Five common misconceptions around nutrient management in hydroponics

After many years of experience as a consultant in the hydroponic industry and interacting with dozens of different customers growing different plants with different systems, there are some common misconceptions that become apparent as time goes by. As a chemist, the ones I remember the most are related with the management of nutrient solution and the diagnosis and treatment of nutritional problems in plants. In today’s post, I want to talk about some of these misconceptions and hopefully shine light into what the more accurate interpretation of these phenomena actually is.

The EC is increasing, my plants are not feeding! One of the concerns I most commonly address is that plants are “not feeding” because the electrical conductivity (EC) of the nutrient solution is not decreasing, but actually increasing after the solution goes through the plants. Many growers think that EC measures nutrients in a solution, so if a plant feeds on nutrients, then the EC should naturally decrease as the plant feeds. This is wrong because the plant consumes both nutrients and water and EC is a proxy for nutrient concentration and not for the absolute amount of nutrients in the water. As a plant feeds it will absorb both nutrients and water but significantly more water than nutrients. Remember, plants are mostly made out of water and also use water to regulate temperature, humidity and nutrient uptake, so they will take way more water than nutrients, increasing the EC as they feed. As a plant grows larger it’s nutrient and water demands grow, but the water demand grows significantly more than the mineral nutrient requirements, meaning the plant will progressively increase the EC more and more as it feeds more and more aggressively.

The plants are yellowing, there must be a nutrient deficiency. As soon as plants start to show signs of yellowing, a significant amount of growers will immediately look and try to interpret this as a sign that there is some form of nutritional deficiency. Most that subscribe to this belief will look for pictures of deficiencies online and do their best to match what they see with a deficiency and then proceed to supplement the solution with some fertilizer that contains the “missing element”. More often than not, this is actually not caused by the composition of the solution at all but by some environmental factor that is not being properly managed. In run-to-waste systems this is most commonly related with a significant pH drift in the media – reason why it is always necessary to measure pH/EC of the run-off – but it can also be related to unnecessarily harsh VPD conditions or even a lack of enough air circulation. I would say that 5/10 times, problems with the plants have virtually nothing to do with the nutrient solution at hand.

If you want more X, then increase X in the nutrient solution. The relationships between the concentration of elements in a solution and the concentration of nutrients in plant tissue is not linear. Sometimes, increasing the concentration of an element in solution can actually lead to less of that nutrient being present within plant tissue. An example of this can be phosphorous, a plant can suffer from a phosphorous deficiency due to the formation of insoluble iron phosphate compounds in tissue that appear when the concentration of these two elements goes above some threshold. As more of either is added, more of these insoluble compounds are formed and less of P and Fe actually gets to the plant. Another example can be Ca, where the amount of Ca in tissue is more dependent on VPD than on the concentration of Ca in solution, changing the VPD by 20% will affect Ca in tissue significantly more than adding 20% more Ca to the solution in some plant species. In these cases you might add 20% more Ca but your VPD drops 20% and you actually see a decrease of Ca in tissue. Sadly nutrient dynamics are not simple and often a more holistic picture needs to be used to approach nutritional management!

Plants need aggressively more phosphorous when they flower. Most commonly used fertilizers in soil tend to have higher P/K values when they target “flowers”, this is because, in soil, phosphorous is not highly available and the supplementation of highly available phosphorous during flower can be very useful to plants. However, flowering plants in hydroponics always have access to significant amounts of soluble P and most actually do not require an increase from this base level when they go into their flowering periods. Many commercial hydroponic solutions used for tomatoes will – for example – keep their P values at 50 ppm through the entire growing period, only increasing K during the flowering period, but not P. Experiments across various commercially grown flowering species have shown that levels in the 50-65ppm range are ideal for many plants during their entire life cycle, this matches the experience of growers in the horticultural hydroponic industry.

There is a perfect nutrient solution. Many growers go on a “holy grail” quest to find the “perfect” nutrient solution that will give them the absolutely best yields. Many commercial fertilizer producers also call me asking to formulate “the best possible formulation” to grow a given type of plant or – even worse – to grow a wide variety of plants. The truth is that the ideal solution to feed a plant will depend on the genetics, the environment, the irrigation system, the growing media, etc. Due to the large amount of variability between growing setups, plant genetics and growing methodologies, more often than not, the nutrient optimization process needs to be carried out for every grower. Don’t get me wrong, a base formulation will probably get you 80% of the way to your maximum potential yields – nutrient solutions are not miracle generators, they are just food – but conquering that final 20% will require a lot of additional effort that will most likely be limited to your particular conditions. This is because most environments are limited by different factors and using the nutrient solution to help overcome some of these limitations will modify the solution in a way that’s probably detrimental for other environments.

I hope the above misconceptions show that the world of nutrient solutions and plant management is not so simple and that there is a lot that goes into understanding how nutrients interact within a plant and how a given growing environment needs to be modified in order to improve crop results. My goal is to help you expand your knowledge about hydroponics and better reach your goals by overcoming some of these misconceptions and tackling some of the true problems within your hydroponic crops.

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7 Comments

  • Udaykumar Bamane
    August 19, 2020 @ 12:55 am

    Thanks for such a important information

  • Keesje
    August 19, 2020 @ 1:35 am

    Dear Dr. Daniel Fernandez,
    Unfortunately I can’t post pictures.
    But there is a well-known overview (mainly known in the cannabis community) which has 4 columns: Water level, EC, pH and The solution to the problem.
    Each column can contain ‘static’, ‘falling’ or ‘rising’. And then the column ‘Solution’ tells you how to act if these circumstances arise. In one of the rows it is written that if Water level is falling, EC is rising and pH is static, the solution is’ plant is drinking more than eating: lower EC.
    In another row it is written that if Water level is falling, EC is static and pH is static, then the solution is’ perfect conditions. EC and pH are at the correct level ‘.
    This seems contrary to what you indicate.
    What is your view on this?

    • admin
      August 19, 2020 @ 8:29 am

      Thanks for commenting. This reasoning is plainly wrong, plants that grow under ideal conditions will always tend to strongly increase the EC of solutions. Under healthy circumstances plants will always uptake significantly more water than nutrients and do so more and more aggressively as plants grow bigger. There is a wide array of research and experience in traditional agriculture that shows this to be the case. Under these conditions, to keep up with the plant’s demand, it is necessary to decrease VPD, increase irrigation frequencies and irrigation volumes. Lowering the EC makes the plant take in less nutrients but also reduces stress in the root zone, so it is a solution that puts the plant at a more comfortable level – meaning there is a lower probability of issues – but the plant is going to yield less if the high EC value was ideal. For highest yields it is necessary to push the plants both with nutritional and environmental inputs. Obviously a very high nutrient concentration is going to be detrimental, so it needs to be as high as the plants can take under ideal environmental conditions.

      The view that an ideal solution has no pH or EC changes at falling volume is also incorrect. A crop grown under condition where a solution behaves like this will be terribly unproductive compared to a crop where plants are fed ideally and the grower properly acts to ensure the output produced is not detrimental to the plants.

      I have never seen the chart you mention but there are a lot of charts produced among cannabis growers that are not correct, including charts related to feeding, pH management, nutrient deficiencies, VPD, etc. These are probably mostly well meaning, produced by the anecdotal experience of growers, but they often cause more problems than they help solve because of their lack of accurate information. Sadly things can often not be simplified to charts like these and often whether to lower the EC, increase the EC or change other environmental inputs requires a good understanding of all the variables involved, not just a narrow view related to one of them.

  • Keesje
    August 19, 2020 @ 9:51 am

    Thank you for the quick and comprehensive answer!
    You are right that many of the charts do not provide the correct information.
    So-called Bro Science is widespread in cannabis culture.
    Indeed, there are VPD charts that deviate enormously from what is common for other cultivated plants.
    However, there is no scientific source or substantiation.
    While a VPD of 1 is already seen as high for tomatoes, cannabis growers believe that 1.5 or 1.6 is also good.
    This seems to be prompted more by the fear of Botrytis than by science. At least, I could never find a source for it.
    Perhaps you can explain this in more detail in the future.
    Again, thank you!

  • Terie Wakeham
    August 19, 2020 @ 2:24 pm

    Hi there,
    We are a professional grower small scale in Queensland Australia growing hydroponic lettuce. Our produce is perfect until we hit the hotter months from December until March when the older leaves develop chlorisis and growth is severely restricted. Leaf analysis test showed very low nitrate levels so we believe we need to increase the nitrate level in our hotter summer months.
    FYI we have hail netting reducing temp by 10% and a heat pump cooling the water which we run over the summer months.
    Could you please advise how we should go about increasing our nitrate level in our nutrient solution (which we dump every 1 – 2 weeks) to reduce the loss of our product, or any other suggestions you may feel appropriate.
    thank you

    • admin
      August 19, 2020 @ 10:11 pm

      Thanks for your comment! Increasing nitrate is NOT going to fix your problem. Lettuce at high temperatures suffers from significant stress that needs to be addressed. Increasing nitrate in the solution won’t increase it in tissue because your problem is related to temperature, not to nitrate availability. It is the plants ability to carry out photosynthesis and get nitrate into tissue what’s wrong (putting more won’t help). If you want my help on this matter feel free to send me an email through the page’s contact form and we’ll arrange for one hour of consulting time to help you fix the problem.

  • Tinghui Jiang
    September 7, 2020 @ 10:20 am

    I’m happy to see the post and discussions here. There are lots of misunderstanding of science in hydroponics. As has been mentioned in this post and others. For example, people believe phosphorus is a rooting/flowering element. The common commercial products have very high P. My Ph D research was on P. I noticed the better P concentration is roughly the half strength of Hoagland’s. Daniel mentioned lower range but still similar to Hoagland’s(in P2O5 I believe). Plants can accumulate much high nutrients than their need as storage or detoxification. Too high P content in produce (not for cannabis products) would result in health problems in human (this is less addressed issue), say high ureic acids. As scientists we should remind community beware of the risk. This is a good platform to start doing this. I recently generated a series products under Dr Hydroponics. Hope they will help people change some misleading concepts.
    As to the Australian friend’s farm, use of unique nitrate nitrogen over hot season will be beneficial to overcome the heat stress.

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