Organic acid for pH adjustment?

I am growing organic strawberries on table-tops in peat moss.  My system is therefore very similar to hydroponics. I have found that my water pH is quite a bit too high, and I wonder if you could recommend the best options for organic pH adjustment?  My operation is small-scale commercial, so cost is a definite consideration.

The term hydroponics is sometimes limited to refer only to water based systems. However, in this magazine, as in most of the world, it is taken to be identical with the more general and descriptive term ‘soilless culture’. Provided that you use a nutrient solution and not slow release fertilisers, your system actually is hydroponic.

You don’t mention whether your produce is certified organic, but here are some general comments upon organic certification. This is quite a confused area, with significant differences in requirements between certifying organisations, countries and states.

The first and most important difference is whether their definition of organic includes growing in the soil. For those bodies requiring the use of soil, no hydroponic produce can ever be acceptable for certification. Often this differentiation is more a matter of philosophy rather than science.

For those certifying organisations who don’t specify that produce must be grown in the soil, their requirement is usually that all inputs must come from natural sources. This can also introduce different interpretations as to what is ‘natural’.

Acetic acid
The most common acid used in organic hydroponics is acetic acid, scientifically known as ethanoic acid (formula CH3COOH). This is best known as the active ingredient in vinegar, which typically contains about 5% acetic acid with the remainder being mainly water. Vinegar results from the oxidation of the alcohol (ethanol), such as found in intoxicating drinks. For example, wine exposed to the atmosphere for only a few days, it will take on a sharp-sour taste as the alcohol is turned into the acid by acetic acid bacteria, which are common in the air. Commercially, culinary vinegar is produced via fermentation from a number of products, which give their name to the vinegar, such as malt, cider, balsamic, etc.

Chemically, acetic acid is termed a weak acid because only a small proportion of the acid ‘dissociates’; that is, splits into charged ions. In this case, they are positive hydrogen ions (H+ the ‘acid ion’) and negative acetate ions.

For industrial use, acetic acid was originally derived from vinegars, but is now produced by reacting methanol and carbon monoxide over a catalyst. Some six million tons are produced annually as the raw material for the manufacture of film, coatings, paints, inks and as a solvent. Industrial acetic acid can be at high concentrations, where it is corrosive and dangerous.

If you can access industrial acetic acid, that would probably be your most efficient and cost-effective product. Of course, if you are a certified grower, this would need to be acceptable to your certifying body. If using vinegar, the specialised varieties would be far too expensive. I have checked out the cheaper retail bottles. These cost only about AUD $0.50 per litre, however, their labels typically don’t list the percentage acetic acid, so it would be a matter of trial and error to check how much is needed to the job and hence its cost.

Citric acid
A probably more expensive choice is citric acid, which is also a weak acid. It occurs naturally in citrus fruits, especially lemons, whose juice contains about 5% citric acid. It is also produced industrially as a white crystalline solid, which can be dissolved to give a 15% solution at 20 degrees C. The production process involves fermentation of inexpensive sugar solutions such as molasses, and then recovering the citric acid produced via precipitation and acidification.

pH management
Just a reminder that the pH of most importance in your system is that of your root zone solution. The best sample of this is the run-off from your medium. A pH of 5.5 to 6.5 would be good, and extension down to 5.0 or up to 7.0 is usually OK for moderate time periods, provided that it then trends back to a safer reading. Strawberries prefer a pH of 5.7-6.3, as they can suffer from iron deficiency at higher pH.

EC measurement
When a mineral fertiliser, such as potassium nitrate, is dissolved in water it splits into two changed entities called ions. One of these has a positive charge, called a cation, in this case potassium ion, symbol K+. The other has a negative charge, called an anion, in this case nitrate ion, symbol NO3-. Similarly, other mineral fertilisers in solution also split into anions and cations.

The electrical strength of ionic fertiliser solutions can be detected by an electrical conductivity (EC) meter. The higher the ionic concentration the higher the level of EC, hence EC can be used as an indicator of ionic solution strength, however, it has its limitations. The first is that the EC gives absolutely no indication of the nutrient balance of that solution. The second is that it does not measure any non-ionic components in the solution. This means that when using organic fertilisers, the solution strength will be higher than indicated by its EC, because most carbon-based compounds are not ionic and won’t register on an EC meter.

Water uptake by plants is influenced by root zone solution strength—the stronger the solution, the lower the water uptake (refer to my column of July 2014). This is the strength of actual dissolved solids, not only the ionic components. Consequently, the influence of an organic fertiliser will be greater than indicated by its EC.

Thanks for advice from Dr Mike Nichols, a retired Lecturer from Massey University, NZ, an expert horticultural scientist with a broad range of hydroponic interests including organics.  Ω

Rick Donnan

PH& September 2014 / Issue 147