May/June – 1994
Story Title: Nutrient Management – Part 3 – Recirculating Systems
Author: Rick Donnan
“Recirculating systems (closed systems) have many advantages. However, one of their major problems is the difficulty of nutrient management, an aspect which is rarely mentioned in books and articles. Consequently many growers do not recognise the need for good nutrient management techniques, and can get into severe difficulties. This is a major reason why currently no more than 10% of the world’s commercial hydroponic crop production uses these systems. This article details the basis of nutrition in recirculating systems and gives practical advice on its management.”
In this article I am only discussing aspects related to the nutrient management of recirculating systems. There are many other aspects of overall management which are also important if a system is to operate well. These include the temperature and aeration of the nutrient solution, and the prevention and control of disease. General knowledge of the plant, how it grows, its pests and diseases, etc., is also vital if you are to grow a good crop, especially on a commercial scale. These aspects are not included in this article, but must always be kept in mind.
If comparing recirculating with non-recirculating systems, there are the following general differences: Watering and EC (electrical conductivity) management are usually easier in recirculating systems. Nutrient management is usually easier in non-recirculating systems. pH management is usually relatively easy, especially with automatic control, but can give difficulties in either system.
Types of Systems
There are two types of closed, or recirculating, systems: those with continuous flow, and those with intermittent flow.
The most common of the continuous flow systems is NFT (Nutrient Film Technique). Here nutrient solution is pumped from a tank into the top of channels. It flows down the channel in a thin film and returns to the tank to be recirculated. Details of this technique are given in most hydroponic books. The best known of these is The ABC of NFT by Dr Alan Cooper.
There was also a detailed article in the NFT – the Channelling Challenge by Rob Smith in the March/April 1994 issue of Practical Hydroponics &Greenhouses. Another technique is water culture, where the plants are grown in a tank filled with nutrient solution. This is now rarely used. It performs similarly to NFT in terms of nutrient management, but not in many other aspects of management.
The most common of the intermittent flow systems is the “flood and drain” technique. Nutrient solution is periodically pumped from a holding tank into a system containing a growing medium. The surplus solution then drains back to the tank. There are many different versions of the technique, but they behave similarly in terms of nutrient management.
Types of Management Techniques
The schematic diagrams in figures 1, 3, 5 and 7 give an indication of the four main techniques for managing watering, EC and pH in recirculating systems. Paired with these in figures 2, 4, 6, and 8 are typical patterns of the change in EC with time in an NFT system operated these ways. Adjustments when water and/or nutrient is added are indicated.
These patterns are typical of summer conditions when nutrient demand is high and the water demand for transpiration is even higher. I will consider the normal case where the aim EC is typical, say EC about 2.0mS/cm (cF 20). Under these conditions, the plant is taking up relatively more water than is in the solution around the roots. If nothing is added to a system, this will result in the EC rising. The background to this was discussed in detail in Part 2 of this series in the Jan/Feb 1994 issue of Practical Hydroponics & Greenhouses.
The patterns for flood and drain systems will vary slightly from those shown because these systems operate on cycles. The holding tank and the growing medium will have different but related patterns. Once draining is complete, conditions in the tank will remain unchanged until the next cycle. However, in the medium are changing similarly to the NFT system. Therefore the typical flood and drain pattern will change in small steps rather than a continuous change as in NFT. Other than this the overall patterns are similar.
For both types, how far and how fast the EC will change will significantly depend upon:
– climatic conditions and their influence upon plant uptake;
– the total volume of working solution per plant; and
– the initial EC.
In the case of flood and drain there will be the additional factors:
– the frequency of the cycles;
– the relative volumes of the solutions in the tank and the growing medium; and
– the degree of mixing and/or displacement of the solution in the medium by the tank solution during the flood cycle.
pH is influenced in quite different ways to EC and has no general pattern. In the short term, high pH is lowered by adding acid, and low pH raised by adding alkali.
This technique is illustrated in Figure 1. Water addition, pH and EC control are all automatic. For convenience, the diagram shows only acid injection for pH control. Alkali may also be used. I comment more on pH control later.
Obviously automatic pH and EC control of continuous flow systems is excellent, provided the controller is functioning properly. A typical EC pattern is shown in Figure 2. Regular manual checking of EC and pH is a sensible safeguard. EC controllers are usually very reliable.
Automatic tank control for flood and drain systems is also excellent, provided control is sensibly tied in with the flood and drain cycle. Refer also to technique 2.
This technique is shown in Figure 3. The water make up of the holding tank is automatic, usually by float valve, i.e. the tank level is held steady. Here both water and nutrient are being taken up, but only water is being replaced. Therefore the EC will fall until the tank solution is brought up to strength by nutrient addition. The EC is periodically checked and adjusted to the required value by adding nutrient to the tank by hand. The pH is adjusted if necessary by adding acid or alkali by hand. A typical EC pattern is shown in Figure 4.
Care must be taken when setting up this technique for flood and drain systems. If too much make up water is added to the holding tank during the flooding cycle, the tank will overflow when the drainage returns. There are a number of ways to avoid this, the simplest being to set the make up float low enough that sufficient height is left for the returning solution.
Technique 3 is totally manual control. There are two main versions of this.
Technique 3A is illustrated in Figure 5. The holding tank is partly, or totally, run down then refilled as a batch by adding water and/or nutrient.
The important aspect of this technique is that the effects of the addition are checked. First the original EC can be checked to allow an estimation of the amounts to be added. Once the tank has been refilled it is allowed left to continue recirculating through the system until its readings have stabilised. The EC and pH are then adjusted to the required value if necessary. A typical EC pattern is shown in Figure 6.
The vital part of this technique is to allow time for mixing. This is because it is not only the solution in the tank being used, but also the solution in the system. Mixing is even more important for flood and drain systems. The first flooding cycle should be extended until the bulk of the high EC solution in the medium has been displaced back into the tank, and mixed. Further adjustments can then be made. Making no attempt to mix in the medium’s solution before the final EC adjustment can lead to severe problems.
Why mixing and checking are important will become obvious when we consider the second version of this technique.
Technique 3B is illustrated in Figure 7. The holding tank is partly, or totally, run down then refilled using a standard strength nutrient solution. However, the resultant EC in the system is not checked or adjusted. This technique really can lead to disaster.
Growers sometimes fool themselves that because they have mixed a tank full of fresh solution, then the whole system is fresh and everything is OK. However, even if the new solution is at a reasonable EC, the old stronger solution is still in the medium. When these are mixed the resultant EC will be higher. If this practice is continued the EC will continue to rise until eventually disaster strikes. A typical EC pattern is shown in Figure 8.
This situation usually occurs where growers do not have an EC meter, or do not use it. When EC rises in a system, then eventually the plant will show symptoms of stress. If this is detected in time, the grower can flush the system heavily with water. This gives quite a shock to the plants, but it is much better than being too late. If not detected in time, the plants will die. I have met cases where the EC has risen more than 5 times higher than the grower intended.
The main cause of this rise is the high water uptake by the plant. However, there is another common cause to aggravate the problem. Often the amount of nutrient added when refilling the tank has been calculated using full tank capacity. However, the tank may be refilled before it is empty. Therefore less water is added than was calculated. Put the other way around, more nutrient has been added to the make up water than was calculated. Hence EC’s get even higher. A variations of this is that the calculated volume of the tank is wrong. Another is that the manufacturer’s specification volume has been used, which refers to the total capacity not the working capacity.
For those patterns where the EC rises, there are a number of factors that can make the effect much more severe. These are:
– higher temperatures;
– longer cycle times between floodings in flood and drain systems;
– longer times between adjustments;
– poor mixing within the system;
– small holding tank volume;
– small solution volume per plant.
The higher the EC aim, or the starting point, then the higher and more rapid the rise will be. Where the aim is very low, say well below 1.0 mS/cm (10cF), the EC pattern will be reversed. That is, the EC will fall because of the large amount of water present. Particularly with technique 2 this can lead to very low EC’s and plant nutrient starvation.
Volume of Solution per Plant and EC Change
The smaller the volume per plant, then the faster and more severe the change in EC will be. This applies to all types of systems. This is illustrated in Figure 9. In this case, it is not only EC for which the changes are more severe. It also applies to other vital aspects such as pH, temperature, nutrient balance, and the build up of problem ions such as sodium and chloride.
Cycle Frequency & EC Change
In flood and drain systems the time between pumping cycles will influence the degree of EC rise. When the time between cycles gets longer, so the EC rises higher. Therefore the rise can be reduced by keeping the cycles shorter, i.e. by pumping more frequently. This is indicated in Figure 10.
Improving Manual EC Control
Studying the EC patterns described earlier can indicate ways in which techniques can be modified to improve the control of EC in manual systems.
The most important is to check regularly to find out what is really happening within your system. Is it behaving the way you thought, or in fact are you actually getting much higher EC’s. Without a meter, good EC control is virtually impossible. A good EC meter is essential for even the smallest of commercial growers.
If you don’t have a EC meter and can’t check your solutions, then probably you will need to discard regularly. Here are some points to consider that should help improve your EC control:
– Check and adjust the EC frequently, a high EC before adjustment indicates it should have been done sooner;
– Consider whether your tank is too small for the number of plants it supplies, perhaps you would do better with a bigger tank or split off part of the system to another tank.
– Collect your solution samples when the system is well mixed, i.e. for flood and drain – after a pumping cycle rather than before it;
– Pump for long enough to get reasonable mixing but no longer;
– Keep your flood and drain cycles short rather than long.
Installing a float valve in your holding tank can avoid the risks of high EC, as you now have automatic water makeup and the EC will fall until you adjust it (Figure 4). It is still important to check the EC regularly. Set the float at a sensible level for makeup sothat the tank isn’t oversupplied with water during the pumping cycle.
The hobby grower who cannot afford an EC meter need not despair. There is quite a wide range of EC for most crops that will have only slight effects on yield and quality, significant only to the good commercial grower. For more detail refer to Part 1 of this series in the Nov/Dec 1993 issue of Practical Hydroponics & Greenhouses. Therefore, many of the steps above will be useful to the hobby grower.
In particular, having water makeup via a float valve substantially reduces risks. An old cistern valve with a new washer would be cheap, easy, and effective. Calculate your nutrient addition to give the maximum EC you are prepared to have. If possible, borrow a meter to check your actual figure.
If automatic water make up is out, then make sure that your initial solutions are no stronger than EC 1.5 mS/cm (cF15). To achieve this add no more than 1.0 gram dry fertiliser per litre of water, or less if your water is already high in salts. Physically check the volumes of your tanks, and use only sufficient fertiliser to match the actual water that you add. If in doubt, especially in summer, occasionally add water only, without fertiliser.
The pH of a solution can influence the availability of the individual ions within that solution. Put simply, as pH changes on particular nutrient ion may gradually become more insoluble, leaving less of that ion available to act as a nutrient. pH is of little influence over a range, but if it goes too far, especially too high, then severe problems result. The direct influence of pH on plants is important in the soil, but seems relatively unimportant in hydroponics. Quoting Salisbury and Ross Plant physiology…
Little is known about why some plants are native to low pH soils and others to soils with higher pH values. Certainly one of the reasons is competition. If we use hydroponic techniques to study the growth of various species apparently preferring different pH levels, we usually find that they do reasonably well over a fairly wide pH range. But in nature, even a slight advantage of one species over another can eventually lead to elimination of the less well adapted one.
There are many different suggestions as to the range of pH which won’t cause problems with availability and some of these are far too narrow. David Huett’s recommendation of 5.2 to 7.5pH is realistic, especially for hobby growers, provided a chelated form of iron is used. Commercial growers would use a narrower range, at a level to suit their particular crop.
Control is done using acid to lower pH, which is the most common. If pH needs to be increased then alkali is used. The acids commonly used are phosphoric, nitric, or sulphuric, and the alkalis are potassium hydroxice and potassium bicarbonate.
The more expensive controllers have separate injectors for both acid and alkali. pH meters and controllers are usually reliable, but the weak link is the pH electrode. They must therefore be checked regularly, preferably daily, and standardised.
Obviously, if a commercial grower can afford pH control, this is a good way to go. If not, then a good pH meter is essential, and should be standardised regularly as mentioned before. For most hobby growers, pH strips would be adequate.
One aspect of pH control needs to be watched whether using manual or automatic control. This is, that the acid or alkali contributes nutrient ions to the solution and must be accounted for. Growers often fail to do this. One case to take special care is where the solution requires large quantities of acid added to keep its pH controlled. If this is done using phosphoric acid the phosphate levels can rise very high.
This is covered in detail in Part 2 of this series in Jan/Feb 1994 edition of Practical Hydroponics & Greenhouses magazine.
As water quality deteriorates it has a increasingly severe effect on nutrient management in recirculating systems. The worst problems nutritionally come from sodium, chloride and boron. Depending on the crop, water quality can be bad enough that recirculation is impossible with untreated water. The most feasible treatment in this case is reverse osmosis.
Nutrient Balance in Recirculating Systems
For more detailed information on nutrition please refer to Part 2 of this series.
The early part of Figure 11 shows a typical buildup with time of non-essential ions in a closed system. This is looking at a recirculating system where the EC is controlled to an aim level. If you start with a certain base level of non-essential ions, such as sodium and chloride, that aren’t taken up by the plant, then they will build up. At the same time nutrients are being absorbed, so their relative proportion is falling. This won’t continue forever of course. However, the further it goes, then not only is the recirculating solution building up towards toxic levels of ions like sodium, but the amount of nutrients is also shrinking. This combination of nutrient starvation and toxicity compounds nutrition problems.
At the same time similar sorts of things are happening with the various nutrient ions within that system. They are getting out of balance.
A classic example of what can happen with poor management is shown in Table 1.
Table 1. Analysis of nutrient solution from a commercial NFT lettuce system showing severe imbalance. Sydney, June 1990 (Finlayson)
Composition mg/1 #
Nutrient Commercial Formula Feed Solution Solution after 2 weeks
– Boron 0.42 0.31 1.35
– Calcium 104 130 276
– Copper 0.04 0.07 0 57
– Iron 3.2 1.1 2.8
– Potassium 255 211 3
– Magnesium25 32 97
– Manganese0.43 0.48 0.08
– Sodium 4 23
– Phosphorus29 31 31
– Sulphur 33 44 194
– Zinc 0.2 0.26 0.75
– Nitrate N 155 146 109
#all adjusted to the same strength as the other feed.
In only two weeks there is virtually no potassium left. Manganese has also almost disappeared, and other elements are rising steeply, especially sulphur. While the sodium level is not yet high, it has risen very sharply from an extremely low initial value.
What is done when nutrients are getting out of balance, or when ions like sodium and chloride are building up? The most common technique is to “dump” some solution from the system, as shown in Figure 11. This is usually done once a critical point has seemed to have been reached. This critical point is often based on a time interval, usually obtained from books, consultants, other growers, etc. Typical suggestions may be “never”, “8 weeks”, “4 weeks”, “1 week”, etc. Which is correct?
I strongly suggest that every system is different. Firstly, there is the question of what the critical point should be. Secondly, the rate of change of the solution balance within an individual system depends upon numerous factors. These include the season, the climate, the quality of the water supply, the geometry of the system, the suitability of the nutrient feed, the crop and its stage of growth, etc.
Consider a system which has a small tank for a lot of plants, i.e. a low volume per plant in the system. In this case nutrient imbalance will happen much faster than in a system which has a large volume per plant. Obviously the grower needs to allow for this, and all the other factors I mentioned. This is a virtually impossible task.
Therefore I suggest that the only way for a commercial grower to really know at which stage to dump is by analysis. When they study the analysis, perhaps with some assistance, they can accurately assess the situation. “Yes, that balance is still quite reasonable, I could keep going a while longer”, or “Oh dear, that is really too far out of balance, I should have dumped weeks ago”. You can really only tell by analysing. Also as the rate of uptake and change is much slower in winter than in summer, you would expect to dump less frequently in winter than in summer.
Once you have some information on the way your system operates you should be able to prepare a timetable for analysis. This will probably reduce the number required. Analysis will also enable you to adjust your feed formulation if it is unsuitable.
If you have a system that is completely empty, then you start with fresh solution containing the minimum proportion of sodium chloride. When you get to dumping from this system while it is operating, you won’t get back to the original level of sodium chloride, even if you have emptied your tank completely. This is because the used solution is not only in the tank, but also in the system, even if you let it drain fully . There is a surprising amount of liquid held within systems. When mixed with the fresh solution this gives some initial imbalance and buildup.
The other way to go I call “bleeding”, which is done in many large systems. Instead of dumping regularly, you bleed from the system a proportion of the amount that is used. This is either done continuously, or at least daily. This is shown in Figure 5. In this case, from a fresh start the non essential ions build up to a level, but then balance out. This avoids the peaks and troughs associated with dumping and allows steadier control. Although rarely used deliberately in small commercial units, bleeding is often used unintentionally. This is done by having leaks in the system. Many growers unwittingly solve their potential nutrition problems in this way.
A later part of this series will deal with the responsible management of nutrient solutions which are discharged from hydroponic systems.
The next part in this series will deal with the detailed management of open, or non-recirculating, types of systems.
About the Author
Rick Donnan is a chemical engineer, who has worked full time in commercial hydroponics since 1981. He manages Hydroponic Consulting Services and Growool Horticultural Systems in Sydney. Rick was foundation president of the Australian Hydroponic Association and is president of the International Society for Soilless Culture (ISOSC).