Compost Extracts 101
Why does a small amount of solid compost go a long way as a liquid? ➕How-to make an extract
As temperatures cool and fall planting season begins, it’s a great opportunity to nurture the microbiome of our soils. Today is a how-to on the basics of making a compost extract.
In this post we’ll cover the following:
Why use a liquid compost
What is a compost extract, how does it differ from compost tea
Easy DIY for making a compost extract
When & How to apply a compost extract
Why use a liquid compost?
As you may know, microorganisms are essential to establishing and maintaining healthy plants and functional soils. Compost can be an incubator for raising beneficial soil microbes.
The solid compost you work so hard to make (or spend good money to purchase) can cover a much larger area as a liquid soil drench than it can as a solid topdressing. Liquid composts essentially provide a medium to transport microbes into the rhizosphere (root zone).
I rather see people making/buying a very small amount of biodiverse compost and applying it as a liquid than making/buying several cubic yards of cheap GOM (ground-up organic matter) i.e. industrial / conventional compost to apply as a topdressing.
In fact, I’d go so far to say that in most cases a topdressing or tilling in of GOM is a disservice to soil health.
Why? Because most of our soils are ecologically disturbed, and the characteristic of disturbed soils is bacteria dominance and low biodiversity.
Do you know what else has bacteria-dominance and low biodiversity? GOM. 🙈
It’s like if we had a “mouse problem” and our approach to fixing it was to introduce MORE mice… nonsensical.
When you know this, you can’t unsee it and my hope is that we stop using something that has not and will not move the needle on soil health.
(You can learn more about GOM and why not all composts are created equal from previous posts)
Why are we applying bacteria-dominant compost that has low biodiversity onto disturbed soils that are suffering from bacteria-dominance and low-biodiversity?
Lack of mainstream awareness.
What is a compost extract?
Extracts are made by “extracting” microbes off of solid compost and releasing them into water.
The intention behind an extract is to get microbes living in a solid compost into water so they can be applied as a liquid to soils and plants.
Teas begin as an extract and then undergo a brew cycle where the liquid is aerated and microbial foods are added.
The intention behind a tea is to increase the microbes by feeding them and providing oxygen.
Why I advise MOST people to NOT make teas:
A tea sounds like a good idea because, in theory, you’re able to increase the amount of microbes before applying them to soil or plants. In reality however, teas are really easy to mess up and require a microscope and tedious monitoring to get right. Furthermore, they can take an unpredictable amount of time to reach peak microbial communities and have a very short shelf-life (hours or less).
IF you’re up for the challenge, teas can be a very useful amendment. Teas are particularly good for foliar applications because the actively metabolizing microbes in the tea produce exudates which makes the liquid “sticky” and more likely to adhere to leaf surfaces - introducing beneficial microbes to the aboveground parts of plants (an often overlooked zone of plant-microbe interactions!).
Take Home Message:
Don’t f*ck with teas unless you’re a detail-oriented nerd with basic microscope skills.
If you’re looking to keep things more efficient and practical, stick with extracts.
How to make a compost extract
Materials
5-Gallon Bucket
Mesh Bag
Biologically Active Compost
Watering can
Fill 5-Gallon Bucket with water: fill to about 4 - 5 inches from the top so it doesn’t slosh everywhere
Put Compost in Mesh Bag (~400 micron openings): fill the mesh bag up to half-way or less of the total bag volume - we want plenty of room to massage the compost without any falling out
Tip: a good rate to aim for is ~6-12 cups per 4 - 5 gallons of water; depending on the size of the bag, this may mean you do two rounds of extractions into one bucket of water
Dry Massage: break up aggregates in the compost by gently massaging for about 1 minute before dunking into the water.
Tip: I like to hold the bag over the water as some compost “dust” will fall out. 🧚♂️
Dunk - Swish - Massage: this is where the extraction happens, for about 5 minutes move the bag around in the water in every which way stopping every now and then to massage the bag of compost under water. When you’re done just squeeze the bag out like a sponge over the bucket and set aside.
Tip: If you’re too delicate you won’t get a good extraction of microbes, if you’re too rough you might kill a lot of the microbes. So, be firm but avoid rigorous friction movements.
That’s it! Your extract is ready to use.
You can put the spent compost back in an active compost or use it to mulch a potted plant.
If you leave extract sitting for too long (several hours or more) it will become anaerobic and unfriendly microbes will replace beneficial microbes, so just plan to make it at the time you’re ready to apply it.
Remember, an extract’s quality will only be as good as the quality of the solid compost it is made from.
If you want to know more about sourcing and / or making biologically active compost, let me know in the comments and I’ll plan for a future post!
📈Of course, this can be scaled up to larger totes for more industrial applications. I’d recommend Hiwassee extractors for farmers looking to make their own extracts at scale.
When & How to apply compost extract
Application Method: for small scale garden applications, you can simply use a watering can. For a larger scale you’ll need to use a sprayer or drip line of some kind. The important thing here is to make sure the sprayer isn’t killing or filtering out the organisms (a thing I help land managers investigate 🔬).
Apply to Moist Soil: The goal of an extract is to get it to soak into the soil, effectively introducing microbes to the root zone. When the topsoil is dry, extracts may run-off or evaporate before being incorporated into the soil, so plan to apply after a good rain or irrigation event.
Timely Considerations: You can apply extracts pretty much any time of the year. Some especially optimal times are during transplanting or seeding events or when established vegetation is emerging from a dormant season. Lastly, be considerate of the environmental conditions. For instance, applying at the hottest time of day in the summer or just before a freeze or big storm may not be the most successful application.
Application Rate: for most cultivated or developed soils, I’d expect ~20 gallons / acre to be a good target application rate especially early in a restoration effort; for healthier, more functional soils you can go as low as ~5 gallons / acre.
Some land managers opt to test their soils and compost and let that inform their application rate and frequency - these are the kinds of things I help commercial-scale sites with through my business, Rhizos.
📱THE MATHS
Most compost topdressing would be applied ~4 cubic yards+ / acre
A generous extract application (~20 gallons of extract / acre) would require only about 30 cups of solid compost / acre or < 1% of a cubic yard / acre!
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TAKEAWAYS
Extracts help us get microbes where we want them
Use a biodiverse compost for making extracts
Extracts are more fool-proof to make than teas
Using just a few cups of biodiverse compost for an extract is a more effective inoculant than topdressing with yards worth of average compost (GOM)
Making extracts is easy - make a batch in a few minutes and use right away
Apply extracts to moist soil (after rainfall or irrigation)
🤎Thanks for reading!
Hi Andy . Thank you for such a wonderful and useful post . I have two comments . It's my understanding that worm castings are also excellent feedstock for the compost extract. The most reliable sources for complete bioactive castings, AKA vermicompost, are large scale worm farms, which will ship castings anywhere, or homegrown worm bins, rather than retail outlets. Second comment: Regarding the type of water for extract, the post does not specify rainwater or dechlorinated, versus tap water. Your recommendation? From Sharon C., Elgin, Texas.
Thank you for this! I'm glad to hear extract is more effective than top dressing. It is certainly easier to apply! I have a question. Why not just add the compost to the water and stir with a stick? I've been basically 'sloshing' the water out of the bucket. I thought the bag was for when you plan to use a sprayer. Also, what do you think about adding a little charcoal, fish emulsion and airariting for a couple hours before distributing? Or does that just add bacteria?