One Plant's Trash Is Another Plant's Treasure: Pondering the Use of Compost Tea as a Hydroponic Nutrient

Posted on October 20, 2014 by Travis Athougies

Recently, I’ve been thinking about hydroponic nutrients and sustainability. Most hydroponic nutrients (both dry or wet) are made from the same stuff used to make regular fertilizer. This is usually a combination of readily available chemicals like Calcium Nitrate (Ca(NO3)2), Potassium Nitrate (KNO3), etc (for a full list see here). These chemicals are synthesized from dissolved minerals that have been mined out of the earth. Although all these chemicals are abundant in the earth, we still have to pay for the transport cost of moving them from the mine to the hydroponics store and also for the energy necessary to remove them from the earth. This story doesn’t fit well with hydroponic’s promise of locally grown food. Rather it seems using these chemicals as hydroponic nutrients means that we’re simply shifting the transportation costs.

But there’s another source of nutrients that I think we may be overlooking: plants themselves. After all, even a successful vegetable crop produces a lot of plant waste. What if we could recycle the nutrients that the plants have already absorbed? Of course we wouldn’t be able to form a complete cycle in this way, but surely we could offset some of the need for inorganic fertilizer?

After doing some reading, I found out that this is definitely possible. Some people have had success using compost tea as a hydroponic nutrient.

What is compost tea? It’s a solution made by steeping compost in water and providing adequate aeration and energy (in the form of sugar) in order to encourage bacterial breakdown of the organic matter in compost. This frees up the nutrients in the discarded organic matter to be released into the water in soluble form. Of course, this also means the solution contains a lot of bacteria, which could end up clogging up your hydroponics system. This means it would probably be necessary to kill and remove the bacteria from the solution before putting it into your hydroponics system. This can be accomplished with either UV light or some hydrogen peroxide to kill the bacteria and an aquarium protein skimmer to remove the dead bacterial parts. The guy in the video I linked to didn’t do this, but he had a simple system. In a more complicated system, this would definitely be an issue.

Brewing a compost tea hydroponic nutrient

So how would you go about brewing the perfect batch of hydroponic nutrients. To start, you will need dechlorinated water, good quality compost, some molasses, two 5 gallon buckets, something to filter the tea (either a few pieces of cheese cloth or an old t-shirt), an aquarium protein skimmer, and an air pump and stone. Here’s what I would do:

 A freshly brewed bag of compost tea. Yummy! From <a href="" class="uri"></a>

A freshly brewed bag of compost tea. Yummy! From

  1. Fill one of the five gallon buckets about one-fifth of the way full with compost.
  2. Fill the bucket with the water, and drop in the air stone. The aeration is necessary to prevent the growth of anaerobic bacteria which can kill the plants
  3. Add 3 tbsp of molasses into the mixture. This is to feed the bacteria and fungus and help them break down the compost
  4. Plug in the air pump and let the tea brew for at least three days. If the tea starts to stink, anaerobic bacteria have started to proliferate. Throw it out away from your plants and start again
  5. After three days, remove the air stone and pump
  6. Put the cheesecloth or the t-shirt over the second bucket and slowly pour the mixture from the first bucket into the second
  7. Leave the bucket for about an hour or two and see if any solids settle at the bottom, if they do, repeat the previous step until the solution is just liquid
  8. Now, add in a 1/2 cup of 3% hydrogen peroxide to kill the bacteria or expose the solution for a few minutes to a strong UV light. This should kill all the bacteria
  9. Use a protein skimmer to remove the dead bacterial bodies from the mixture. This should take about a day.
  10. The compost tea is done! You can use it to fertilize your plants!

Does this actually work?

I’ve yet to try it on my own systems (stay tuned for a future post), but it seems that some people definitely have luck with it. Personally, I doubt it could totally supplant a fully put together nutrient solution, and even in the youtube video I linked to, there were some problems with the final plant. However, I think that using the tea means that you would be able to use a weaker fertilizer solution.

For a more scientific answer, I decided to research how much nutrient actually gets dissolved back into the water during the brewing process. I couldn’t find any good non-pay-walled sources, but this article has a good explanation. Basically, compost has an NPK ratio of about 2.6-0.9-2. If you dilute this by one fifth (since we only added one gallon of compost), you get an NPK ratio of about 0.5-0.2-0.4. To put that in perspective, hydroponic nutrients have NPK ratios around 9-9-9 (9% Nitrogen, 9% Phosphorous, and 9% Potassium). This brings me back to my point that compost tea makes a good supplement, but probably couldn’t serve as the only hydroponic nutrient.

However, there may be some other benefits to using compost tea like encouraging the growth of beneficial bacteria. The only way to find out is to run an experiment, which is what I’ll be doing with my next round of vegetables. Stay tuned!