Recirculation Basics – Part 2

What All Hydroponic Growers Need To Know About Nutrient Recirculation

Reprinted from Urban Garden Magazine

Last issue (click Link below this article), Michael Christian discussed the fundamental fluid of a recirculating hydroponic system… water. Without pure clean water (low EC), we are going to be scratching our heads when it comes to troubleshooting problems when they occur. This time, we look at water analysis, maintaining nutrient balance and strategies for managing pH in a recirculating system.

Michael Christian is president of American Hydroponics, a hydroponic system designer, and consultant to commercial growers worldwide. Photos courtesy of AmHydro.

• require a custom nutrient formula that takes that element into account and compensates for it (you may need professional help with that)
• reverse osmosis to take your water to ideal and better (water treatment store)
• prepare for frequent reservoir dumps

If your water is coming from a well or a questionable ground source where there may be pathogens like E. coli, then request a coliform test also. Don’t risk it. You can use ozone (O3) to treat your source water. As long as ozone comes in contact with every drop of water you are guaranteed that all organic life forms get crisp. Your local high tech garden supply or water treatment supplier should have these ozonators and injectors.

Nutrient balance and pH

Assuming that your source water is A-OK, this next section is on understanding nutrient balance and pH, how to steer plant performance by manipulating EC at different stages of growth, and controlling pH fluctuations.

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

This article focuses on ‘inorganic’ nutrients: minerals derived from mineral salts*, which are primarily inorganic elements in the form of ions or ‘magnetically charged particles’ (the only form a plant can absorb anyway). These ions must be available (dissolved in water) for the roots to be able to absorb them by the process of osmosis. *High quality mineral salts are mined and processed, bagged, and sold as agricultural/solution grade with a guaranteed analysis.

It’s interesting to note that out of 2.2 lbs (1 kg) of plant material, 95% is water while 5% or 2 oz (50 grams) is dry matter. Of that 2 oz (50 grams), 95% of that is sugars and carbohydrates and only 5% or 0.08 oz (2.5 grams) are nutrient elements. Not a very large percentage? At these small weights, the balance and relationship of each element to one another is crucial for high performance plant growth.

Learning from Nature

When growing in soil, experienced growers know that by adding high quality organic materials to a loose arable soil, plants grow well and resist disease. Of course, the ‘good’ of the soil is the abundant microbial life contained therein. Microorganisms (fungi, bacteria and protozoa) are ceaselessly at work breaking down organic material by secreting enzymes and acids, consuming each other and releasing ions from their waste. It has been discovered that plants can spend 25% of their growing energy excreting exudates (sugars) to feed microbes, a fantastic symbiosis of Mother Nature. Microbes in turn feed the plant ions specific to the exudate. How cool is that? A healthy soil full of microbial life leaves no room for pathogenic microflora, pythium, fusarim, phytophera, etc. to colonize.

Often in hydroponics, specifically Nutrient Film Technique (NFT), Deep Flow Technique (DFT), aeroponics and any other strictly water culture system, this process is interrupted. Plants rely exclusively on ions being delivered in solution and readily assimilated by the roots AT WILL. Plants do not spend as much energy feeding or teasing microbes to colonize; instead they focus on growing, fruiting, flowering … fulfilling their genetics. Even in water culture, microbes will show up … and colonize the root systems. It’s inevitable. This is a good thing if they are fed copious amounts of oxygen.

When using a media such as rockwool, perlite, coco, grow rocks, etc. and running inorganic nutrients, biology inevitably shows up as airborne fungi, bacteria, and spores that are brought in on entry vectors, bugs, friends, shoes, hair, etc. It has been discovered that microbes show up in as little as 24 hours after plants are introduced to a media based system. Plant roots know they’re there and will exude sugars to entice them to colonize. This all means that the inorganic nutrient solution that percolates through media arrives back at the reservoir with biology thriving. This is a good thing: it’s symbiotic, meaning there’s a healthy microbial population in a recirculating system naturally outcolonizing pathogenic microflora. This is a great reason NOT to disinfect a recirculating nutrient solution unless a variable goes radically out of balance and root die-off occurs, which attracts pathogens. In this case, enzymes would be the first line of defense to digest dead root material before active sterilization (hydrogen pyroxide or ultra violet).

With media based systems if you are using a doser (highly recommended to keep EC and pH right on), be sure to compensate for the EC and pH at the root zone. You will find that the return solution to your nutrient reservoir likely has a higher EC. Take a reading of the leachate where the solution leaves the media. It is not uncommon to have nutrient running into rockwool at EC 2.2 and leachate at over EC 2.7 due to the concentration of salts in the rockwool. If EC 2.2 is the target, lower the incoming EC to 1.8. Keep an eye on this discrepancy as you will want the duration of your feed cycle long enough to flush out accumulated salts … usually 10-20% runoff back to the nutrient reservoir for top feed irrigation. Shorter irrigation cycles during the fruiting flowering cycle (creating a ‘just moist’ media) forces roots to dry out more, which increases osmotic pressure. This triggers plants to speed up the fruiting flowering process.

With either method, measuring conductivity of the nutrient solution is critical. The universally accepted method is EC. The EC test is a measurement of the electrical conductivity of water. Pure water (with no dissolved minerals) does not conduct electricity, so the EC is 0 (EC 0.0), but as mineral salts are dissolved into water the electrical conductivity increases.

We can use this to our advantage when growing plants: if the plants remove minerals from the nutrient, the EC value falls, so we add more minerals. If the plants remove only water from the system (on a hot day, for example), we only have to add water, as the EC value will rise. That’s why a float valve is so important, as well as a doser to manage these fluctuations automatically.

EC, CF vs ppm: Which is a more accurate measurement of nutrient strength?

It’s common knowledge that 1 ppm is the same as 1 mg/ liter or 1 gram of nutrient in 1 million grams of water. The universal method of measuring the strength of a nutrient solution (where any one in any country will be speaking the same language) is Electrical Conductivity (EC) or (CF) which is really EC with the decimal point moved one digit to the left. For example: 0.8 EC = 8 CF. Stating the solution strength in ppms, (which many growers do) can be misleading, as different salts may weigh the same but have different ppms when dissolved in water. The ppm measurement actually came from waste water treatment or TDS (total dissolved solids), where there are several conversion factors where 1 EC equals either 600 ppm, 640 ppm, 700 ppm, or 750 ppm. So which one is which? Very iffy. Good luck if you stick with ppm!

EC is important to plants because a solution that is too strong can burn the roots and causes reverse osmosis. Osmosis is the natural process whereby water, including dissolved minerals but not solids, is moved through a semi-permeable membrane, such as the cell walls in plant roots: the weaker solution flows to the stronger. This is how plants take in minerals. However, reverse osmosis occurs when solution is drawn out of the roots because the solution on the outside of the roots is stronger than on the inside: this leads quickly to plant death. If you’re not measuring correctly or not calibrating your meter often, this could sneak up on you… such a simple variable to control. Get a good meter or a doser.

EC levels are different for many crops, even at different stages of growth of the same plant. Lettuces like ECs around 0.6-1, tomatoes: 2-4, fast growing flowering annuals: 1.2-2. Different plants, depending on their genetic history (i.e. where they came from) are used to growing in native soils that exert a unique pressure on the roots: clay, loamy, dry, wet, and so on. Drier climate plants can take a higher EC than tropical plants in vegetative and flowering growth. But the rule of thumb is: The lower the EC, the more loose (vegetative) the growth; the higher the EC, the tighter, more compact, the growth.

Ultimately, experience with your plants will tell you what EC levels they prefer. If your plants are thin and leggy, and provided there is sufficient light, then the EC level may be too low and you need to raise it a couple tenths at a time. Observe plant response. If your plants are short, thick and stunted with sufficient light, then the EC level may be too high: back it off a couple tenths. This is one method by which you can steer plant performance on a fundamental level. Vegetative growth uses a lower EC, while flowering growth uses a higher EC as a rule. Many high phosphorus (P/K) or mineral salt amendments actually create higher EC in the nutrient solution. If you hadn’t added them and just increased the EC, you would most likely get similar results. Plants will take what they want. Keep the solutions balanced.

EC levels are different for many crops, even at different stages of growth of the same plant. Lettuces like ECs around 0.6-1, tomatoes: 2-4, fast growing flowering annuals: 1.2-2. Different plants, depending on their genetic history (i.e. where they came from) are used to growing in native soils that exert a unique pressure on the roots: clay, loamy, dry, wet, and so on. Drier climate plants can take a higher EC than tropical plants in vegetative and flowering growth. But the rule of thumb is: The lower the EC, the more loose (vegetative) the growth; the higher the EC, the tighter, more compact, the growth. Ultimately, experience with your plants will tell you what EC levels they prefer. If your plants are thin and leggy, and provided there is sufficient light, then the EC level may be too low and you need to raise it a couple tenths at a time. Observe plant response. If your plants are short, thick and stunted with sufficient light, then the EC level may be too high: back it off a couple tenths. This is one method by which you can steer plant performance on a fundamental level. Vegetative growth uses a lower EC, while flowering growth uses a higher EC as a rule. Many high phosphorus (P/K) or mineral salt amendments actually create higher EC in the nutrient solution. If you hadn’t added them and just increased the EC, you would most likely get similar results. Plants will take what they want. Keep the solutions balanced.

Of course, everything stated in the previous paragraph is all predicated on satisfactory transpiration rates, which will be covered next time. Transpiration makes nutrient absorption possible. If you don’t have fresh air movement and ambient or injected CO2 available at all times, or if humidity is too high, EC manipulation is not going to make much of a difference.

Getting to Grips with Nutrient Deficiencies

It’s important to understand how the elements work…especially Calcium and Nitrogen. Deficiencies in either one can be easily detected and corrected.

Calcium is a non-mobile element, critical for building strong cell walls as well as activating enzymes that push auxins into new growing tissue. Calcium must be constantly supplied from roots to new tissue. If humidity is too high (90% and up), plants stress as they cannot transpire and Calcium does not get to meristems (growing tips) and tender shoots, resulting in tip burn, leaf curl, blossom drop, and so on. You will see a dried out look in new tissue as cells have collapsed. No bueno.

Whereas Nitrogen is a mobile element. If a plant cannot absorb enough Nitrogen through its roots, Nitrogen will be drawn from the lower leaves along with chlorophyll to newer, higher priority growth. Once again, if there is adequate Nitrogen in solution and humidity is too high, transpiration will be low and Nitrogen, instead of being drawn up from the roots, will be drawn from the low priority growth: older leaves. It’s a matter of survival priority how a plant steers its course under stress. If your plants are thin and leggy, and provided there is sufficient light, then the EC level may be too low and you need to raise it a couple tenths at a time. A plant’s top priority is developing a flower to reproduce, second is shoot and leaf development, and last is root. Roots will die off first, leaf and shoot second, flower last.

Nutrients

I’ve found that the highest quality mineral salt based nutrients in the market, powders and/or liquids, all work. The most reliable ones have been around the longest, because they are CONSISTENT and EASY TO USE. They all appear fairly well balanced among the elements and will grow plants well. We’ve used most of them with good results. Of course, good nutrient availability depends on the water the nutrients are mixed into … but you know about that.

The large commercial operations we work with use 2-part powder (dry) nutrients and add water to make their own stock (concentrated) nutrient solutions. Buying liquid nutrients is not cost effective at the volumes they use; they will not pay extra dollars for shipping water, which makes sense. Eventually they may create their own formulas on site.

Mixing Your Own Nutrients

Two-part dry nutrients typically are used like this: In two 10-gallon containers filled with pure (preferably warm) water, dump one part (Bag A, pre-weighed) in one container, and another part (Bag B, pre-weighed) in the second container. Bag A has Calcium Nitrate, Potassium Nitrate and sometimes Iron. Bag B has Potassium Nitrate, Magnesium Sulfate, MKPhosphate, etc. and all the micros carefully measured. Bags A and B are equal in weight. Stir thoroughly until salts are totally dissolved. The reason they are not mixed together in one container at those concentrations without being chelated is that Calcium would react with Sulfur and Phosphorus making Calcium Sulfate and Calcium Phosphate… AND drop out of solutions as a precipitate. No bueno.

Sediments that fall to the bottom of the containers are inert carriers with no consequence to the purity of the stock solutions. To use the stock solutions: pour equal parts of A and B into the nutrient tank to the desired EC and adjust pH slowly to pH 6. Viola, a balanced nutrient solution on the cheap. That’s how it’s done in the commercial growing arena.

When choosing a nutrient to use, if you buy liquids, the manufacturer does all this for you, no mess no fuss, but then you pay for that convenience. If you have friends or associates who recommend a nutrient because it is producing good results for them, you may want to go with that. It’s usually best to start at a level that is known to be effective and recommended by a trusted companion grower. If you go to a hydro shop and the owner doesn’t have any growing plants or the plants he/she does have look stressed and sick, it may not be wise to take the owner’s recommendations as you have no evidence he knows what he’s talking about. Good luck if you do.

We always have our commercial growers test source water, add nutrients, and then test that fresh nutrient solution. After two weeks we test the nutrient solution again as well as a plant tissue analysis. We can tell exactly what the plant is taking up and what is accumulating in the nutrient solution. Then we reformulate the nutrient to compensate for any element that is out of balance in the solution with the demand of the plant. In this way a nutrient solution can be recirculated for a month or longer depending on the size of tank and use of a nutrient doser.

For small commercial or hobby growers, it is not practical to fine tune the nutrient to this level. Since every growing environment is different, ultimately we have to know our plants and be able to read them: this takes at least three or four cycles, each time learning from the last. (Actually, it takes a lifetime and still things happen that keep us scratching our heads!) When experimenting, make small adjustments in nutrient… or try bio stimulants, which assist the natural plant processes without affecting the nutrient balance significantly. Plants like consistency, no big swings in EC or pH, a balanced nutrient solution, and stable water temp – especially when you are refreshing the solution and use super cold water on roots after they’re used to warm.

pH

Plants can survive in the pH range 4.0 to 8.0. Below 4 there is a danger of the roots being burnt and some minerals are not available to plants. Above 8.0 some of the minerals can be precipitated or are not available to the plants. If roots are ever exposed to extremely low or high pH, turn off irrigation, bleed 50% of the tank, add fresh water, get pH spot on and then turn irrigation back on. Most times you can save a crop with this method.

The most important thing to remember is to keep pH between 5.5 and 6.5. Aim for 6. All the elements are available in that range. When plants are growing in good light and warm conditions, the normal trend is for the pH to rise and we have to add a pH lower (acid solution). In cool, dark, short day conditions, it can be normal for the pH level to fall and we have to raise the pH with pH raise (alkali solution). As a rule, as plants feed, their root waste (sometimes in the form of ethylene gas) is basic and raises pH. In media based systems, microbes eat most of this up so pH is fairly stable. In water culture, root exudates raise pH, making the addition of phosphoric acid a regular occurrence.

With the addition of either phosphoric acid, pH down (try to stay away from sulfuric acid as it accumulates Sulfur and takes up precious EC) or Potassium Hydroxide, pH up. Always mix with water at least 100 to 1 before adding to solution. Adding pH adjuster full strength causes all kinds of mischief in the nutrient solution. Elements exposed to a pH below 4 (even temporarily) may precipitate and you won’t know it until a deficiency shows up: even then you won’t know what caused it. Be careful. If possible use a pH doser to incrementally dose on demand. In this way you avoid spikes in pH.

As adjuster is added to nutrient solution, either phosphorus or potassium is being added. It does affect the nutrient balance. Custom formulated nutrients can take that into account, but if your tank is big enough, that is enough to mitigate the problem by sheer volume.

When all else fails and you are having performance problems with your plants, having checked off every other variable, the last being your nutrient tank, purge your tank. Drain off 25 or 50%, top up water, use fresh nutrient, adjust pH and see how that goes. If you normally use a consistent amount of pH adjuster daily and all of a sudden the nutrient is not demanding adjustment anymore, you can bet your solution is out of balance and needs to be purged or dumped. As you go though complete growing cycles, you will begin to see the signs and patterns.

If plants lose their sheen or start cupping leaves and if EC and pH are right on, then there’s a strong possibility the nutrient is out of balance and plants are either hungry or toxic. Purge or dump the tank. Resist the temptation to add something else to the solution. Better instead to check transpiration rates, light, temp, relative humidity, CO2 and air movement.

Ok that will do for now! Next time we will look at the last of the five basics – plentiful oxygen availability and optimum light/temp/humidity/air circulation/CO2.

Article from January/February 2010 issue of Urban Garden Magazine