Compost Teas in Hydroponics

What Are Compost Teas—and Should You Use Them in Hydroponics?

In this blog, we’re diving into the murky (literally!) world of compost teas—a microbial brew that’s making waves in organic and regenerative agriculture. But before you go spraying it on your crops, there are some crucial facts you should know.

And no, this isn’t a drinkable tea—compost teas are strictly for plants.

What Is Compost Tea?

Despite the name, compost tea often contains little or no actual compost. Instead, it’s a brewed solution made from:

• Humic and fulvic acids
• Seaweed extracts
• Molasses (as a sugar source)
• Yucca (a natural wetting agent)
• Live bacteria cultures or compost inoculants

Brewed in a tote or IBC (Intermediate Bulk Container), compost teas can be aerated, agitated, heated, or left completely undisturbed—methods vary widely across growers.

What Does Compost Tea Do for Plants?

When applied to crops—usually as a foliar spray—compost teas are believed to:

• Boost micronutrient uptake
• Improve resistance to pests and diseases
• Enhance tolerance to environmental stress, like UV and ozone exposure

I’ve personally seen ornamental growers in Belgium use it with great success.

Risks of Using Compost Teas in Hydroponics or Food Crops

While compost teas have their fans, they’re not without significant risks, especially when applied to edible plants:

Contamination Risks

These are not sterile brews. In uncontrolled conditions, you risk cultivating:

• E. coli
• Clostridium
• Anthrax
• Legionella
• Even bacteria commonly found in… well, the human “nether regions”

That’s why I do not recommend using compost tea on crops close to harvest or on any fresh produce that won’t be cooked.

Spray Nozzle Blockages

Another issue? Physical clogging.

• Filamentous microbes and fungi can grow large enough to block spray nozzles (150–200 microns)
• Biofilms from bacterial interactions can form sticky masses that gunk up your system
• Even originally soluble additives (like seaweed and molasses) may clump or react after brewing

This is a big headache for large-scale farmers spraying 7,000-litre tanks across their fields.

The Science of Quorum Sensing: Why Compost Teas Might Actually Work

One exciting aspect of compost teas is quorum sensing—a phenomenon where microbes communicate and change behaviour when they sense a crowd.

• Planktonic bacteria (floating solo) express different genes than bacteria in biofilms (living communally)
• When multiple microbial species interact, they can activate beneficial traits not present when isolated

This might explain why compost tea effects seem greater than the sum of their parts. And why critics may overlook their potential benefits.

✅ When Should You Consider Using Compost Teas?

If you’re an ornamental grower or have long lead times before harvest, compost teas might offer real value. But if you’re producing leafy greens, salad crops, or microgreens—steer clear unless you have full control over microbial inputs.

💬 Final Thoughts

Compost teas are fascinating, complex, and—when used with caution—potentially powerful. They’re not without downsides, but understanding how they work biologically can unlock serious benefits.

Article by Dr Russell Sharp

If you would like to keep up to date with subjects just like this, you can listen to both our podcasts! Links can be found bellow:

Hydroponics Daily Podcast: https://open.spotify.com/show/207T7p7fw9sPjINfSjVXW2

Cereal Killers Podcast: https://t.co/eSEbBkTVHl