Five tips to succeed when doing Kratky hydroponics

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Passive hydroponic growing has become very popular during the past 10 years as it has a very low starting cost and uses no electricity. However, growing without active nutrient circulation, aeration and solution monitoring can cause significant problems, many of which can lead to crop failure. In this post I want to give you five tips that should help you with your passive growing experience and should allow you to go through your first Kratky crop with hopefully less problems.

1. It’s all about height and volume per plant. In a Kratky system, successfully growing plants requires the level of the solution to go down with time to allow the roots to develop structures to obtain oxygen from the air as the solution level drops. Have too much volume per plant and this does not happen quickly enough and the plant dies from water logging, have too little volume and the solution goes down too fast and the plant dies. The exact volume per plant and container dimensions depend on the environmental conditions – which determine the plant’s demand for water – but some rules of thumb have been established. For your first experience, a 4 liter bottle can be used to successfully grow a head of lettuce through its entire lifetime. You can check this and more suggestions for more complex setups in Kratky’s 2008 paper.

Figure taken from Kratky’s 2008 paper, cited above.

2. Be careful about the starting level. Another critical issue for a Kratky system is to make sure that the water level just barely touches the bottom of the receptacle where the seedling is placed or germinated. If the pot where the seedling resides is soaked with nutrient solution then the roots will never have access to enough oxygen and the seedlings will die. It is fundamental to allow the media where the plant is placed to wick water but to allow enough air space for the seedling at this stage.

3. Start with a lower nutrient dosage. Since the passive system will concentrate the nutrient solution as a function of time, the strength of the nutrients will go up a lot which will fit nicely with the ability of the plant to deal with more concentrated solutions. Starting with a nutrient solution that is too strong can cause the solution to become unbearable for the plant as the solution becomes more and more concentrated. This is why it is necessary to start at a lower strength. In general, starting with a solution with an EC of around 0.6-0.8mS/cm is good since the solution will become around 4-5 times more concentrated by the end of the growth cycle.

4. Starting at a lower pH can be better. Plants like lettuce will generally want to try to increase the pH of a solution as a function of time, as they will absorb nitrates more aggressively, causing the nutrient solution to become more and more basic. Lettuce can be grown at lower pH levels with fewer problems than at higher pH levels, reason why it can be beneficial to start the solution at a pH of 4.5-5.0 so that it can increase gradually and reach 7-7.5 by the end of the growing cycle.

ronmauer.net » Food – Off Grid Hydroponics (Kratky Method)
Example of Kratky lettuce, taken from this blog.

5. Disinfect the water before preparing nutrients. The Kratky method is very vulnerable to plant pathogens due to the fact that the solution remains unchanged through the entire growing period. If the solution contains any bacteria or fungal spores, these can prosper aggressively within the growing cycle. If you’re aiming for a purely hydroponic experience with no bacteria or fungal content, you can alleviate this problem by disinfecting the solution before preparing your nutrients. This can be done by adding a couple of drops of household bleach per liter – allowing the solution to rest for a day after that before preparing nutrients – or by running the water through a UV treatment. Inline UV treatment filters for aquariums are cheaply available online, you only need to pass the solution through them once. Boiling the water is not something I would recommend, as this also removes all the dissolved oxygen from it, which can be hard to recover without a lot of aeration, which can reintroduce pathogens into the water.

There are many more things to consider to run a successful Kratky setup but I hope the above tips do help you avoid some common pitfalls and establish your first completely passive, hydroponic growing method. All the above mentioned issues can get substantially harder when growing larger plants, so starting with smaller plants that are easier to handle – such as lettuce – is always a sure way to increase your chances of success.

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4 Comments

  • Toni
    February 8, 2021 @ 6:32 am

    How about adding small portions of hydrogen peroxide to increase the level of oxygen from time to time? Would also H2O2 work as disinfection instead of bleach prior to adding nutrients? I have Kratky tomatoes and I do add water and nutrients once or twice, depending on EC and temperature.

    • admin
      February 8, 2021 @ 8:07 am

      Thanks for your comment! Hydrogen peroxide is not very good to add dissolved oxygen, it enriches the solution to a low degree and for a short period of the time at the concentrations you can use without killing plants. The oxygenation of the solution should not be an issue in a correctly designed Kratky setup since the roots should uptake all oxygen from the air gap. Peroxide is also neutralized pretty fast by living organisms – due to the presence of catalase in most of them – reason why it is significantly less effective for the initial disinfection compared to bleach. For large plants the recommendation is to fully exchange the solution at least a couple of times, because a large plant in a relatively small container will create big imbalances in nutrients if the solutions are not changed. However this can be avoided if a very large container is used (think 50 gallons for one tomato plant). If you look at the setups proposed by Kratky for larger plants, they are very different compared to the setups proposed for smaller plants like lettuce and most do allow for solution exchange. I hope this helps answer your questions!

  • Syarif
    July 4, 2022 @ 9:33 pm

    Is UV treatment have negative effect on hydroponic nutrient?

    • admin
      July 5, 2022 @ 3:55 am

      If UV light is shone permanently on solutions it will destroy any metal chelates. Use of UV in kratky is not recommended.

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