic, 7 is neutral, and above 7 is basic.  Most fish will do best with a pH between 7.5 and 8.  A pH level below 4 or above 11 is fatal to most fish.  PH levels of 4-6 and 9-10 can affect fish growth and reproduction.  The nitrifying bacteria do best with a pH between 7 and 8.  The nitrification process is hindered outside of that range.  And plants do best with a pH between 6 and 6.5.  Each nutrient a plant uses for growth is available to the plant at different pH ranges.  When the pH falls outside of the 6-6.5 pH range nutrient uptake for certain nutrients is hindered.  This can lead to a deficiency even when the nutrient is present.  And a pH lower than 4.5 or higher than 9 can severely damage plant roots.
     Because all three components function and thrive at different pH levels a compromise must be met in an aquaponic system.  A pH level between 6.5 and 7 is ideal.  This allows for healthy growth of the fish, an adequate rate of nitrification by bacteria, and healthy growth of the plants in a system.  The nitrification process naturally lowers pH, so most often a base will have to be added to raise it.  Hydroponic pH adjusters should not be used.  They are made from salts that will negatively affect an aquaponic system.  Hydrated lime is an excellent choice for raising the pH because it is often supplemented into aquaponic systems to make up for a lack of calcium.  Potassium hydroxide is another choice for raising the pH.  If the pH of your system rises above 7.5 it can be lowered with the addition of an acid.  Nitric, phosphoric, or acetic acid can be added in very small amounts to a system until the desired pH is acquired.
     A good method for pH adjustment is to take a clean container and fill it with a gallon of your system's water.  You can adjust the pH of that water to find out how much acid or base you will have to add to get your desired pH.  Once you know how much it takes to adjust that gallon you can simply multiply your system's water capacity by however much it took to adjust the pH of the gallon sample.  For example, if .1 teaspoon of hydrated lime was used to adjust the pH to 7 in a sample gallon from a 500 gallon system it would require approximately 50 teaspoons (or 1 cup) to adjust the pH of you entire system.  Note that these numbers are for demonstration purposes only and do not reflect real pH adjustments.

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