Five tips to successfully manage your nutrient solution in a recirculating hydroponic setup

Although a significant portion of hydroponic growers use run-to-waste setups – where the nutrient solution is ran through plants and then lost – the industry is now moving towards the implementation of recirculating hydroponic systems in order to reduce both water usage and the unnecessary dumping of fertilizers into sewage systems. A recirculating setup has many advantages and can provide better yields than run-to-waste setups, provided the solution is properly managed and changed through the growing cycle. In this post I’m going to talk about five tips that will help you successfully manage your nutrient solution when using this type of system.

Ensure the volume of the reservoir is at least 10x the volume necessary for a single irrigation. The total volume of a reservoir is key in a recirculating setup because you want the bulk of the solution to be unaffected by whatever nutritional changes are caused by the plants during each feeding. This means that you want most of the solution to be inside your tanks and not inside the media when every irrigation is done. A simple rule of thumb is to make the volume of your initial reservoir at least 10x the volume that it would take to carry out a single irrigation of your entire crop. If you do this the water and nutrient absorption effects will happen slowly and will give you time to manage your solution without any harm coming to the plants.

Bato Bucket Systems & Supplies | Hydroponic Buckets | CropKing
A recirculating hydroponic tomato system using dutch buckets

Circulate your solution until your pH and EC are constant. After an irrigation cycle starts, the solution will first mix with the remnants of the last irrigation cycle within the media, which will make the pH and EC of the return different from those of the main tank. In order to ensure that the plant’s root system is being subjected to the desired nutrient concentrations, make sure you carry out the recirculating process until the EC and pH of the tank remains constant and matches the return pH and EC. Once this happens you know that the conditions within the media have now been equalized with the larger body of solution and you can stop the irrigation process. Constant monitoring of the pH and EC within the tank are therefore necessary within this type of setup.

Add water and not nutrients when the EC increases with every irrigation. In a normal recirculating setup the EC of the solution in the main tank will tend to increase with every irrigation while the total volume of the solution will decrease. This happens because healthy plants always absorb more water than nutrients, which means any measure that’s proportional to concentration – such as the EC – will tend to increase as the amount of water goes down. You want to add enough water to bring the EC down to the desired EC but you do not want to add nutrients with this water and this would increase the EC or contribute to nutrient imbalances within the solution. Note that you will need to add less water than the amount that was absorbed by the plants, because the plants also take some nutrients with them, meaning that the amount of water needed to reestablish the EC to what it was before will be lower. If an initial solution has 1000 gallons, the volume might go down to 950 gallons on the first irrigation but you might only need to add 20 gallons to bring it back to the original EC, making the total in the end around 970 gallons. Make sure the pH of the tank is also corrected after every irrigation and water addition.

Replenish water with nutrients when volume is down 40%, use this as an opportunity to shift the solution. As discussed in the last tip, the volume of solution will go down with time, even if some water is added to return to the original EC. At some point more than 40% of the volume will have been spent and it is at this point where you should fill the tank back to its full volume with water plus nutrients. You can also use this opportunity to change the nutrient ratios and skew them in the direction that you want your plants to follow nutritionally. For example in a flowering crop it is common to increase the amount of potassium during the blooming stages of the plant, so this can be done as nutrient solution is replenished after it’s consumed by the plants. Note that this process cannot be carried out indefinitely because both nutrient imbalances and plant exudates will accumulate within the main solution. Most recirculating crops will fully change the solution every 3-4 weeks to avoid these problems although the life of the solution can be extended further when chemical analysis is done – to customize nutrient replenishing – and adequate filtering is implemented to remove substances contributed by plants.

Add in-line UV filters and carbon filters. It is fundamental to ensure no microorganisms contaminate your nutrient solution. For this reason, online UV-filters are necessary to keep the nutrient solution as sterile as possible. Carbon filters are also very useful as they remove plant exudates that can contaminate the solution and cause problems within the crop itself. Many of these exudates are food for microorganisms, others are plant hormones that might cause unwanted responses in the plants. However both carbon filtration and UV filters can cause some issues – such as the destruction and adsorption of heavy metal chelates – so it is important to use chelates that are more resistant to UV and have less affinity for carbon filters to alleviate these problems.

There is certainly a lot more to the management of recirculating hydroponic solution than what I have detailed above, it is important to note that some of these tips are simplifications and much better tailor-made solutions are possible with a proper analysis of each situation. These are just some simple tips to hopefully make your change towards the use of recirculating systems a lot easier and should greatly increase your chances of success in the world of recirculating hydroponic setups.

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10 Comments

  • chivas
    August 17, 2020 @ 12:56 am

    Thank you for the article. I find your take on topping up with water to the target EC to be very interesting. Do these recommendations apply to NFT lettuce as well? A potential problem I see in my tropical climate is as the reservoir level drops, the nutrient solution will not be able to resist temperature rise as well as a full tank of reservoir.

    Also, maybe ultra-filtration can be used instead of UV to avoid oxidation of chelated micronutrients.

    • admin
      August 18, 2020 @ 8:58 am

      Thanks for your comment! These recommendations would also apply to NFT lettuce. You’re right that in a tropical climate having a large reservoir volume might take priority so in this case topping up a reservoir and then increasing the EC by adding nutrients might be a better approach, this however makes management a bit more complicated unless you automate both the volume replenishing and the EC adjustments. About ultra-filtration, I have no experience with it, it could potentially work but at a significantly higher energy cost than UV filtration. If you have any experience comparing the two, please let us know!

      • chivas
        August 19, 2020 @ 12:04 am

        Thank you for your reply. I only have experience using ultra-filtration as a prefilter for my RO system to prevent the RO membrane from fouling or clogging due to bio-film and such. However, to my knowledge, the working pressure is much lower than that of the RO system and with no waste water. It won’t filter any dissolved solids but is fine enough to filter all microbes except for some viruses. Might try using it for my recirculating nutrients and let you know how it goes.

        Another draw back of the UV system is that ideally the water should be relatively clear and flows slow enough for it to work properly.

  • Keesje
    August 18, 2020 @ 3:06 am

    You say: “”However both carbon filtration and UV filters can cause some issues – such as the destruction and adsorption of heavy metal chelates – so it is important to use chelates that are more resistant to UV and have less affinity for carbon filters to alleviate these problems.””
    But how do us simple people – who buy ready made nutrients like General Hydroponic for example – know if their nutrient brand contains chelates that are more resistant to UV or have affinity for carbon filters?

    Side question: I saw by accident that you replied to my comment. Is there a box I can check so I will get a notification?

    • admin
      August 18, 2020 @ 8:56 am

      Thanks for commenting! Commercial hydroponic nutrients like General Hydroponics are not created with this in mind, so you can assume that those chelates will get destroyed by UV light quite efficiently. This does not however mean the heavy metals disappear, but just that they are bound to precipitate within the media or reservoir with time.

      About your side question: the website does automatically send an email whenever someone replies to your comment but most mail providers just block them as spam. I’ll try to find if there’s a way to solve this issue.

      • Keesje
        August 18, 2020 @ 9:26 am

        Thank you for your reply.

        I do not get any notification. Also not in my spam.
        I do however receive your newsletters, so my email account is not blocking you.
        Weird. But ok. I just have to look more often then.

        • admin
          August 18, 2020 @ 10:43 am

          Thanks for your reply! The newsletter is sent using an independent emailing service, so the emails aren’t actually sent by the website’s server, while the ones related to comments are. I see they are being sent from the server but email providers just block them outright – I don’t receive them either – I’ll need to research further to figure it out.

  • Billy
    September 13, 2020 @ 8:28 am

    Hi Daniel,
    Thank you for a wonderfully informative website and great calculator.

    I’m running a recirculating flood and drain system. The system floods with around 300L of solution, the plants are in pots so that’s not media volume, that would be circa 200L of small loose rock wool cubes.
    My reservoir is 1000L. It’s not quite the 10x volume you suggest. So I’m wondering if the 40% depletion rule before topping back to 100% and adding nutrients would be good to use in this setup, and would I still be able to go 3-4 weeks between changeouts?

    Regards
    Billy

    • admin
      September 13, 2020 @ 8:42 am

      Thanks for writing Billy! I would need to learn more about your system, the plants in it and its water consumption in order to make an assessment. Feel free to contact me using the website’s contact form to book an hour of consulting time so we can address your issue in detail.

      • Billy
        September 13, 2020 @ 9:11 am

        Thanks Daniel!

        Regards
        Billy

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