What are hydroponic fertilisers? (Nutrient management Part 3)
What is special about hydroponic fertilisers and how are they used?
A hydroponic fertiliser must supply all the nutrient elements required by the plant at that stage of its growth. Most hobby growers will use pre-prepared fertilisers, but most commercial growers will make up their own mix using individual fertilisers. Hydroponic fertilisers are different from soil fertigation fertilisers.
For convenience, or when using automatic controllers, most commercial growers use concentrated nutrient solutions, which are typically 100 times stronger than normal feed strength. These are then diluted 100 times just before feeding into the dripper pipes. At the high concentration, it is essential to use a two-part fertiliser, typically called parts ‘A’ and ‘B’. The same applies to retail hydroponic fertilisers, which are liquid concentrates.
The reason for splitting the fertilisers into separate parts is to prevent precipitate falling out of solution, as will happen if a single concentrated solution is used. This is because the calcium ions (positively charged when dissolved) in the concentrate react with sulphate ions (negatively charged when dissolved) 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.
As a rough analogy: 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, 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.
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.
Beware of ‘complete’ fertilisers
I recently met yet again the severe problems a grower experienced by mistakenly using a ‘complete’ fertiliser. By the term ‘complete’ the suppliers really mean 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 are essential in hydroponics, but would normally be present and available in the soil, especially calcium (Ca) and magnesium (Mg). There are also other problems with these fertilisers if used in hydroponics due to them containing high levels of ammonium and /or urea.
In the early years of commercial hydroponics, growers had major problems keeping enough iron (symbol Fe, which comes from the Latin ‘ferrum’) in solution. This was because the form they were using was ferrous (iron) sulphate, which would quickly oxidise to rust and drop out of solution. This problem was solved with the development of chelates, which provide a protective shell around the iron ion.
The common form used is Fe-EDTA, which contains about 13% Fe; however, even this degrades gradually, especially at pHs higher than 6.0. Increasing numbers of growers now use more complex chelates with better stability, such as Fe-DTPA and/or Fe-EDDHA. Fe-DTPA contains about 7% Fe and is safe to pH 7.0. Fe-EDDHA contains about 5% Fe and is stable to any pH possible to find in practice. With all chelates, the % Fe varies between manufacturers. Commercial growers often use a combination of chelates, such as combining EDTA for cost with some higher chelate as insurance.
Many of the other micro-nutrients (trace elements) are also available in chelate form. While these are increasing in use, most growers have no difficulty with using the standard non-chelated forms (which are mostly sulphates).
The plant roots not only take up ions from the root zone solution, but they also exude ions (plus a wide range of organic compounds). The plant keeps itself electrically neutral. Therefore, if it is taking up more positively charged ions (such as ammonium NH4+) than negatively charged ions, it will exude positively charged hydrogen ions H+. H+ is the ‘acid’ ion, and hence the pH of the root zone solution will fall.
The opposite effect takes place when there is a higher uptake of negatively charged ions (such as nitrate NO3-), and hence, the pH of the root zone solution will rise. This is common for many crops, especially during their early vegetative growth stage.
Because ammonium ions are taken up by the plant very much quicker than any of the other nutrients, increasing the proportion of ammonium in the feed will result in a relative lowering of the pH. Reducing the proportion of ammonium in the feed will result in a relative raising of the pH. This applies to all systems whether recirculating or not. For example, the pH of your root-zone solution may be over 7, even though you have been pulling your feed solution pH down to well below 6. Note that in this situation the high acid input, especially if using phosphoric acid, can cause your root zone solution to get well out of balance as mentioned earlier. Increasing the ammonium in the feed should allow you to feed at about pH 6 and to maintain that level with time without adding excessive acid.
Whether using acid/alkali or ammonium for pH control, do not overdo trying for tight control. Often the safest approach is to allow a reasonable degree of pH drift rather than use excessive correction. In hydroponics, tight pH control is rarely needed.
Beware that high levels of ammonium can be toxic to plants, in particular causing root death. Also, too high a level of added nitrogen can lead to soft growth. Up to 10% of the total nitrogen in the ammonium form is safe for most plants, dependant upon their stage of growth. It is very risky to go above 20%. Your calculated figure should include the ammonium that comes in the calcium nitrate (typically 1.1% N as ammonium).
Part 4. Next month I’ll look at the influence of water quality. Ω
1 March 2014