Well, we thought enough was enough. So we’ve put together this quick, no-nonsense and impartial guide to understanding how to measure the strength of your nutrient solution so we can all be clear about what we’re talking about – once and for all!

Worldwide, there is one standard parameter for measuring pH, but there are many more for measuring the strength of a nutrient solution. The two major measurements in use today are:

• EC  – Electrical Conductivity
• TDS – Total Dissolved Solids

EC

First, some basic concepts: when we add nutrients to water we create a nutrient solution. The more nutrients we add, the more concentrated the solution, and the more readily it will conduct electricity. So, the electrical conductivity (EC) of your nutrient solution can be seen as a quick and easy measure of how much nutrient is dissolved in it overall. Put another way, measuring the conductivity of a solution means measuring the electrically charged ions. Pure water will not conduct anything, but tap water already contains other minerals, metals and salts so it does conduct a small amount. Remember, it’s always important to measure your source water to see what you’re dealing with.
To measure conductivity we can use an EC meter, also known as a conductivity meter. It has two electrodes that, when dipped in the solution, measure its electrical charge by passing a small charge between them.

What is EC measured in?

Siemens are to “electrical conductivity” what feet or meters are to “length” – it’s the unit of electrical conductance. It’s important to get this distinction really clear in your head right now. EC is the scale (also known as the ‘parameter’) and siemens are the units. When dealing with the very low amounts of conductivity associated with EC in nutrient solutions, the preferred units are mS (millisiemens; one thousandth of a siemen) and µS (microsiemens, one millionth of a siemen) per centimeter.
EC is the most widely accepted measurement for the strength of nutrient solutions, and is the standard in Europe and many other parts of the world. The one notable exception is North America which prefers to use TDS.

TDS

TDS (Total Dissolved Solids) is the preferred scale for measuring the strength of a nutrient solution here in North America. It quantifies the concentration of dissolved solids contained in a solution. TDS is arguably a better parameter for measuring nutrient concentration, since it measures by quantity or weight.  In other words, you can have two glasses of water with equal parts TDS but different EC levels, since one glass may have more or less conductive elements (say salt vs. calcium.)
The problem is that a true TDS measurement is difficult to achieve (and would also defeat the purpose since evaporation is required).  Therefore, if one wants to eliminate the estimating that the conversion factor does, an EC meter is better.  If we lived in a perfect world, and every nutrient company and TDS meter used the same non-linear scale, a TDS meter is preferable.  But since there are so many different variables, an EC meter lends itself to more consistency.

What is TDS measured in?

Once again – make sure you get your head around this – TDS is a scale, or a parameter, just like time, length, temperature and volume. The unit of TDS is ppm (parts per million.) A TDS reading of 50 ppm means there are 50 milligrams of dissolved solids in each liter of water, or 50 mg/l.

How do TDS Meters work?

If EC meters (conductivity meters) work by measuring conductivity in a nutrient solution and expressing this in siemens, how to TDS meters work out how many parts of nutrient there are per million of water? Sorry to break it to you, but the answer is, they don’t.
TDS meters work in actually the same was as EC meters! Both measure the electrical conductivity of the nutrient solution they are dipped in. The difference is in how the information is displayed.
A TDS meter will measure the electrical conductivity, and then use a conversion factor to display the strength of the nutrient solution in ppms. Now here’s the slightly tricky bit. The conversion factor from EC to TDS varies from meter to meter.

Conversion Factors

TDS NaCl

NaCl is a conversion factor based on Sodium Chloride (regular table salt.)
The conversion factor range is 0.47 to 0.5.
Non-linear meters based on NaCl typically use: 0.5 x the EC level (if converting from µS to ppm or mS to ppt) or 500 x the EC level, if converting from mS to ppm.
TDS 442™

442™ or Natural Water™ is a proprietary scale based on properties of naturally occurring fresh water.  The 442™ part is an abbreviation of 40% sodium sulfate, 40% sodium bicarbonate, and 20% sodium chloride.
The conversion factor range is 0.65 to 0.85.
Non-linear meters based on 442™ typically use: 0.7 x the EC level (if converting from µS to ppm or mS to ppt) or 700 x the EC level, if converting from mS to ppm.

TDS KCl

KCl is a conversion factor based on Potassium Chloride.
The conversion factor range is 0.5 to 0.57.
Non-linear meters based on KCl typically use: 0.55 x the EC level if converting from µS to ppm or mS to ppt) or 700 x the EC level, if converting from mS to ppm.

TDS 640

A less popular conversion factor.
The conversion factor range is 0.64 to 0.67.
Non-linear meters based on 640 typically use: 0.64 x the EC level if converting from µS to ppm or mS to ppt) or 640 x the EC level, if converting from mS to ppm.

Yes, four different possible conversion factors means that four different meters that give measurements in ppm may all give different readings from the same solution! However, all EC meters should give the same reading in the same solution as there’s no conversion factor necessary.
I know, I know … TDS sounds like a confusing thing – but it’s really just a measure of the total ions in solution. For every gallon of water you have X mg’s of stuff in it. If one of your friends starts talking about their nutrient solution in terms of TDS, be sure to find out what scale they are using. Many growers, especially in Europe, in an effort to avoid confusion, use EC. If you are still confused, contact the manufacturer of your nutrients and find out what they recommend. Remember to ask them what TDS scale they use if they give you dosages in terms of ppm.
Likewise, if you are working with a TDS meter that only has a ppm display, remember you need to be sure of the conversion factor being used. TDS comes into its own when you need to measure individual elements in applications such as nutrient and water quality, tissue analysis results and soil analysis. Results from these laboratory tests will give individual elemental readings in ppm or mg/l. Remember, a TDS meter will only give you an approximation of the overall nutrient concentration, based on the conversation factor used.
Below is a table to show the relationship between the various methods of displaying the strength of a nutrient solution.

Jargon Buster

• EC = Electrical Conductivity
• TDS = Total Dissolved Solids
• PPM = Parts Per Million
PPT = Parts Per Thousand
• µS (or µS/cm) = micro-Siemens (one millionth of a siemen.)
• mS (or mS/cm) = milli-Siemens (one thousandth of a siemen.)
• NaCl = Sodium Chloride (EC-to-TDS conversion – EC x 0.5)
• KCl = Potassium Chloride (EC-to-TDS conversion EC x 0.55)
• 442 = 442 Natural Water™ (EC-to-TDS EC x 0.7)  (The “442” is an abbreviation for 40% sodium sulfate, 40% sodium bicarbonate and 20% sodium chloride.)

Making Sense of your Meter

Here are some popular TDS meters along with their conversion factors, where applicable.

Towards A Clearer World

There is a drive towards some standardization in the hydroponics industry to create less head work for all concerned. Nutrient manfacturers, if you specify dosage with in ppms, please also state what TDS scale you are using. This includes calibration fluid!

by Everest Fernandez

Rockwool is a mainstay of commercial hydroponic growers – and for good reason. It takes up a minimal footprint and, when used correctly, yields like crazy. We asked Dr Lynette Morgan, a world authority on hydroponic vegetable production, to give us some expert advice on growing tomatoes in rockwool. There’s LOTS to be learned here as Dr Morgan takes us through how to develop irrigation strategies for your particular growing environment.

Rockwool, also known as stone wool or mineral wool, is the most widely used substrate for the commercial production of hydroponic tomatoes. It is also a great tool for smaller growers who can benefit just as much from its use in a range of different systems and situations. While rockwool is relatively easy to set up and use, it does require some monitoring and irrigation adjustment to make the best of its ability to hold high levels of moisture and aeration at the same time.

Rockwool originally started as a thermal insulation material in the construction industry: its lightweight but highly aerated nature helps keep heat in buildings, while being easy to handle, cut and install. However, towards the end of the 1960s, trials were carried out in Denmark to test the possibility of using stone wool as a substrate for plants. Things went well and since then rockwool as a growing media has seen some continuing development of the substrate and the tools used to manage it.

Rockwool comes in a range of sizes from propagation cubes to large slabs and even a granulated product.

Rockwool is manufactured by melting basaltic rock and spinning this molten mix into thin fibers which are then cooled by a stream of air. Although rockwool is a man-made substrate it is essentially made from rock and considered by many to be a natural product. Grodan dominates the rockwool market world-wide and is the most common brand used by large and small hydroponic growers alike. Grodan rockwool is highly advanced and is not a single product – growers can select from a number of different Grodan rockwool types such as `Grotop Master,’ `Grotop Master dry,’ and `Grotop Expert,’ all of which have slightly different properties and uses. `Grotop Master Dry,’ for example, maintains a slightly drier root zone and is used by tomato growers to steer crops away from overly vegetative growth. `Grodan Classic’ is used for multi-year use, while `Grotop Expert’ is designed for ultra quick root growth and development. Along with these product differences, rockwool of many brands comes in a huge range of sizes from tiny propagation plugs for seeds to larger cubes for cuttings, mega sized cubes for large plants, a wide range of slab sizes, and as a granulated product as well.

Setting up to grow with rockwool

1. Sit the rockwool down

Whether you are using the standard rockwool growing slabs, large cubes, or even pots of granulated rockwool, basic preparation is important. Slabs and cubes in particular need to be on a flat, even surface as any indentations will cause the material to sink and create pockets of unwanted moisture. Next, realizing that nutrient solution will be draining from holes cut in the slab’s plastic wrapper or from the base of cubes, some consideration for drainage of this solution away from the slab is important. There is no point in having well placed and made drainage holes if the solution can’t be channeled away from the slab and the material ends up sitting in a pool of stagnant waste nutrient. Many small hydroponic systems on the market these days designed for use with rockwool have trays and channels designed to do just this and these are a good choice for inexperienced growers.

2. Settle the rockwool in

Rockwool, whether it is slabs, small propagation blocks, or large growing cubes, needs to be prepared correctly by fully wetting the substrate before use. Some growers like to adjust the pH of their water to 5.5 before wetting up rockwool, but generally for small systems it’s not necessary with good quality brands (unless you have a very `hard’ water supply in which case acidification of the water before making up any nutrients would be a good idea). The rockwool should be fully saturated so that all of the material is wetted and then left to drain. Some growers pour water into the rockwool slab before the drainage holes have been cut to make sure everything has had a good drenching, while others just pour water on or run the irrigation long enough for saturation to take place.

3. Remember the holes

Rockwool slabs need drainage – holes or slits should be cut in the plastic sleeve the material comes in. Several cuts are required along the base of the slab. Granulated rockwool should be placed into containers or pots with plenty of drainage holes in the base.

4. Irrigation programs

The most common way of applying nutrient to rockwool slabs or large blocks is with the use of dippers. A simple drip irrigation system should use a dripper with a capacity of 2 litres/hour, with one dripper per plant. Because a standard rockwool slab may hold four tomato plants, four drippers per slab are required, which also means that if any one dripper becomes clogged, the entire slab will still be getting enough irrigation until the problem is fixed.

Developing an Irrigation Strategy for Rockwool – The Moisture Gradient

Rockwool is the most widely used substrate for hydroponic tomato production.

The irrigation program for any hydroponic plant is vital for successful growth, development and optimal yields. The most common problem experienced by smaller or new growers is over watering, and usually the grower is totally unaware that it is their irrigation program causing problems with plant growth. Flushing vast amounts of nutrient solution through the root zone in a substrate-based system often equates to plant murder – more is not necessarily better when it comes to nutrient application. This type of mistake is easy to make. After all, many new growers get enthused about hydroponics after seeing a well-run NFT or other solution culture system and assume that plants are more than happy to grow and thrive in a flooded root zone environment. However, solution culture and substrate systems are completely different and need to be managed in different ways for the plants to get the optimal root zone conditions they need. In NFT the roots should never be flooded: they sit in a very thin film of nutrient flow (2-3 mm or about 0.1″ deep), hence the roots have moisture at the base of the root system, but many of the other roots are sitting up in the moist air, accessing all the oxygen they need without being submerged. In a rockwool slab the plants are in a similar situation – at the base of the slab there is plentiful moisture, usually at media saturation levels, while in the upper layers of the rockwool slab the roots are in drier conditions and hence have access to plenty of aeration and oxygen for root uptake and respiration. It is this moisture gradient from the top to the bottom of the rockwool material that makes it such a good substrate. At the same time, growers who are not aware of this property can make the mistake of thinking the rockwool is too dry on the surface and over-irrigate their plants despite having plenty of nutrient solution being held deep down in the root system. Rockwool growing media, when being irrigated correctly, should not sit in a pool of nutrient and be completely saturated from top to bottom like a sponge. It is essential that the rockwool is allowed to completely drain so that excess nutrient leaves the slab or cube under the pull of gravity after being applied– in doing so, fresh air is drawn into the top layers of the material, providing fresh oxygenation for the root zone. By allowing the rockwool material to drain freely, over-watering becomes more difficult, although vast amounts of nutrient drainage from the base of rockwool slabs or cubes is not an ideal situation either.

Setting up an Irrigation Program

Obviously the amount of nutrient required is going to depend on factors such as the size of the plant, the growing conditions, light, temperature and, in particular, humidity, which drives plant transpiration and water uptake. So the irrigation program is going to change as the plants develop. Also an irrigation program needs to be developed and adjusted by each grower for their particular system, environment, and set up and this has to be monitored and adjusted as required. Just following guidelines for the amount of nutrient to apply at certain times will eventually lead to over or under-watering, as each plant and situation is different when it comes to nutrient and water requirements.

Commercial hydroponic rockwool growers have some good tools for fine-tuning their irrigation. The Grodan water content meter allows growers to measure the water content, EC and temperature in the rockwool slab root zone using hand-held meters or a continuous monitoring system hooked up to the computerized irrigation program. However, these sorts of high-tech tools are not often used by smaller growers and a successful irrigation strategy can be put together with just observation, some innovation, and a little time.

Remember the Moisture Gradient

Rockwool propagation cubes and slabs are designed to be used together to minimize root disturbance. Excellent moisture holding capacity and good aeration of the root zone are features of rockwool substrates.

Irrigation of rockwool is a little different to other solid substrates because of the way the material is manufactured to have just the right degree of moisture gradient, and because it does give quite a limited root zone for plants that eventually grow fairly large. For this reason, rockwool is best irrigated with short, frequent applications of nutrient, with just enough at each irrigation for the rockwool to reach ‘field capacity’. Field capacity is a term that means the substrate has drained fully but is still holding a good level of moisture for the plant roots to access until the next irrigation. At each irrigation, there should be some drainage from the rockwool material. However, this doesn’t need to be excessive. Even in closed systems where the drainage solution is being collected and reused, it pays not to over-water and not to run the irrigation continuously. Having around 10-15% of the nutrient solution fed to the plants, drain from the slab at each irrigation is considered to be optimal. This amount of drainage of solution flushes fresh nutrient solution right through the slab without too much wastage and usually keeps the EC in the slab fairly stable.

When rockwool is irrigated and allowed to drain naturally, it will then contain 80% nutrient solution, 15% air pore space and 5% rockwool fibers. A typical rockwool tomato growing slab actually holds around four gallons (about 15 liters) of nutrient solution immediately after irrigation, despite the drainage holes allowing free drainage of excess solution. Four gallons is a good reserve of moisture for four plants, so drying down to wilting point could take a long period of time for small plants.

How much solution should be given at each irrigation?

Having a drainage collection tray or channel under each slab allows growers to see how much drainage they are getting after each irrigation (even if this has to be poured off and measured in a jug) and the irrigation program can be increased or decreased to keep this at the 10-15% level. By doing this, the amount of solution to be given at each irrigation can be worked through and adjusted as the plants grow. Keep cutting back the irrigation amount until only 10-15% of the solution volume applied drains from the slab, and then the amount of irrigation has been fully adjusted for.

How often should nutrient be applied?

Rockwool needs small frequent irrigations, particularly under hot or low humidity conditions when the plants are taking up a lot of water. However, the frequency of irrigation can be as low as once per day (or every other day) for small plants under cool conditions, to over 10 times a day for large plants in a hot or dry environment. It can be hard to judge just how much moisture the rockwool material may be holding at any one time to determine when to irrigate. Smaller propagation blocks and even larger cubes can be gently picked up – the weight will soon tell you if the cube is saturated (it will be comparatively heavy and moisture will drip from the wet base), or whether it has dried out considerably, in which case it will feel very light (compare an unused dry cube to one in use). Rockwool is an unusual material in that, even when the slab has lost 50% of its moisture to plant uptake, the plants are still able to very easily keep extracting water until the slab is almost completely dry – so plants in rockwool can’t get water stressed until the rockwool is almost completely dry, by which time the cube or slab has become much lighter in weight. For granulated rockwool in pots or containers, a similar method can be used, either by gently lifting the pot to see what the weight might be (a light pot is a dry pot) or by a light tap or kick: if the pot moves, the rockwool has become quite light and potentially too dry.

Another method to try and gauge the moisture status of the rockwool and how often to irrigate is to carefully remove a small piece of the wrapper plastic and examine the moisture gradient of the slab from top to bottom. Like all growing media, moisture in rockwool can be gauged manually. Lightly touching or pressing the rockwool at the base of the slab will soon determine if there is still a good level of nutrient held in the base of the slab or whether it has become too dry. The top and middle layers of the slab should always appear drier than the base where the reservoir of moisture is naturally held, so only the base of the slab should be checked. Even if the top of the slab appears to be dry, this is not important as the moisture gradient has been designed to give these sorts of root zone conditions – only ensure the base of the slab has sufficient moisture.

This process of working out how much moisture is still in the rockwool material is not something that needs to be done for long. Growers will soon become quite skilled at working out their frequency and amount of irrigation for each stage of plant growth and may only need to do this for their first crop provided growing conditions remain stable. Other times when it might be important to have a quick check of the amount of solution drainage or amount of moisture in the slab is when conditions suddenly change – addition of more grow lamps, sudden changes in temperature or humidity, or rapid growth spurts can all change the irrigation requirements of the plants.

Generally, good brands of rockwool are quite forgiving compared to other substrates – the material is naturally well aerated and doesn’t suffer the compaction issues that some substrates do during the life of the crop. It does hold high levels of moisture, so the chance of drying out is not as severe as it might be with other substrates and being sterile gives young plants, seedlings and cuttings an advantage as well. The irrigation program and water holding capacity of the substrate depends on the fiber density and arrangement, which can differ from brand to brand.

More Advanced Irrigation Practices

With tomatoes and similar crops, growers have the option of using the EC and moisture content of the rockwool slab to help ’steer’ the plants into either more vegetative or ‘generative/reproductive’ growth, depending on what is required. Drying the slab back between irrigations and allowing the EC in the root zone to increase pushes tomato plants into a more generative or reproductive state with less leaf growth and more assimilate being directed into the fruit. A higher level of moisture maintained in the rockwool and a lower EC pushes the plants towards more lush vegetative growth. Skillful growers use these techniques to direct their crop and control leaf, flower and fruit growth at different times, and rockwool is a great substrate for this sort of control via the root zone.

Other Rockwool Tips

EC Levels and Management

Checking the EC in the root zone is important with rockwool just as it is with any media. The EC of the nutrient solution in the growing substrate changes as plants extract different ratios of water and nutrients from the root zone. The EC in the drainage solution coming from the base of the rockwool cubes or slabs is the best indication of the EC the plants are actually experiencing in the root zone. As a general rule, the EC in the drainage solution should be the same as or only slightly higher than that applied to the plants in the feed solution. If the EC is becoming much higher in the drainage than what was fed to the plants, then the EC in the feed solution should be dropped back – this is common under hot growing conditions when the plants might be taking up far more water than nutrients, hence concentrating the nutrient solution.

Rockwool Reuse

Rockwool for tomato crops can be reused – some commercial growers get many successive crops from rockwool slabs by steaming these after the plants have been removed and then replanting. Smaller growers can also do this – a few slabs can be heat treated by pouring hot water through them. Solarization is also possible, as is using chemical disinfectants, although care should be taken to rinse the rockwool well with plenty of water after using these. Commercial Grodan users have the option of the Grodan recycling service, which picks up the used slabs and recycles them into new product. However, smaller growers with just a few slabs of used rockwool can recycle the material by shredding it and reusing it as a growing media, as a component of potting mixes, or by incorporating it into outside soils and gardens.

Real World Rockwool Q&A

Q: What pH should I adjust the nutrient solution to and how do I monitor and adjust accordingly? For instance, keeping the tank pH at 5.8 and the run-off at 6.0 is perfect, but what happens if the pH starts to come back higher or lower than expected? What could / does this mean? And what should be done to correct it? How much should a grower raise or lower the pH of the tank with pH adjusters – when does a situation become ‘too extreme’ to use pH adjusters?

A: There are many factors that affect pH in the nutrient: some are normal like plant uptake and nutrient formulation salts (NH4 in particular), and some are not so good, like root disease. Water plays a big role and can range from very hard to very soft and hence needs to be handled differently depending on what a specific grower is dealing with. Chemicals for pH adjustment are also a huge topic! The nutrient solution pH is usually optimal at around 5.8 – 6.0 for commercial tomatoes; however, for small systems pH in the range of 5.5 – 6.8 is usually fine and having tight control at 5.8 is not necessary. The main problem with pH is with growers who might have a `hard’ water source, which is highly alkaline. In that case, acidifying the water with acid (nitric or phosphoric) before making up any nutrient will give better and longer term control of pH swings (in any growing media). pH should not need to be raised in most situations unless the water supply is very acid: in that case, potassium hydroxide should be used.

Q: I understand that rockwool can be prone to salt build-up if you don’t know what you’re doing like the commercial guys. Most hobby rockwool growers I have talked to flush either one day a week, throughout the whole grow and bloom cycle, or when they dump the res. (They will commonly give their plants 24 hours of either very low nutrient solution (if so, what EC?) or pure water, or even pure water with a product like GH Flora Kleen. What do you think of these flushing techniques? Do you have any better advice?

A: Rockwool is actually one of the better media for preventing salt build up as it tends to be drip irrigated from above and not bottom watered like with ebb and flow. Flushing is another subject that really needs a whole article to cover the theory, practice and problems with it. Flushing with straight water after a plant has been sitting at normal or high EC is not recommended: it causes the plant cells to suddenly take up huge volumes of water (because the osmotic pressure has been dropped in the root zone). This can cause cells to burst and create major physiological problems – splitting of tomato fruit is one common one; many other fruits and vegetables do the same. Even low strength nutrient can do this. Any changes in EC in the root zone should be done slowly (i.e over days), so a gradual dropping back of the EC over a few days should be done rather than flushing with water. Or better still, don’t let EC build up in the first place!

Q: What is the disadvantage of watering rockwool for a minute and getting 50% run-off in a closed system with adequate drainage, as opposed to watering for a minute and getting, say, 15% runoff? If you are only achieving 15% run off, is it not the case that the rockwool is already fully saturated and any additional runoff will just wash out the excess salts more thoroughly? In short, how difficult is it to over-water rockwool? I also can’t see what the problem would be for the plant if more run-off was created unless, of course, you were irrigating for several minutes to achieve this much run off, but even then surely the plant won’t feel any effect having its roots flooded for, say, 10 minutes, then allowed to drain freely?

A: Rockwool is a media which has been specifically designed for commercial growers who aim to have the recommended 10-15% run-off with the slabs spending as little time as possible at saturation levels – when doing this, the structure of the rockwool has been manufactured so that the root zone will remain at the correct moisture status which is why it is recommended. Also, with rockwool systems, the feed nutrient should be applied so that ‘excess salts’ don’t occur and therefore don’t need continual flushing. If the EC is getting high in the drainage solution, drop it back in the feed solution and/or increase the frequency of short irrigations. Rockwool, like any media, can be over-watered if flooded and is best kept below the saturation level for balanced growth.

Q. What’s the scientific explanation behind the influence that irrigation strategies have (or, to be more precise, the levels of moisture in the root zone) on generative / vegetative growth? Is this peculiar to tomatoes or is it applicable to other species?

A. Crop ’steering’ as it’s called is a technique used by commercial growers to manipulate the natural growth pattern of the plant. It’s widely used by skilled growers of tomato crops, but also on capsicum and many other plants as well. It’s quite a complex topic as there are a number of tools a grower can use in a controlled environment to direct the growth of the crop – commercial growers will use a combination of DIFs (day/night temperature differentials), EC, CO2, moisture control in the root zone and directional heating (i.e. directing heat towards the fruit or tops of the plants) to manipulate the growth of the plant. Different techniques force the plant to send the assimilate produced in the leaves into flowers/fruits when required or direct the plant back to some more vegetative growth if that was what was required. Various temperature techniques are sometimes used to keep seedlings or older plants as short and compact as possible (i.e. prevent stem elongation) and to get the plant to hold back on the production of overly large, succulent leaves. Commercial tomato growers use tools such as measurement of stem diameter to determine if their plants are getting overly vegetative or too generative at certain times of the year. The basic scientific explanation of why this works is that when a flowing plant encounters ’stressful’ conditions such a drying back of the root zone, high EC, high light and temperatures, it triggers a response – the plant wants to hurry up and flower, and to set seed to make sure it reproduces before the harsh conditions can kill it. We sometimes see this effect on lettuces which, under high light, temperature and moisture stress, can flower (or bolt) while the plant is still only a seedling and far from maturity. A plant with plenty of moisture under no particular stress is happy to go on producing a lot of large leaves with no hurry to set fruit and seed, which is great for vegetative crops such as lettuce but not so much with fruiting crops like tomatoes and capsicums. The ‘controlled stress’ commercial growers use to direct plants into more generative growth is often via the root zone because with Grodan rockwool very precise control of moisture content in the substrate can be controlled – particularly with the use of the Grodan moisture meter. And in hydroponics, control over EC is also fairly easy and precise. For this reason, Grodan Rockwool has different products for growers who might need to steer their crops towards more generative growth by having a drier root zone. It makes it much easier for the grower to then restrict irrigation and moisture levels in the root zone to steer the plants towards more generative growth and generally the technique is very effective. However, commercial growers use high tech tools likes moisture meters linked to their computerized irrigation program so that the crop is not at risk of being damaged by delaying irrigation to long. Smaller growers can certainly use similar techniques and allow the rockwool to run a little drier between irrigations and keep their nutrient run off to an absolute minimum if their plants are getting a bit too vegetative. Running a lot of nutrient through the rockwool on a frequent basis means the slabs or media are at saturation for much longer, and that favours vegetative growth (although we should also remember a lot of other factors, such as the growing environment, play in a role in the vegetative/generative balance as well).

Dr Lynette Morgan holds a B.Hort.Tech(Hons) degree and a PhD in hydroponic greenhouse production from Massey University in New Zealand. Her PhD thesis focused on hydroponic tomato production in both NFT and media systems and improvement of fruit quality aspects. Now a partner in SUNTEC International Hydroponic Consultants, Lynette is involved in many aspects of hydroponic production, including remote and on-site consultancy services for new and existing commercial greenhouse growers worldwide as well as research trials and product development for manufacturers of hydroponic products. Lynette is also the author of 5 hydroponic technical books: Hydroponic Lettuce Production, Hydroponic Capsicum Production, Fresh Culinary Herb Production, Hydroponic Strawberry Production and her latest release, Hydroponic Tomato Crop Production.

Is your back aching from lugging endless sacks of soil, coco or other growth media in and out of your indoor garden? Then check out our latest blueprint, aptly named “The Water Room.” The idea is to grow monster tomato plants directly in a nutrient solution using a cutting-edge, modular Deep Water Culture (DWC) system called The Under Current™. But the liquid theme doesn’t end there. Water is also used to cool the garden using an ingenious chiller-based system created by Hydro Innovations.

Everest catches up with Dan and Stephen, the co-designers of this blueprint, to find out what logic exists beyond all this liquid!

GROWING IN WATER

Everest: Hi Dan. Let’s start by looking at the systems themselves. Am I right in thinking each 16 pot system requires both an air pump and a water pump?

Dan: That’s right, Everest. The inline water pump powers the negative solution displacement, which drives the Sub Current Culture (SCC) method. The linear, high efficiency air pumps provide the active aeration which supercharges the nutrient uptake.

Everest: So it runs 24/7 – even during the night cycle?

Dan: In properly aerated and balanced nutrient solution, plant roots can stay submerged 24/7, even through the dark cycle. Plants continue to metabolize nutrients and exchange gases in the dark, so keeping the solution moving aids in these processes. And remember, no timers for pumps means no worries!

Everest: How much solution is in each module?

Dan: We recommend an operating volume of approximately six gallons per module. That makes 100 gallons +/- in a 16XL (6 x 17 modules). A very small volume of solution is held in each joint (conduit) between the modules as well.

Everest: Is it the same for the bloom cycle?

Dan: We advise growers to drop the operating level to about four gallons per module during the fruit and flowering cycle. This helps ensure ample atmospheric oxygen uptake by the non submerged roots within the module. This oxygen exposure aids in proper fruit set and essential oil production as the plants mature. This technique can also mimic “drought conditions,” which triggers the plant to produce more oils as a means of reducing transpiration rates.

Everest: What about nutrient top-up?

Dan: The return module (epicenter) comes equipped with a high quality float valve built in for easy auto top-off. Each system also includes a bulkhead adapter for plumbing straight to your reservoir.

Everest: What about developing this set-up further with an auto-dosing system?

Dan: This system would work perfectly with an auto-doser like the Intellidose from AM. In this case you would plumb the Under Current (UC) float valve directly to a pure water source and let the Intellidose do the rest. Of course, you’ll need to set the doser to your specs, but then it’s on like Donkey Kong. The likelihood of a zero dump out run increases exponentially when a doser is used.

Everest: What EC should the top off res be balanced to?

Dan: When operated properly, top off should be balanced the same as the solution in the system. Traditionally hydro growers have been instructed to top off with half strength or pure water to avoid nutrient toxicity, but because the UC runs best with half strength nutes there is less of a chance of salt build-up. Ideally the solution in the system should stay balanced even as the plants use the nutrient and water. As a rule of thumb, if the nutrient EC/TDS rises as the solution is depleted you are likely running your levels too high to begin with. Conversely, if your EC/TDS drops it indicates you’ve started too low. Ultimately, as solution levels drop in the system the EC/TDS should stay stable; this is a good indicator that you’re dialed in. This EC/TDS stability will translate into improved plant health and greater pH stability to boot.

Everest: What if I experience drift in my nutes?

Dan: Correct it with your top off solutions. For example: a system started at 500ppm but has crept to 625ppm as the solution level has decreased. That’s a 25% increase, which can be easily offset by a top off res balanced at 25% below the initial 500ppm. This results in a top off res balanced at 375ppm to compensate. Ideally solution strength should stay constant as the plants consume it. This is a good indicator that minerals and water are being used at equal proportions.

Everest: What solution temperatures are optimal?

Dan: The system operates well anywhere from 65-80°F. We recommend maintaining a temperature between 68-72°F. This is a happy medium between optimum dissolved oxygen capacity and not chilling the nutrient solution so much that it slows the plant’s metabolism. If necessary, the water chiller can be easily adapted to the return pump.

Everest: Besides high water temps, what else can reduce dissolved oxygen levels in the system?

Dan: Elevated levels of dissolved solids can displace dissolved oxygen as they compete for real estate in the nutrient solution. So cool, half strength nutes are a perfect environment for high dissolved oxygen levels.

Everest: What dissolved oxygen levels should growers aim for in the UC?

Dan: We’ve tested on average +/- 9ppm of D.O. in solution. Water temps and quality will influence levels. As a point of reference, Dr. Elaine Ingham recommends no less than 6ppm to brew actively aerated teas.

Everest: You claim nutrient solution can last several weeks in the UC, but what about nutrient schedules that change by week?

Dan: Given that we encourage zero nutrient change outs, this does complicate things a bit. Best technique is to dilute any primary supplement into the top off reservoir.

Everest: How do you veg for the system?

Dan: Quad Tops are now available for the UC which allow up to four juvenile plants to be grown in each bucket. You can transplant our 5.5” heavy duty net pots right into your blooming UC rig. Other systems that veg well for the system include the GH Aeroflo2, AmHydro’s N.F.T., or transplant straight out of any aero cloner. Veg times in the UC are notoriously quick so start your fruiting cycles early to avoid overgrown madness. WE MEAN IT!

Everest: What grow media works best in the net pots?

Dan: Any non-wicking inert grow media tends to work best. Expanded clay pellets, growstones, silica stones, lava rock, sure to grow … to name a few. When using a wicking media like rockwool be sure to adjust the solution level to a point where it is not in contact with the media.

Everest: How much longer will nutrient stay viable vs. traditional ebb ‘n’ flow set-ups?

Dan: Time frames vary but typical change outs in E/F are about 7-10 days. In the UC, change outs should be necessary no sooner then 21-28 days. Many variables influence this time frame, so adjust your time frame to best meet your needs. Change nutes once they destabilize or become murky.

Everest: Is it a pain to clean in between crops?

Dan: Disassembly is not necessary. A bottle brush, green pad, biogreen and some elbow grease is all you need.

COOLING WITH WATER

Everest: Right, let’s talk about cooling this room with water. Many of our readers will be unfamiliar with using water chillers. Stephen, can you explain the basics of what a water chiller actually is and how it works?

Stephen: Sure thing. Firstly, water absorbs heat. And a water chiller cools water. So the basic idea is to use water to absorb heat from your indoor garden, and then a water chiller to get rid of it – similar to a regular air conditioner but with greater efficiency. A pump drives cool water through a manifold pipe and into a heat-exchanging device called an Icebox. The Icebox can be located on the exit duct of an air-cooled hood and provides increased surface area for the cool water to absorb heat from the hot air that passes over the grow lamps. The warm water then returns to the reservoir where it is re-chilled.

Everest: Why is water chilling more efficient than air conditioning?

Stephen: It’s down to the heat exchange capacity of water compared with air. The thermal conductivity of water is 23 times greater than that of air! A chiller will exchange the heat in a given space much more quickly than an air conditioner, allowing it to run less to get the same results. This is where you save electricity. With an air conditioner, air is passed over the evaporator instead of water. Since the air is less conductive, the evaporator can’t draw out as much heat as it can with water. The chiller evaporator is significantly smaller than an air evaporator because of the increased thermal load of water. In nearly all cases, the evaporator in a chiller will be significantly more efficient than that of an air conditioner, again allowing it to run less to get the same amount of cooling.

Everest: What type of chillers should be used?

Stephen: You need an industrial chiller – not a nutrient or aquarium chiller. Nutrient chillers might be more affordable, but they were not designed for battling against a constant source of heat! Only an industrial chiller is able to cope with constant loads and most can be placed outside if desired. Generally speaking, the larger your chiller, the more efficient it is.

Everest: How do you calculate the correct size of chiller for your room?

Stephen: Good question! Obviously this is really important to get right! First you need to decide whether you are going to use the water chiller simply for offsetting the heat generated by your grow lamps (i.e. keep your room at the same as the ambient temperature) or if you want to actively lower temperatures in your indoor garden further. It’s important to note that both heating and cooling are measured in BTUs (British Thermal Units). The first thing to do is measure how many BTUs are being generated from your equipment. In general, 1000 watt bulbs produce 4000 BTUs and 1000 watt digital ballasts produce around 2500s BTU of heat. (Exact figures vary.) That’s why ballasts should always be housed OUTSIDE of the garden.

Everest: So what sized chiller would this room need?

Stephen: 8 x 1000W lamps generate 32,000 BTUs. Each horse power of the chiller gives us around 12,000 BTUs. This room would need a 3HP chiller to cool the room entirely without A/C. Otherwise, a 3 ton A/C could be used in combination with a smaller (e.g. 2HP) chiller.

Everest: How is the cooling regulated / controlled?

Stephen: Cold water circulates around the system constantly. Regulation of the cooling effect is achieved through the fans that blow over the heat exchanger coils inside the Iceboxes. The fans are plugged into a thermostat controller. As it gets warmer, the fans speed up. As it gets colder, they slow down. The thermostat has a night and day setting.

Everest: Okay, now it’s time for the nitty gritty. I want to ask you about humidity. Surely cooling hot air rapidly through an Icebox creates condensation?

Stephen: A room full of transpiring plants is going to create humidity. Every indoor gardener has to deal with this and it’s easy to overcome with a dehumidier. As for condensation, the dew point changes with room temp and humidity levels. If you cool things down, water drops out of the air. Check out dew point calculators online. Typically, if you keep your humidity at below 50% then you will have no condensation.

Everest: How does the grower know how much the water needs to be chilled? I guess what I’m asking is, does the number of lights correlate to the water temp?

Stephen: Water temp is irrelevant to number of lights. You need to compare your water temperature with your room temperature. Assuming you have the right sized chiller, if the water temp is 10°F less than the room temp then you will maintain the room at that temperature. If you chill your water more than that it will create an A/C effect. 20°F difference will create active cooling in the room. It’s all about heat exchange and surface area, Everest, not just about how cold your water is. If you have three lights daisy-chained to just one Icebox, you can get the same results from three Iceboxes but you have to get your water a whole lot cooler. When you take away heat exchangers, you take away efficiency. But also, you need to take into account the volume of the room.

Everest: So you’re saying that a good rule of thumb is: the more Iceboxes (or heat exchange surface area), the better.

Stephen: You got it. The best efficiency is achieved when your water temperature is above the dew point and as close to your room temperature as possible.

Everest: Ok guys – that’ll do I think. I like the look of this room. Thanks for sharing!

What do you think of The Water Room? Have you used a similar set-up? Did Everest miss any questions? Post a comment below!

One of the most appealing aspects of hydroponics for any grower is the ability to recirculate water and nutrients.  Hydroponics can reduce water consumption by up to 80%!  Not to mention the financial savings to be made on nutrients and additives too.

However, recirculating nutrients brings with it additional challenges that the grower must meet in order to maintain maximum production.  So we asked Michael Christian, an expert consultant to the commercial hydroponics industry, to share his insights into recirculating nutrients effectively to achieve high performance plant growth while conserving water and nutrients.

The days of run-to-waste or open irrigation in horticultural operations are numbered.  Not only is pure water an essential resource that is becoming more and more precious as demand increases, but the minerals dissolved in water are also becoming increasingly scarce as they are mined from a finite resource, processed and distributed over long distances. We are quickly approaching the point where they must be recirculated in closed systems.

As food production becomes more localized, horticultural operations in controlled environments are being constructed in and near cities where food is grown short distances from consumers. Produce that is grown for freshness, nutritive value and purity is winning the day for people who care more and more about their health, their family’s health and where and who grows their food.

It is becoming more evident by the size and number of horticultural operations springing up all over the world, that hydroponics is the technique of choice. Why? Because it is not dependent on soil fertility and is therefore not limited by geographic location. Parking lots work well for hydroponic operations, as does hard pan soil and rooms inside buildings.

There are four basics elements of successful nutrient recirculation.  By “successful” I refer to the creation of optimum conditions in the root zone while still enjoying the efficiencies of maximum reuse of water and nutrients.

First, let’s state the common goals in any horticultural operation:

• Create and sustain an environment to generate healthy, vital, fully realized crops on a CONSISTENT basis.
• Avoid CROP LOSS at all costs. Crop loss can be defined as ANY condition or situation that detracts from our first goal. (Aiming for less than 10% crop loss is standard operating procedure in commercial operations.)

In addition, any successful hydroponic growing operation using a closed system (nutrient recirculation) must adhere to these fundamental basics:

• Pure water source
• Balanced nutrient ions/anions (CF)
• Optimum pH
• Plentiful oxygen availability
• Optimum light/temp/humidity/air circulation/CO2

Just to reiterate, if ANY one of these basics is out, plant performance will inevitably suffer.  It really is as simple as that. That’s why it’s important to understand each one individually and then how they operate in unison.  In this article, I’m going to focus on the first of these fundamentals.

To dial in any system is to get a handle on the variables and control them, period. Each one of the basics is a variable that must be managed… as any grower well knows, plant life has a way of beguiling even the most experienced growers. The better the understanding we have of each basic element, the faster we will be able to determine the one that is out and correct it with minimal drop in performance and / or recovery from crop loss.

Water is a universal solvent designed to carry minerals to the ocean and feed life forms on the way. It is hungry and will pick up any element it runs across and dissolve it in itself. It is guaranteed that the water that runs from your tap has a unique cocktail of minerals which may be fine to drink…but in a hydroponic system, it could be the kiss of death. You won’t know until you find out by analysis.

WATER

Water is the heart of a hydroponic system. If you don’t know what’s in your source water and you’re adding nutrients to it in a closed system, AND if plant performance suffers, you won’t have a clue if your water is the problem.  In addition, you will most likely spend a lot of time, money and effort taking ineffective actions to correct it.  This predicament is easy to avoid.  Simply obtain a water sample and get it analyzed. Actually, a simple analysis measuring the mg/l or ppm of, N,P, K, S, Ca, Mg, Cl, Na, Mn, Fe, B, Cu, Zn, Mb, Bicarbs, pH and EC in your water is all you need. If your plants require an EC of 2.0 and your source water is at .7 EC, you have only 1.3 EC “spare room” in which to add actual plant food. The rest is, who knows?  It’s what you don’t know that usually gets you.

All successful recirculating systems have plastic or stainless steel float valves… why? As water is transpired by plants, additional water is required to top up the tank.  Plants uptake more water than nutrients so if additional top-up water is not added to replace transpired water, the nutrient solution becomes more and more concentrated. Not a great situation if you are aiming for high performance. Large, fast growing, annual plants can drink up to a gallon of water a day especially when it’s hot. If it’s REALLY hot, plants will spend all their energy transpiring and NOT feeding which really adds to nutrient imbalance without a float valve.  So use the biggest reservoir you can handle AND a reliable float valve. (Remember that flood and drain systems will require the float valve to be installed at the drain level in the reservoir.)

With pure, low EC top-up water coming in through the float valve you’ll have no worries.  But if you have source water with a high or unknown EC you can be fairly confident that non-plant food minerals will start to accumulate.  This is because they are not being taken up by the plants. And unwanted or unknown nutrients take up valuable EC… in terms of chemistry, you can bet that there is mischief going on with the precious ion balance that you are trying to achieve with your spare no expense nutrients… plants will only tolerate this situation so long before plant performance suffers. So, TEST YOUR WATER… and avoid all that drama.

If you find your source water to have 40 ppm or more of Cl (chlorides from chlorine) you can off-gas it before adding to your tank or run through an activated charcoal filter. If Calcium and / or Magnesium are high and your water is hard then you will need to use a reverse osmosis (RO) system. Just be sure to run your water through a water softener pre-filter to take out the Ca so your RO membranes last longer. Check with you local garden/hydroponic store… if they are knowledgeable, they’ll have RO units and prefilters in stock. Determine how many gallons per day your plants will be transpiring (say 100) and size one with 25% greater capacity (125) than you need.

Go for a large volume reservoir. Rule of thumb… if you are growing 100 plants and, at their optimum size, they are transpiring half a gallon of water per day, or 50 gallons total, make sure your tank is ten times that (500 gallons). Why? Larger volumes of water stabilize temperature, help nutrient stay in balance longer, and enable the grower to make more subtle adjustments (top-up water added as well as nutrient and pH adjuster) to avoid any spikes in EC or pH that upset ion balance. A good rule of thumb for reservoirs – the bigger, the better.  We have growers with 12,000 plants in their systems running off of 1500 gallon reservoirs who dump their tanks every two or three months with no loss is crop performance. The water in their 1500 gallon reservoir will have been replaced completely with top-up water more than 12 times. This is what you want to aim for. These growers have pure, low EC source water, balanced nutrients, correct pH, large reservoirs, float valves and EC/pH dosers… the ingredients for successful, long term nutrient/water recirculation.

During the life of a plant, as it goes through vegetative growth, flowering and / or fruiting load, different nutrient ions are taken up at different rates. High Nitrogen (N), low Potassium (K) for vegetative growth, and low N, high K for fruiting / flowering growth. Rather than getting anal and freaky and adding all kinds of amendments and extra salts in anticipation of their shifting needs (and perhaps killing them with kindness), go easy! Large reservoirs have enough buffer built in and enough ions to take care of these phases without the balance shifting to detrimental levels and requiring frequent dumps. Particularly if you’re using a nutrient/pH doser (highly recommended), a well balanced nutrient added incrementally to a large volume of pure water will produce phenomenally healthy and robust plants all the way through flowering.

Continue with part 2, where Michael looks at nutrient balance and pH, how they work with pure source water, and how to manage them to steer plant performance.

Greetings Urban Gardeners.

Let’s take a look at how we can get the most out of our hydroponic nutrients, otherwise known as “nutrient management.” Now, to the skilled grower, nutrient management represents an opportunity to enhance plant growth, enjoy bigger yields and achieve higher overall crop quality. However, to the novice it can represent a difficult challenge or even a complete mystery! The difference is in knowledge, understanding, environment and equipment.

Here are six questions to test your nutrient knowledge. Do you already know the answers?

Your nutrient solution should be maintained at around 68 °F (20 °C) for the best combination of oxygen content and uptake by the roots.

Q2) What is the “dissolved solids” content of the water you use to mix your nutrient and does this content vary greatly from season to season? Does your water supplier provide you with good water from one reservoir at one time of the year and bad water from a different reservoir at another?

Test your water with an EC meter before adding anything to it.

Q3) Are there any components (such as high levels of calcium or magnesium) in your water that could affect the availability of other nutrient  elements? Have you considered the presence of sodium chloride from sea-water contaminating your water supply?

Ask your water supplier to provide you with an analysis of your water. Some grow stores also offer this service.

Q4) What is the “EC” or strength of your nutrient? Do you mix special nutrient blends for different kinds of plants and for each stage of the crop’s lifecycle, or in response to different environmental conditions like high temperatures and low humidity?

If you are unable to prevent temperatures in your indoor garden from rising too high, you can decrease the stress levels for your plants by decreasing the strength of your nutrient solution.

Q5) Does the pH of your nutrient stay within a reasonable range, or does it drift up and down significantly? How quickly?

It’s quite normal for the pH of your nutrient solution to rise (say from 5.8 to 6.5) over the course of a few days – but greater changes could indicate the presence of pathogens in your nutrient solution from contamination or from sick plants that may spread disease to the rest of your crop.

Q6) Do you change your nutrient often enough to prevent imbalances from salt accumulation or deficiencies from nutrient exhaustion?

It’s really important to change your nutrient solution regularly because it helps to minimize the wastes your plants discard into the nutrient. Did you know that, as plants transpire and nutrient levels drop in your reservoir, the EC or strength of the nutrient can rise to dangerously high levels?

What’s In Your Water?

Water quality is a crucial issue for all gardeners. Success comes easier with soft water. Just add the right combinations of nutrients to the water and you’re off to a great start. However, if you have very hard water, or water contaminated with sodium, sulfide, or any number of heavy metals, your first step may be to filter your water using “reverse osmosis.”

The best way to find out what’s in your water is to obtain a seasonal water analysis through a lab. If you’re on a municipal water system, call your water district and request a copy of their most recent analysis. Keep in mind that a good analysis at one time of the year does not mean that the water quality will remain good throughout the year. During dry seasons water suppliers often switch to a different reservoir with different water quality.

Another approach – highly recommended – is to check your water regularly yourself with a dissolved solids meter, also called an electrical conductivity (EC) or parts per million (ppm) meter. These instruments are one of the most important tools for a grower to use regularly. By measuring the EC (ppm) of your source water routinely before adding nutrients, you will be able to tell if your water supply is consistent, or changing.

How Does a Conductivity Meter Work?

All dissolved solids instruments work in essentially the same way: they measure the electrical conductivity of the water. It is the dissolved salts in most water that allows it to conduct electricity. Pure water is a poor conductor since there are none of the conductive salts found in impure water. Purified water will show no, or very low, salt content (conductivity) when tested with a dissolved solids meter.

It is not uncommon to find high levels of salts in well water or municipal water supplies. Calcium and magnesium carbonates are among the most common ingredients in tap water and in well water. In fact, water “hardness” is defined as a measure of the water’s content of calcium and magnesium carbonates. In some regions sulfates can also reach high levels in water supplies.

Since calcium and magnesium are important plant nutrients, water with reasonable levels of these elements can be just fine for hydroponic cultivation. However, even a good thing can become a problem if the levels are too high. Generally, a calcium content of more than 200 PPM, or 75 PPM for magnesium, is on the verge of excessive for most hydroponic applications. An excess can cause other important elements in the nutrient solution to “lock-out” and become unavailable. For example, excess calcium can bond with phosphorus to make calcium phosphate, which is not very soluble and therefore not available to the crop. If magnesium bonds with phosphate it becomes completely insoluble and unavailable to the plant. The key to success is to start with decent water and add the right combination of nutrients.

Nutrients – A Question of Quality

What makes a quality nutrient? Here are some questions to ask. Some answers may be harder to find than others!

• How pure are the ingredients?
• How consistent is the product?
• Is it properly labeled with ingredients and NPK?
• How reputable is the manufacturer?
• What ingredients are in there and how are they combined?
• Does the product contain contaminants from poor quality control?
• If it is a liquid, did the manufacturer use R/O or purified water in the blend?
• Does the manufacturer have a quality control (QC) program like safety sealed bottles so you know it is pure and has not been tampered with?
• Are there any hidden or ‘mystery’ ingredients?

Too Hot, Too Cold

The temperature of your nutrient solution is another important factor. If your solution is too cold, seeds won’t germinate, cuttings will not root and plants will grow slowly – or stop growing and die. If it’s too hot, the same seeds won’t germinate, cuttings won’t root and plants will die from oxygen deficiency, or they will succumb to pathogens that thrive in higher temperatures, or simply bite the bullet due to temperature stress. Most plants prefer a root zone temperature range between 65 and 72 °F (18 to 22 °C), cooler for winter crops, warmer for tropical crops. When adding water to your reservoir it is a good idea to allow it to come to the same temperature as the water in the root zone before starting circulation pumps.

Remember, plant roots have evolved in a soil environment where temperature changes occur slowly, tempered by the thermal mass of the earth. Rapid temperature changes in the root zone can cause shock and invite root disease.

Water pH

A subject that is often discussed but rarely understood by many growers is nutrient pH. Plants normally grow best within a pH range from 5.5 to 6.8. Generally we worry about pH and its affect on nutrient availability. For example, if pH is too high, iron may become unavailable. Even though your nutrient solution may contain an ideal amount of iron, your plants may not be able to absorb it, resulting in an iron deficiency – the plant’s leaves will yellow and weaken.

On the other hand, hydroponic plant foods usually contain special “chelates” that are designed to assure iron availability, even at higher pH ranges. The result is that your crop will grow reasonably well even at higher pH levels. Nonetheless, high pH can damage plants in other ways. The cause of a high solution pH can be fairly complex. Most city water supplies contain calcium carbonate to raise the pH of the water and prevent pipes from corroding. As a consequence you are starting with water that has an abnormal pH: typically 7.5 to 8.0 for city water.

One method of dealing with the high pH of city water is to mix in fresh nutrient, let it stand for a while to stabilize, then test and adjust the pH. With city water supplies you will often have to add a solution of pH down (usually phosphoric acid) to lower the pH to the suitable range for most plants.

As your plants grow it is a good idea to occasionally test the pH and adjust it if needed. You can safely allow pH to drift between 5.8 and 6.8 without adjustment. In fact, constantly adjusting the pH in your system to maintain a perfect pH of 6.2 can do damage. It is common for pH to drift up for a while then down and up again. This change is an indication that your plants are absorbing nutrient properly. Adjust pH only if it wanders too far, below 5.5 or above 7.0 for example.  A pH below 5.5 or above 7.0 can spell trouble but don’t overreact. An apparently sudden and dramatic shift in pH can be the result of a malfunctioning pH meter. If in doubt, double-check with a reagent (color match) pH kit before adjusting your solution. Also remember that pH meters are temperature dependent. Read and follow all of the instructions that came with your meter or test kit. I know of at least one case where a grower lost a crop due to a defective pH meter after over-correcting nutrient pH.  Use a meter as well as a color-match test kit to double-check your pH.

Big pH swings can also indicate strong microbial life in the nutrient solution and root zone.  Microbes can change the pH to meet their needs.  The best way to manage this is to introduce beneficial microbes into your nutrient solution and the plant’s root zone. These microbes are nature’s little plant helpers.  The beneficial fungi typically help plants grow bigger, stronger and more effective root systems. The beneficial bacteria typically populate the root zone and protect the roots from bad microbes and environmental stress factors.

Time For a Change?

How often should you change your nutrient solution? That’s one of the most common questions asked and one of the most difficult to answer. Many people have tried to come up with a simple, easy-to-follow rule … once a week, every two weeks – but they’re all wrong! They’re wrong because there is no simple answer. It all depends on the plant variety, the number and size of your plants, their stage of growth, the capacity of the reservoir, the kind and quality of nutrient you use, water quality, environmental conditions such as temperature and humidity, and the type of hydroponic system used; there are so many factors that the answer is not obvious. Instead of a simple answer, what we need is a procedure that takes many of these variables into account and is responsive to changing conditions.

It sounds complicated, but it’s actually quite simple. All it takes is a little monitoring and some basic record keeping.

• Start with a fresh reservoir of nutrient and make note of the date, pH, and EC or PPM of the solution.
• As you run the system, the level will drop in the reservoir. Note the EC/PPM level then top-up the reservoir with fresh water. Test again for nutrient concentration. If the nutrient strength has dropped significantly, add a bit of nutrient to bring it back up to specs. Be sure to record how much water you added to top-up the reservoir. Repeat the procedure every time you top up the system, carefully recording the amount of water added.
• When the total amount of water added equals the capacity of your reservoir it is time to drain and replace all of the nutrient solution.

For example, imagine a hydroponic system in a cool greenhouse in the spring with 24 strawberry plants and a nutrient capacity of 20 gallons. Typically, such a system might require about five gallons of added water each week. After four weeks the plants will have transpired 20 gallons – the capacity of the reservoir. You need to completely drain and replace the nutrient every four weeks in this example.

Now imagine five tomato plants with a 20 gallon reservoir. They are fully mature, growing and producing quickly. It is hot and dry. Each plant pulls in a gallon a day so you are adding five gallons a day. After four days it is time to change the nutrient. It may be best to use a larger reservoir since the EC flux in this case will be pretty high.

Fascinated? Read on in the second installment of this feature in Issue 5 …

General hygiene

Together with disinfection of the nutrient solution, the importance of the above-mentioned aspects of hydroponic practice is usually grossly underestimated and, without due emphasis, good and consistent results will not be achieved. I guess downplaying the need for hygiene is an intuitive logic thing which is based on the rationale that because plants grow in dirt, dirt is OK!!

Absolute cleanliness of the growing area is a must to achieve maximum growth and minimum problems from pests and diseases. Thus, diseased foliage should be promptly removed from plants and, along with general debris, removed from the growing area, with surfaces kept clean from dust, dirt and spillages (Fig 11.1). Minimizing personnel traffic in the area, no smoking, and filtering the air supply to the area are other very worthwhile precautions (Fig 11.2).

Nutrient disinfection

It’s common to hear gardeners blaming their nutrient for poor growth results. However, in many cases, the true cause is the failure to regularly disinfect the nutrient solution.

Unlike soil culture, hydroponic nutrient solutions are exposed to the atmosphere and are therefore a perfect breeding ground for many types of disease (e.g. pythium, fusarium). To prevent disease ingress, the nutrient solution, medium, roots (etc.) should be regularly sterilized (Fig. 11.3).

‘Sterilizing agents’ must yield a residual chemical when dissolved in the working nutrient so that the entire system is treated each time plants are watered. Historically, chlorine dioxide, sodium hypochlorite and monochloramine are used for this purpose.  However, monochloramine has the advantage of possessing a long half-life, is gentle on roots, and is compatible with most of the ‘organic’ mediums and growth promoters used in hydroponics.

Replacing Nutrients

In recirculating hydroponic systems, the nutrient solution must be regularly replaced. That is, it should be completely drained and replaced with fresh nutrients.  This is done to maintain the nutrient’s balance and prevent the build-up of nuisance and harmful salts (e.g. sodium, chloride), pathogen, dirt, etc.

In both winter and summer, generally dump at least every 2 weeks. The dumping frequency can be less if using rain or RO (reverse osmosis) water.

Salty water: More frequent dumping (e.g. every 7 days) may be necessary when using salty make-up water because nuisance chemicals build up more rapidly to toxic / precipitation levels – especially during hot, dry weather.

Method of dumping: Poorly designed hardware can make dumping a tedious and messy task such that there is a temptation to delay or perform less frequently than necessary. So consider this at the design stage – or before you buy. Unfortunately, most system designs are not sympathetic to the hassles of dumping. Consider the advantages of the following design features:

1.  Install an in-line 2-way valve between the pump and feed outlets to divert the nutrient flow to waste (Fig 6.1a).

2.  Ideally, design a sloping floor into the tank which drains towards a sump from which the nutrient is drained (Fig 6.1b). This will help remove the last few liters containing the bulk of the sediment. Another simpler method can be to tilt the tank towards the outlet.

3.  ‘Sump’ pumps are convenient for draining tanks (Fig 6.1c). They are light, portable and easy to prime; however, they will typically only drain to a depth of around 1 inch. Hence, a sloping tank floor or built-in sump is needed for best results.

Where to dump: Utilize the remaining nutritional benefit by placing it on your garden or applying over a large area of grassland, etc.  Do NOT put down drains, toilets or in waterways or pour into sand as this can cause environmental damage (e.g. algae bloom).

Flushing of root zone with fresh water

Hydroponic systems must be regularly flushed and cleaned with fresh water. (Also, note that for disease control, external hardware cleanliness is as important as the inside of tanks / channels.) This is done to remove the build-up of too much calcium (white precipitates – causing blockages) and unwanted / harmful salts (e.g. sodium, chloride), root exudates, algae, pathogens, etc. from the root zone, medium and other system parts.

Pay particular attention to flushing of the root zone and feed circuit. Further, inspect filters, inlets, and outlets, etc. prior to replenishing the system with fresh nutrient because they are prone to becoming blocked with solid material dislodged during the flushing process.

Re-circulating systems: Flushing is done immediately following each dump cycle. Firstly, do any necessary manual cleaning, i.e. remove any obvious build-up, etc.  Partly fill the reservoir with fresh water, then operate the pump with the aim of flushing the feed circuit and root zone / medium (flushing can be enhanced by spraying with a garden hose). Discard waste using the methods advised for dumping. Repeat process until waste water is clear and conductivity is close to that of the make up water.

Run-to-waste systems: Although it is relatively common for many hobbyists to flush only every 7-14 days, some commercial growers consider it necessary to flush daily! The frequency ultimately depends on salinity, temperature, medium, plant variety, and other factors.

Flushing methods are:

a) If flushing can be scheduled to occur when the working nutrient tank is empty (i.e. between nutrient batches), then the existing system hardware can be utilized. Place low alkalinity* water in the reservoir and operate the nutrient pump until the EC of the run-off water is significantly lower than the normal operating EC or no higher than ~0.5mS above that of the water in the reservoir. Where the surface of the medium is readily accessible, it can be beneficial to do additional flushing with a garden hose.

* Lower the pH of tap water to ~5.0.  RO or rain water will not need adjusting.

b) If flushing needs to be conducted more regularly than in the scenario above, then the same procedure applies. However, it will be necessary to have a dedicated reservoir and pump for flushing (Fig 6.3). This can be connected to the existing feed circuit at a junction controlled by a 2-way valve. This valve is simply diverted to this reservoir to apply flushing whenever flushing occurs.

Post harvest clean-up

Two separate procedures are required to ensure hardware is clean prior to replanting:

Disease Prevention

At the end of each crop, it is necessary to sterilize the entire hydroponic system to help prevent disease problems in the next crop. The following guide will help remove organic build-up from pathogen, algae, slimes, and dead/decaying plant matter (Fig 11.4):

Step 1. Remove all plants and media, then do as much manual cleaning as possible. External cleanliness is as important as internal.

Step 2. Partly fill system with water. Lower the water’s pH to below 5, then, with subdued light conditions, add household chlorine bleach** (50g/L chlorine) at ~5ml per liter (4 tsp per gallon).

Step 3. Mix well, then soak system for 24-72 hours. (Note that chlorine bleach will not dissolve algae or general solid material. Only wet brushing will remove those contaminants.) Suitable treatment over that time includes:
-  For re-circulating systems, run the pump for at least 15 minutes every hour.
-  For run-to-waste systems, run the pump for a short burst once every hour.

Step 4. Afterwards, discard this solution, then flush the whole system several times with small volumes of fresh water to remove all traces of chlorine, dislodged material, etc.

Step 5. Where fine drippers, sprayers, and so on are used, it may be necessary to individually dismantle and clean each unit.

Precipitate Removal

Over the long-term, it is sometimes useful to conduct an acid** flush to help remove precipitates (white precipitates of calcium sulfate and phosphate – see Fig 11.4b) that cannot be dissolved with plain water or wet brushing.

Step 1. Firstly, treat the system as detailed for “disease prevention” above.

Step 2. To tank, add water and enough hydrochloric acid to achieve pH 2. For example: if using rain or RO water, dilute 30% (i.e. normal commercial strength) by around one thousand fold, or 1 ml per liter (3/4 tsp per gallon).

Step 3. Soak system for 24-72 hours. Suitable methods might include:
-  For re-circulating systems, run the pump for at least 15 minutes every hour.
-  For run-to-waste systems, run the pump for a short burst once every hour and collect the discharge.

Step 4. Afterwards, neutralize solution up to pH 5-6 with soda ash before discarding.

Step 5. Flush the whole system several times with fresh water to remove all traces of acid, dislodged material, etc.

Step 6. Where fine drippers, sprayers, and so on are used, it may be necessary to individually dismantle and clean each unit.

** Be sure to follow necessary safety precautions and contact no metal parts.

UGM would like to issue a huge thank you to Bob Taylor and his colleagues at Flairform (www.flairform.com) for allowing us to publish this article.