What acid should I use for pH control?

From a New South Wales lettuce grower.

I once grew lettuce in the soil, but have recently bought an operating hydroponic lettuce farm. The system is a recirculating NFT (nutrient film technique) system with channels on tables. I have been given the previous owner’s fertiliser formula, which had been quite successful, and he also used phosphoric acid for pH control. I recently visited a tomato grower and he is using nitric acid, which he said had been recommended by you. Which acid should I use?Answer

When an acid is used for pH control, the acid in the solution has split into two charged ions. One is the positively charged hydrogen ion, which is the ‘acid’ ion (the H in pH stands for hydrogen ion). This is paired with a negatively charged ion, nitrate in the case of nitric acid, and phosphate in the case of phosphoric acid. Both nitrate and phosphate ions are an important part of the fertiliser solution that you use.

Nutrient balance
However, there is a significant difference in the normal level of nitrate and phosphate ions in a well-balanced nutrient solution. Typically, there would be at least three times the quantity of nitrate ions compared to phosphate ions. Therefore, it has less of an impact on your nutrient balance to add nitrate rather than phosphate ions.

Consequently, in terms of nutrition management, the acid I would normally recommend to use for pH control is nitric acid. There are some safety concerns which I will cover later.

Whichever acid you use, it needs to be included in the calculation of the nutrient analysis of your fertiliser.

How important is pH?
In many books and articles there is a strong emphasis on the importance of pH. Sometimes the view is expressed that there is a specific figure (often 6.3), which is the essential and critical level. This is often supposedly backed up by a diagram showing the optimum availability of a range of individual nutrients. Most of these diagrams are soil-based and nowhere near as accurate as indicated. In practice, anywhere between pH 5.5 to 6.5 is OK, and often between 5.0 and 7.0 is also acceptable, dependant particularly upon the type of iron chelate that you use.

The downside of concentrating only upon pH is that achieving tight pH control can come at the expense of sending the nutrient balance of your hydroponic solution out of kilter, especially in recirculating systems such as yours. This is virtually never mentioned in books promoting tight pH control, but it is very important.

One of the worst examples I have seen was a strawberry grower who had leaves turning yellow. He was using phosphoric acid for pH control, and when I suggested that he measure how much was being used, it was a significant proportion of his total feed. I also had him send a sample of his recirculating nutrient solution for analysis and a leaf sample for tissue analysis. The aim for the solution was 40ppm P, but the analysis showed over 200 ppm P and this rise had come in only two weeks. The P levels in the plant tissue were extremely high.

Safe use of acids
All strong mineral acids are extremely dangerous. Those most commonly met for hydroponic use are phosphoric and nitric. Hydrochloric acid (muriatic, or ‘pool acid) is readily available, but should never be used in hydroponics because it adds unwanted chloride ions. Sulphuric acid (battery acid) is also common, but rarely used in hydroponics, although its use is possible if you know what you are doing, and never try ‘used’ battery acid which contains significant amounts of lead.

Strong mineral acid will cause extreme damage to your skin and especially your eyes. On a gruesome note, concentrated sulphuric acid was the product of choice for prohibition era gangsters who wanted to disfigure their ‘molls’. It is critically important when handling acids, to use full protective gear. That is, full length chemical resistant apron and gloves, and full face shield.

If acid gets onto the skin or eyes, immediately flood with copious amounts of water for at least 15 minutes. Eyelids are to be held open. Speed is essential, so have equipment nearby such as a drench shower or a hose with a shower rose attached. First aid is critical, if you wait for an ambulance or try to get them to a doctor or hospital, the damage will already be permanent. Damage to the eyes may cause permanent blindness.

If you need to dilute a concentrated acid, never add water to the acid, because immense heat is generated, the water boils instantly and spits hot acid—very nasty. Always slowly add acid to water while stirring and don’t let the mixture get too hot.

Nitric acid
Nitric acid has an added safety issue, in that the concentrated acid gives off unpleasant and dangerous fumes. This is a major reason why phosphoric acid is used. I recommend that you use a lower strength solution, 30% is a commonly sold strength. This will be slightly more expensive per unit of acid, but avoids the problem of fumes and is nowhere near as dangerous.

September 2013 / Issue #135