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Thirty years of growing and consulting experience have left AmHydro with a wealth of knowledge. We’ve learned that hydroponic farming is one of the most important steps in fostering a healthy, sustainable world for future generations. This world is a possibility. It is our mission to make it a reality. To do this, we need to educate each other and expand our collective consciousness by sharing our experiences and wisdom.

Over the years, we’ve written volumes of information that are still useful for growers today! For this reason, we will be reintroducing archived articles with relevant lessons for today’s growers during our Throwback Thursday series.

Today’s article is part of a three-part series on nutrient recirculation written for Urban Garden Magazine by the former president of AmHydro, Michael Christian. Enjoy!

What All Hydroponic Growers Need To Know About Nutrient Recirculation

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 percent! 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 efficiency 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 absent, 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 knows well, plant life has a way of beguiling even the most experienced growers. The better understanding we have of each basic element, the faster we will be able to determine the area that is faltering 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 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 your 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.

Tune in for our next Throwback Thursday, where Michael looks at nutrient balance and pH, how they work with pure source water and how to manage them to steer plant performance.

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