Why is my hobby hydroponic system giving problems?

From a nursery propagator. Why is my hobby hydroponic system giving problems (edited)?
I have a small specialised propagation nursery propagating difficult to strike shrubs. Once I have struck the cuttings I transplant them into large tubes containing an open potting mix, which includes a small amount of wide range slow release fertiliser. I normally lightly water several times per day and occasionally give a feed using a complete soluble fertiliser.
I have had an interest in hydroponics for some time so I decided to set up a hobby system. What I bought was a small NFT (nutrient film technique) system with a table holding five 100mm wide x 50mm high channels 6 metres long. I grow mainly lettuce and herbs, but some other vegetables as well.It has a tank underneath with a water make-up float valve. On most days I check the EC and pH and make the EC back up to about 1.5mS/cm using my nursery fertiliser. I find that the pH is always low so I add ‘pH up’ to bring it back to about 6.3.

My problem is that the plants don’t look healthy and are weak. Do you have any suggestions as to my problem and its solution?

I can be reasonably sure of your problem and its solution. This is a very useful question for intending hydroponic growers because it involves two fundamental aspects of hydroponic management.

‘Complete’ fertiliser
Your first and major problem is that you are not using a hydroponic fertiliser. Your ‘complete’ soluble fertiliser is a single part fertiliser intended for soil fertigation. This has two critical differences to a hydroponic fertiliser.

Firstly, these ‘complete’ fertilisers are actually a supplement containing all the nutrients, which are thought likely to be deficient in a soil. They usually don’t include any significant amounts of nutrients, which would normally be present and readily available, such as calcium (Ca) and magnesium (Mg). Secondly, they contain high levels of urea and ammonium and I will come back to this later.

Using this as a feed for your very young plants is OK, helped by the slow release fertiliser, because they have very low nutrient demand. In comparison, the plants in your hydroponic system are ‘gross feeders’ with a very high demand for nutrient. Consequently, in any hydroponic system it is essential to use a hydroponic fertiliser, which will contain all the nutrients that the plant needs to grow properly, and in reasonable proportion.

Therefore, the most obvious probable cause of your problem is that the plants are severely deficient, especially in calcium and magnesium. Calcium plays a major role in the strength of the plant structure, so an extreme shortage of calcium in the recirculating solution will result in weak plants. If you continue unchanged, things will only get much worse.

Hydroponic fertilisers
Hydroponic fertilisers for use as concentrates come in two parts. This is to prevent precipitate falling out of solution if a single concentrated solution is dissolved. What would happen is that the calcium ions (positively charged when dissolved: Ca++) react with sulphate ions (negatively charged when dissolved: SO4— ) to form insoluble calcium sulphate. Similarly, calcium and phosphate ions precipitate as insoluble calcium phosphate. Both these compounds are actually slightly soluble—enough that they remain dissolved at normal hydroponic solution working strength. It is only in the concentrated form that this precipitation becomes a problem.

I have a rough analogy for this: It is easy to dissolve one teaspoon of sugar in a cup of coffee. However, it is impossible to dissolve 100 teaspoons of sugar in the same cup and most remains in the undissolved solid form.
Therefore, to avoid precipitation, two concentrates are needed. These solutions are commonly called Part A and Part B. Part A consists of the calcium nitrate and usually, the iron chelate. Part B consists of all the other fertilisers. By this means the calcium ions are kept separated from the sulphate and phosphate ions.

In your case, if you buy liquid hydroponic fertiliser, it will come as Parts A and B and you add equal amounts of both.

Commercial growers who dissolve their own fertiliser often make the following adjustment. If Part A contains only calcium nitrate and iron chelate, it is much less concentrated than Part B. Often, ammonium nitrate and some of the potassium nitrate are moved from Part B to Part A in order to bring their concentrations closer together. This enables the same volume of similar strength solutions to be used in each of the A and B tanks.

Falling pH
With ‘complete’ soluble fertilisers, the nitrogen (N) is usually supplied as large quantities of ammonium and urea and very little or none as nitrate. This is not good when used in hydroponics because nitrate is the main form of N required. Urea should not be used at all in hydroponics and ammonium kept to a maximum of 20% of the total nitrogen.

Note that when used in the soil, ammonium is locked to the soil particles and not available to plants until it is converted by bacteria to nitrate (‘nitrification’). In hydroponic systems, any ammonium present is freely available and taken up rapidly by the plants. The ongoing large pH drop in your system is the typical result of having excessively high levels of ammonium in the nutrient feed. This was the major means of identifying the source of your problem.

This property of ammonium, when used in moderation, can be used as a major means of controlling pH drift. A significant fundamental difference between nitrate and ammonium ions is that ammonium is a positively charged ion (NH4+), and nitrate is a negatively charged ion (NO3-). When nitrate is taken up by the plant, to avoid building up a charge within the plant, it will exude negatively charged ions into the solution, which raises the pH.

Similarly, ammonium ion is taken up very easily and will consequently lead to the plant exuding positively charged hydrogen ions (H­+ the ‘acid’ ion), which lower the pH. If used in moderation, increased ammonium in your fertiliser can offset upward pH drift, rather than add excessive acid.

Reader Inquiries – PH&G April 2013 /Issue 130