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Everything You Need to Know About Compost Tea

AlanGardenMaster
Published on May 12th 2022

by AlanGardenMaster. All rights reserved

A man holding a dilutor while standing in a garden
Compost Tea (CT) is a loosely used and often abused term that means different things to different gardeners! In this article I'll unwrap the mystery of just what good Compost Tea is and how to make it.

What is Compost Tea?

  • It's a brew of plant friendly organisms that is used as a micro feed but also as a way of growing healthy plants.
  • Well brewed compost tea is full of friendly bacteria, fungi, mycorrhiza, protozoa, nematodes and other beneficial organisms.
  • CT provides the same microbes and nutrients that well made garden compost does. Of course, where it differs from garden compost, is that it's in a more accessible liquid form.
A house with bushes in the background
AlanGardenMaster's old garden where compost tea was routinely applied

What is it not?

  • Compost Tea is not a feed made by steeping nettles, comfrey or any other plant material in water.
  • These homemade feeds have their place but, beyond being a free way of growing your own plant feeds, they lack the plant protecting properties that well made compost tea has.

Is Compost Tea new?

  • Compost tea is by no means new!
  • J Arthur Bowers (a famous name for producing potting compost) was one of the first in modern times to experiment with CT.
  • But Pliny the Elder beat Bowers to it! Apparently even Pliny wrote about the use of CT all those years ago!
  • Whilst not being new, CT has been used to grow healthy pesticide free plants in modern times.
  • With very stringent water restrictions, the Lowlands of the Netherlands and Belgium have lead the revival in the use of CT in Europe.
  • In USA there has been a widespread interest in the use of CT both by commercial and amateur growers. But a quick search of the internet reveals that there is much confusion and misnaming of plant based teas there.
A person spraying plants in front of a building
Diluted compost tea being applied to plants

How do I know about Compost Tea?

  • Back in 2003 I started to use CT to produce thousands of garden plants with dramatically reduced pesticide use.
  • My nursery and garden centre was in the forefront of the use of CT.
  • With around 600 varieties of hardy garden plants in production we were able to test CT extensively and found the results outstanding!
  • Regular visits to Belgium, Holland and USA gave me an insight into how to make and use good Compost Tea.
A close up of a flower garden
Healthy treated garden plants

Making Good Compost Tea

  • Obviously good ingredients are essential!
  • I sourced my CT ingredients from one of Holland's biggest green waste composters. We used a Belgium lab to monitor our brew.
  • But just as important is to brew under aerobic conditions.
  • We pumped fine bubbles for 24 hours into the water before adding ingredients.
  • With well oxygenated water we then added the ingredients and continued to bubble air through the solution for 72 hours.
A commercial compost tea brewing unit
A commercial compost tea brewing setup
  • Whilst our supplier would never divulge the exact ingredients we deduced that they were best quality composted green waste, fish meal, dried kelp, molasses a small amount of dried herbs.
  • We added a splash of cooking oil to reduce frothing.

Applying the Compost Tea

A group of people in a plant nursery
Compost tea being diluted and applied to nursery plants
  • CT can be applied to the whole of the plant above ground but it can also be applied to the soil.
  • After filtering the solids we diluted the tea 1 part compost tea to 10 parts water.
  • Again rainwater is best but not vital. The water temperature is not critical but should be close to the ambient air temperature.
  • This can be applied to plants by spraying onto the foliage until surplus runs off. A hand sprayer is perfect for small areas.
  • Since our nursery was quite extensive and covered several acres we used an air assisted back pack sprayer to apply the diluted brewed product.
A man spraying plants in a nursery
Air assisted back pack sprayer used to apply compost tea
  • Since weeds can sometimes harbour plant diseases I treated them with CT too!
  • Application to enliven the soil can be done at the same dilution rate and with a watering can.
  • Various home brew compost tea kits are available online and from Symbio

Benefits of Compost Tea

  • Firstly I must say that I don't really know how it works, just that it does!
  • I think that surfaces of the plant are inoculated with friendly organisms from the brewed tea and that when plant disease spores and infections arrive they find the plant surfaces already occupied.
  • Does it really matter if we don't know how it works? I don't think so because the end result for me was superb quality plants!
  • These were produced with the minimum of pesticides applied. Unfortunately powdery mildews were not controlled so some fungicide was used. We used baking soda and milk with some success in mildew control.

Powdery Mildew

Erysiphaceae

  • It's remarkable how much plant friendly wildlife returned to our nursery when we used CT! Frogs, hedgehogs, ground beetles, grass snakes and all manner of bird life.
  • But the big benefit that surprised me was that I could use more biological predators to control pests since the pesticides didn't interfere with them.
  • Since CT is a benign mix there is no need to wear the sort of protective clothing that is recommended when applying pesticides.
  • Hellebore leaf spot disease - a very difficult disease to control without fungicides - is eliminated with regular compost tea usage.
Hellebore leaf spot disease
Hellebore leaf spot disease

Why isn't CT more widely used?

  • Because small companies are involved in making CT recipes they, unlike the big chemical manufacturers, can't afford to finance extensive research testing. As a consequence growers remain skeptical.
  • The confusion over steeped plant feeds and true compost tea has lead to a slow uptake.
  • CT is a live product. It needs to be brewed and used within hours of switching off the brewing machine. The half-life is reckoned to be 10 to 12 hours so has no real shelf life.

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