January/February – 1994
Story Title: Nurient Management in Hydroponics Systems – Part 2
Author: Rick Donnan
Part 2 – General Principles
There have been many articles written on hydroponic nutrient formulation and it is a topic discussed at length in most hydroponic books. However, almost nothing has been written about managing nutrition in hydroponic systems, which is a much more important topic. In practice there is no such thing as a perfect nutrient solution. Even if a nutrient feed solution app apparently close to ideal, if not managed properly it will eventually have nutritional pro:) MS. The first part in the series introduced the principles involved in nutrient management. Part 2 describes the detailed management of “closed” systems, and “open” systems.
The quality of the raw water you use will have a major impact on the type of crop you can grow, the system you grow it in, and how you manage it. In terms of nutrient solutions, by quality I mean the salts (ions) dissolved in the water. This can only be found by having the water chemically analysed. If you use town water the water authority will normally be able to provide this for you. If you use bore, dam or stream water, etc, you need to send a sample for analysis.
Problem Ions in Water Supplies
The most common ions in water supplies which influence plant nutrition are:
Sodium (Na+) and Chloride (Cl-) – the constituents of common salt. These normally occur together and are not taken up to any degree by most plants, especially sodium. Therefore, they tend to accumulate if present in significant amounts. This is shown in Figure 3. Chloride is actually a trace element (micronutrient), but is usually available in far greater concentration than required for nutrition. This is why it is virtually never included in formulations.
Iron (Fe … ). Although iron is a micronutrient, in this form it rapidly oxidises and precipitates as rust, which makes it unavailable as a nutrient. In practice this can give problems, especially by blocking drippers, so the precipitation is best speeded up by aerating the water, followed by settling or filtration.
Calcium (Ca++) and Magnesium (Mg++). These are constituents in hard water. As major nutrients (macronutrients) they are usable in the nutrient solution. Their presence should be allowed for in calculating formulations.
Bicarbonate (HCO3). Also a constituent in hard water. It is not a nutrient but is alkaline and will raise the pH. It will need to be neutralised by acid, typically phosphoric or nitric. The amount of equivalent phosphorus or nitrogen added should be allowed for in calculating formulations.
Boron (B). Boron is the micronutrient with the narrowest range. If present in the water it can be omitted from your formulation. It can become a problem if its concentration is over 1ppm, or lower for sensitive crops. This only occurs in a few water supplies.
High Salt Levels in Water Supplies
The level of salts which can be tolerated in the water depends on their composition and which crop is to be grown. Even 50ppm of sodium can be toxic to plants such as lettuce, straw berry and rose. In contrast tomato could cope with over 200ppm. Because of the accumulation that will develop, water supplies with increasing dissolved solids will make management more critical. Beyond a certain limit, dependant upon the crop, recirculating systems will become unmanageable. Non-recirculating systems can continue to be used but with increased percentage run-off. Eventually,, water becomes so bad that it is unusable in any system. In this case the only possible solution is to remove the bulk of the salts from the water, typically with reverse osmosis.
If managed correctly reverse osmosis is capable of reducing the dissolved salts to very low levels. The major and most expensive component of a reversed osmosis (RO) machine is the membrane. The efficiency and life of the membrane is very dependant upon the water quality being pumped through it. Often, chemical pre-treatment, such as removing iron, is necessary for the equipment to work effectively, and to give reasonable membrane life. The discharge system from an RO machine is more concentrate than the input and this may be unacceptable in some circumstances.
An alternative is to collect rainwater. This may be used direct if sufficient is available or mixed with the poorer water supply to make it more acceptable. Unfortunately, in many areas of Australia, rainfall is so irregular that when you most need fresh water is when there’s none available.
There are other contaminants in raw water that need attention, but I will mention them only briefly.
Suspended matter should be settled out if possible and removed. Good final filtration with a sand filter for example, can avoid many of the dripper blockages experienced by some growers. the water should be free of diseases (pathogens). Town water is usually safe but other sources need to be sterilised, most commonly by chlorinating. Water also needs to be free of plant poisons (phytotoxins).
There are a very wide range of commercial fertilisers available” and an even wider range of formulations published. So, how do you tell which ones to use? I’m obviously riot going to recommend any individual commercial brands. What I’ll do is give some principles for guidance.
Mix Your Own, or Buy Premix?
Firstly, do you mix your own, or do you buy a prepared fertiliser? For the beginner, especially, it is best to buy a premix. For hobbyists, if part of the appeal of hydroponics is mixing your own nutrient, then go ahead. The hobbyist without this interest is probably advised to stick with the premix. Setting up to mix your own requires buying a moderately accurate capacity scale plus a very accurate small scale for the micronutrient fertilisers.
For the commercial grower there are similar choices. The grower who manages the system well, and is getting solutions analysed regularly, should obviously mix their own. In fact, failing to do this removes the opportunity to make the adjustments indicated by the analysis.
However, there are growers who never have any analysis done and have no interest in accurately weighing fertilisers. They are probably better off buying a premix with which they have experience and are confident about. Mixing individual fertilisers gives a saving in direct nutrient costs, however, there are extra labour costs involved. More importantly, if even one serious mistake is made it can prove more costly than any number of years savings on fertiliser. Unfortunately, I have seen several cases of total loss of crops through mistakes in mixing.
Types of fertilisers
For the inexperienced grower there are several major traps to avoid. Firstly, it is essential to buy a “hydroponic” fertiliser. Normally these would be the only types stocked by a hydroponic retailer or wholesaler.
Unfortunately, salesmen will sometimes promote a “complete” fertiliser as hydroponic. These are usually soil fertilisers containing trace elements.
Unfortunately they are quite unsuitable for hydroponics. Typically they contain little or no calcium (Ca++) or magnesium (Mg ++); they have a very high proportion, often 50%, of nitrogen in the ammonium (NH4+) form rather than predominantly the nitrate (N03) form as with hydroponic fertilisers; also they have a different balance of trace elements.
There are other fertilisers designed for liquid irrigation of soil crops by trickle irrigation. This is also known as fertigation. For our purpose these fertilisers are similar to the “cornplete” types, and should also be avoided.
How do you recognise these fertilisers? Check the following: the calcium (Ca) content should be from about the same to 30% lower than the nitrogen (N) content. The magnesium (Mg) content should be about 20% to 30% of the calcium content. No more than 10% of the nitrogen should be in the ammonium (NH4) form, and none should be urea.
Occasionally growers unknowingly, or even deliberately, use these fertilisers. The first indication of potential problems is the pH of the run-off or recirculating solution. Because of the high ammonium content this drops quickly and severely, often to under pH4. With time the plant shows obvious general symptoms of stress poor growth, and lack of vigour.
Preparing feed solutions
There are basically two choices of ways to mix and use the feed nutrient solution. Firstly, the solution can be prepared at working strength, say at an EC of about 2mS/cm (milliSiemens/centimetre). This is usually done in a suitable sized tank which is filled with water and the fertiliser added. The solution is pumped and bypassed back to the tank for some time in order to thoroughly mix the tank contents.
When mixing is completed the EC and pH of the solution should be checked and adjusted if necessary. If you don’t have a meter, then one trap to avoid is to take care to add only enough fertiliser for the extra volume o22 f water you have added to the tank. Say you add sufficient fertiliser to make a full tank of the strength you require, but the tank was not fully empty. The actual solution strength will then be higher than you planned. Not only that, but it gets progressively stronger each time.
This system is also used by small commercial growers. Particularly if they expand, the choice eventually becomes one between the cost of a new mixing system, versus the inconvenience of frequently mixing tanks.
Concentrated food solutions
With the second method, usually installed for convenience, concentrated fertilisers are used.
These are typically fed by injectors or injection pumps into the water or recirculating nutrient solution. A feedback controller is often also installed to keep pH and EC controlled.
A complication arises when using concentrated solutions, which are typically 100 to 200 times stronger than the normal nutrient solutions. At this strength calcium and most of the phosphate combine to precipitate insoluble calcium phosphate. To a lesser extent, calcium sulphate is also precipitated, as is iron phosphate. These reactions are not a problem at normal feed strengths, because these compounds are sufficiently soluble at these much weaker strengths. As a rough analogy, it is easy to dissolve one teaspoonful of sugar in a cup to tea, dissolving 100 teaspoonful is impossible.
To avoid this complication the concentrated solution so split into two parts, usually referred to as “Part A” and “Part B”. This split is to separate the calcium and iron from the phosphate and sulphate. Therefore Part A usually contains the calcium nitrate and iron chelate. Part B contains all the other fertilisers. These are then injected separately into the water flow to give a working strength solution which is fully soluble.
It would obviously be more convenient and halve the cost of injectors, etc, if a single concentrated mix was possible. There have been numerous attempts to do just this, but I don’t yet know of any that have been successful. In simple terms, there have been two basic approaches to this.
One is to use a fertigation solution, as described in the previous section. this leads to the same results as noted before. The other is to use a more typical hydroponic formulation and try to keep the precipitate form forming . I don’t yet know of anyone who has done this successfully.
This has implications for buying nutrients in liquid form. For buying concentrated nutrient solution I would recommend a two part product. If you want a single part mix, check the label as suggested earlier. If it seems to be a typical hydroponic formulation it will probably have precipitated. It is essential to shake this product well, immediately before pouring it into your working tank. This then gives you some chance of getting a balanced nutrient solution. Failure to do this will give a phosphate deficient solution, which will result in pretty stunted purple tomato seedlings, for example.
Nutrition in open and closed systems
Table 1. Comparison of nutrient composition of root zone and the equivalent recirculated and non-recirculated feed solutions.
(a) Corrected to the same strength for each solution for the comparison purpose.
(b)Totals for the columns only – they do not indicate total TDS ppm.
(c)Based on “Nutrient solutions for vegetables and flowers grown in water or substrates” 1992. Sonneveld & Straven, Glasshouse Crops Research Station Naaidwijk.
There are two fundamental types of soilless systems, mainly open and closed. The nutrient feed formulations differ considerably and I discuss this here. Other aspects of man aging these systems will be covered in later parts of this series.
In closed systems the nutrient solution is recirculated. They may be operated either continuously as with the Nutrient film Technique (NFT), or intermittently as with flood and drain systems. In closed systems the nutrient solution is recirculated. They may be operated either continuously as with the Nutrient Film Technique (NFT), or intermittently as with flood and drain systems. In closed systems the nutrient solution is not recirculated. Also known as run to waste systems, they use dripper irrigation of soilless media in containers.
Nutrition in closed systems
A simplified background to the feed nutrition differences between these systems is as follows:
A healthy plant will take up the nutrient balance it requires provided the solution around the roots contains nothing at a deficient or toxic level. In a totally closed system the only nutrient usage is what the plant takes up. if the nutrient solution in the root zone is to remain in balance, then what is fed in must have exactly the same balance as the plant uptake. If not, the solution within the system gets out of balance and continues to get further out of balance. If this happens then solution must be discarded from the system.
The concept many find difficult to grasp is that the solution in the system may have quiet a different balance to the feed solution. Please refer to the Dutch guideline figures in Table 1.
The “Root zone solution” is the same for both recirculating and non-recirculating feeds.
Why is it different to the feed solutions? The answer is related to the rate of uptake. For growth rates there needs to be maximum nutrient uptake. However, upta e is more difficult and slow for some nutrients than for others. Increasing the strength of root zone solution for those nutrients will increase the driving force and boost their rate of uptake.
Fundamental Management Techniques for all Systems
Sample regularly, preferable daily. Sample the feed and, especially, the solution around the plant roots (or the run-off in open systems).
Analyse all samples for pH and, especially, EC.
Occasionally get a full chemical analysis done. If need ed this could be tied to a leaf analysis.
Record the results in a diary. Include other information such as weather conditions, crop performance and symptoms, pests, diseases.
Manage by watching for trends and adjusting gently to correct them. Try to avoid taking severe action.
Referring to Figure 1, compare the recirculating feed solution, which is the actual plant uptake, with the root zone solution. Calcium, magnesium and sulphur are far higher, up to double the strength of the feed. These are nutrients which have a slow uptake. In comparison, nitrogen is about the same strength but potassium is lower indicating a faster uptake. Finally, phosphate and especially. ammonium are much lower to compensate for their very fast uptake. in fact ammonium uptake is so fast that its concentration may well drop to almost zero.
The intended balance in the root zone may be built up gradually or it may be done by having a special starter solution. Starter solutions will obviously be higher than normal in calcium, magnesium and sulphur.
Nutrition in open systems
With the non-recirculated feed there is not only plant uptake, but run-off as well. The feed is therefore part way between supplying the uptake and providing the required root zone solution strength. By the time it runs of it has built up to the required balance in the medium.
If and adequate run-off is maintained then any imbalance goes out with it. For this reason the feed composition is nowhere near as vital as with the recirculating system. Therefore, nutrient management is far easier in a non- recirculating system. This is a major reason why until the pre sent over 90% of the world’s hydroponics systems are open type systems.
The next two parts in this series will deal with the detailed management of each of the open and closed type systems.