Mini Kratky Jars - Easy Hydroponics




Introduction: Mini Kratky Jars - Easy Hydroponics

About: Welcome to my Instructables channel where I'll share my wacky and unique creations that hopefully others find useful, or better yet, inspire an evolution of even better ideas!

UPDATE 12 APR 2021:

A second new design has been completed that further addresses determining the water level and adding additional nutrient solution.

Even with the new wide mouth non-threaded lid it was not easy to add nutrient solution without spilling and seeing the water level was still difficult so I added an automatic water level indicator.

The water level indicator is visual and will indicate when water level reaches the bottom of the net pot and also when the the jar is fully topped off (this is about halfway up the net pot), which makes it super easy to simply add water directly into the net pot and know when to stop. This drastically reduces the chances of spilling.

See Step 12.


UPDATE 12 DEC 2020:

So I had a lot of trouble getting plants to grow past the seedling phase. They'd germinate just fine and send roots down into the liquid, but wouldn't continue to grow. The problem was with the jars being in the window sill and absorbing too much infrared energy. Once I realized this may be the problem I used a laser thermometer to check the temperature of the glass. It was almost 130F! D'oh!!

CAUTION: If placing in windowsills - Use cloth, cardboard, etc. to shade the jar to prevent solar energy from overheating the glass/nutrient solution/root system.

I also made a few modifications to the 3D printed lid design. Based on these findings:

  1. The soda bottle cap turned out to be a real pain. You have to have both hands free to remove or install it, which made topping off the nutrient solution quite a chore. The hole was too small to be able to see where the water level was, which also made it difficult to add solution without spilling. The new design removes the screw on soda bottle cap and replaces it with a simple press in lid that also has a much larger opening.
  2. The bamboo skewers being inserted into the net pot was a poor design. Once the plant got large enough to need the skewers for support, the weight of the plant would simply tip the whole thing over. (Since there is nothing to hold the net pot to the 3D printed lid.) As a result of this, I added skewer holes into the 3D printed lid.

See newly added Step #11 for the revised design.

END UPDATE 12 DEC 2020. (original post starts below.)

Project Summary: Unique way to grow small plants hydroponically according to the Kratky Method, using Net Pots, Clay Pebbles, Rock Wool and a Custom 3D Printed Part I designed in Fusion 360. (Perfect for herbs on a kitchen windowsill.)

This project is an entry in the Mason Jar Contest. Please vote if you like it.

So, I had several ideas for the contest, but in doing research for one of them, I came across the Kratky Hydroponic Method. I don't know how I haven't heard about it before now. Its extremely cool, and I'll be learning more about it as I continue experimenting with it. The coolest part is it requires NO ELECTRICITY!

Here are a few good links to get you started if its new to you as well:


  1. Pint Size, Wide Mouth Mason Jars
  2. Rock Wool 1" Cubes
  3. Clay Pebbles
  4. 2" Net Pots
  5. Bamboo Skewers
  6. Soda Bottle Caps
  7. Hydroponic Fertilizer Solution
  8. Seeds or Seedlings (Your Choice)


  1. 3D Printer
  2. Sharpie Marker

Step 1: Kratky Method Primer

Here's a brief explanation of the technique developed by Dr. Bernard Kratky of the University of Hawaii.

(Image above is from the Youtube Video from Epic Gardening linked on the previous step.)

  • As the seedling grows it sends roots down into the nutrient rich water. As the roots develop, and the plant grows, eventually, the water level will drop, and the plant will use the portion of the roots that are submerged in the water for hydration and nutrition, and the portions that are in air to supply oxygen.

To maintain plant growth simply continue adding nutrient rich water but only to the lower level of the container. (Never completely submerge the entire root system.)

From what I've learned so far with this method; the key is airflow around the upper roots, so the part I designed has a special feature to assist in airflow while at the same time keeping sunlight out of the water solution.

NOTE: Light entering the water solution will cause bad bacteria and algae growth and is the surest way to kill the plant.

Step 2: Fusion 360 Design

This was a fun challenge to design using Autodesk's Fusion 360 CAD/CAM Software. (Free for Hobbyists by the way.)

It was a bit challenging with two different threads, a sweep feature for the breathing holes, and a lofted feature to blend the mason jar opening up to the smaller 2" diameter for the Net Pot. All of which would have been much more difficult with the tools I use at work.

The Mason Jar thread was the most difficult because I couldn't find any documentation online that detailed the type/size of the thread. I tried several on Thingiverse, but they were all wrong and not worth the effort to try and fix. So, I just took measurements with calipers and did trial and error Engineering. I think I got it perfect on the 4th or 5th iteration. (The smaller thread is simply soda bottle thread. Any soda bottle cap from 2L size on down will screw onto this thread.)

The scalloped edges around the perimeter serve two purposes. First, for better grip when tightening and loosing it from your Mason Jar. Second, it gives a bit more material to allow the breathing holes to be present.

NOTE: This is designed for the large mouth mason jars. Not the standard size.

Step 3: Slice in CURA

The unique design of this component allows it to be 3D printed entirely with no supports and absolutely no post processing.

  1. 0.2mm layers is fine, but I chose to do the first few at 0.12 just to get some good examples
  2. 20% Infill
  3. Rafts (I ALWAYS use rafts, to me its cheap insurance to ensure good adhesion)

NOTE: For those unfamiliar with 3D printing; slicing is simply software that takes a 3D model and "slices" it into very thin layers. The outputed file can then be read by the 3D printer to interpret how to move and deposit the filament and generate your component. CURA is a brand of slicing software made by Ultimaker.

Step 4: Gather All the Materials

  • Wide Mouth Amber Mason Jar
  • 3D Printed Component
  • 4 Rock Wool Cubes
  • Clay Pebbles
  • 2" Net Pot
  • 3 Bamboo Skewers (these are 10" long)
  • Soda Bottle Cap (missing from picture)

TIP: The amber colored Mason Jars block out the harmful light. If you use clear glass Mason Jars you need to cover or paint them to do the same thing. (One really cool idea I saw was using chalkboard paint, which you can then use to write information about the plant on.)

Step 5: Insert Skewers Into Net Pot

Insert three (or more depending on the plant) Skewers as shown into the Net Pot to serve as supports once the plant starts to mature.

NOTE: This step in not needed for lettuce and probably many other leafy greens. Herb plants will benefit from it though.

Step 6: Add Clay Pebbles

Add a few Clay Pebbles to the bottom so that when you add the 1" Rock Wool Cubes they will be flush to the top of the Net Pot.

Step 7: Add Rock Wool

Add four of the 1" Rock Wool Cubes as shown.

NOTE: I think four may be a bit much as its rather tight. I'll play with adjusting this between 2 and 4 depending on the plant and how strong each plant's root system is. But this is good to start seeds from for transplanting later on.

Step 8: Final Assembly

  1. Screw the 3D Printed Component onto the Mason Jar
  2. Add the prepared Net Pot into the top hole
  3. Fill with Nutrient Solution until about 1cm of the bottom of the Net Pot is submerged
  4. Seal with Soda Bottle Cap
  5. Add seeds or seedlings of your choice

NOTE1: I initially adjusted the placement of the Mason Jar thread in the 3D Printed Part so when fully screwed onto the Mason Jar it lined up facing the front (so the fill cap lined up with the "Ball" logo on the thread), but when I got another box of jars they were different, so each one will likely be different depending on the jars you use.

NOTE2: The lime green cap is from Diet Mountain Dew

NOTE3: I plan on marking the soda bottle caps with numbers and then keeping a separate log with each jar's information. I'm going to be making a bunch of these so keeping them straight will be important. (Just have to make sure not to mix up the caps and jars ;)

Step 9: 2" Vs. 3" Net Pots

Some of you with more experience in this may be questioning why not simply put a 3" Net Pot in the Mason Jar?

This is true. A 3" Net Pot fits perfectly into wide mouth Mason Jars without the need for any type of custom additional part. (See image above.) Reasons for using a 2" Net Pot:

Reason 1:Mason Jars on Amazon are crazy expensive. A 4-Pack of the Pint Jars shown in this Instructable were only $10 at my local Ace Hardware and actually all they had available. (I guess people are hoarding Mason Jars too!) These same ones on Amazon? $23! (And that's if they are in stock) Quart sizes are obviously even more expensive. So I had to work with the Pint size jars.

Reason 2: I primarily wanted to start with smaller plants to get experience in this method and felt the 2" Net Pots would keep the plant smaller and thus more stable when the plant matures. If 3" Net Pots were used in these Pint Jars, the larger plants may make the jar top heavy and prone to tipping.

Reason 3: Due to the restriction of using the Pint Size Mason Jars, I felt the 2" Net Pots would provide a better balance of volume for the plant's root ball and growing medium in relation to the volume of nutrient solution that the jar could provide. (Not only would a 3" Net Pot likely be unstable in a Pint Jar, you may also have to top off the nutrient solution much more frequently and/or there would be insufficient space for the roots.)

Reason 4: By making the transition from the wide mouth Mason Jar to the 2" Net Pot and offsetting it to the back it provided just the right amount of space to allow for the "Nutrient Solution Level Check and Fill Port" feature capped off with an easily recyclable soda bottle cap. This allows for checking the water level and topping off as needed without having to lift the entire plant up and disrupting the roots.

At least those were my thoughts going into this project with no experience in the Kratky Method (but a LOT of experience with standard hydroponics and gardening in general.) Time will tell if this reasoning was accurate or not. (I'll try and post updates in the future.)

Step 10: Additional Photos

I've just been playing around with these as they come off the printer (up to 5 now). Images from left to right are:

  • Corn started from seeds
  • Baby Thai Basil started from seed
  • Transplanted mint plant (rinsed off soil from rootball and secured into Net Pot with some of the Clay Pebbles
  • Funnel, also designed in Fusion 360. It serves to easily top off the nutrient water and as a "dipstick" to check water level

Thanks for taking the time to read through my Instructable. Please send me any questions or comments you might have. I try to answer them all. Stay safe and healthy! Happy Hydroponicing!

Remember to vote in the Mason Jar Contest!

Step 11: Revised Design (UPDATE 12 DEC 2020)

Revised design. Details at start of Instructable.

(Peas on the left and cucumber on the right.)

Step 12: 12 April 2021 UPDATE

A second new design has been completed that further addresses determining the water level and adding additional nutrient solution.

Even with the new wide mouth non-threaded lid it was not easy to add nutrient solution without spilling and seeing the water level was still difficult so I added an automatic water level indicator.

The water level indicator is visual and will indicate when water level reaches the bottom of the net pot and also when the the jar is fully topped off (this is about halfway up the net pot), which makes it super easy to simply add water directly into the net pot and know when to stop. This drastically reduces the chances of spilling.

See images above for how it works and a new STL file for printing the new lid.

This is the pipette I used:

NOTE: Maintaining the water level for the net pot is primarily important when germinating new seeds to ensure the seedlings don't dry out if you happen to not notice the water level drop too low. Once the plant has roots extending out of the net pot and into the mason jar you actually want some airspace for the roots to "breathe". (That's why I didn't just snip the pipette at the "low" level. I'll have another mark higher up to indicate when to add water for established plants.)

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    2 years ago

    Your explanation for using the Kratky method is very easy to follow the concept and understand. I'm not sure what rock wool is as it sounds like an oxymoron, but would any other type of material work, too - like peat moss or other fibers?


    Reply 2 years ago

    Thanks. You know, it was new to me too, but its actually rock that they somehow heat up and separate and spin into wool. Not sure about other materials. I believe they use rock wool specifically because its completely inert (no bacteria or organism like are present in soil) so I think that eliminates peat moss. Its actually not very expensive, and I just ordered it along with the clay pebbles and net pots all from Amazon. Let me know if you try it out.


    2 years ago

    This is so cool! Great project :)


    Reply 2 years ago

    Thanks so much.