The pH of hydroponic solutions is an important factor to regulate for optimal plant growth. The term pH comes from the French "pouvoir hydrogen", meaning the "power of hydrogen". Pure water has a pH of 7.0 and acidic solutions have more free hydrogen atoms and a lower pH (<7) and basic or alkaline solutions have a higher pH (>7) and less free hydrogen atoms (Bailar, et. al., 1989).

In hydroponics, the pH value has a greater significance with respect to nutrient availability than in regards to cellular damage which can be inflicted by extremely acidic (pH=<4) or basic (pH=>9) nutrient solutions. This is because molecules of nitrogen, sulfur, etc. become insoluble at lower pH values and at higher pH's, ions such as iron, copper, and phosphorus can precipitate out of solution. Therefore, proper solution pH must be maintained to ensure proper plant growth.

As mentioned in my previous article, the best way to check the solution pH is to periodically ( 1ce a day or so) check it using a cheap pH test kit. The more expensive meters work, but are unnecessary as rapid pH fluctuations should not occur, and if they do, you likely have some other problem such as supplying nitrogen as nitrate (NO3) alone, rather than applying it in combination with ammonium (NH4) which will help to eliminate quick rises in the pH (Taiz & Zeiger, 1991).

As the plants remove nutrients from the solution, the pH will naturally rise slowly and become more alkaline, which must be counteracted with acids; nitric, phosphoric and sulfuric being the most common used. In contrast, if solution acidity is a problem (which happens rarely) potassium or calcium hydroxide can be added, or lime added to the growing medium will often help and is good practice to buffer pH fluctuations.

Different plant species have been experimentally shown to grow best in different pH ranges, with acid loving plants such as Azaleas, Rhododendron and Cranberries growing best in the pH 4-5 range. Plants such as Soybeans, Tomatoes, Cowpeas, Cucumbers, etc. prefer to be grown in solutions with an optimum pH of 6.0 and other plants such as Lettuce, Cabbage, Carrots, etc. prefer a slightly higher pH of 5.5-8 (Brady, 1990). As a general rule, a pH of 5.7-6.0 is best for most food crops.

Go to The Best Company: Hydroponics http://www.ncf.carleton.ca/~de440/ for more information.

PS: According to differential solubilities, most plant nutrient ions are most readily available in solution from ~pH 5.5-8 depending. They can precipitate out of solution or become bound outside of this range causing the TDS to drop. Fertilizers are also usually acidic and will lower the pH when added along with raising the TDS. When plants selectively deplete ions from the solution lowering the TDS, the pH will usually rise accordingly.

References:

Bailar, et. al.. Chemistry. Orlando, Florida. Harcourt Brace Jovanovich, Inc.: 1989.
Brady, Nyle C.. The Nature and Properties of Soils. New York, New York. Macmillan Publishing Company: 1990. Taiz & Zeiger. Plant Physiology. Redwood City, California. The Benjamin/Cummings Publishing Company: 1991.

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