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Compost Tea

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Growers have long added nutrients to their gardens by soaking manure or compost in containers of water and pouring the resultant brew on their plants. But according to soil microbiology expert Dr Elaine Ingham, this traditional method of making “compost tea” can produce mixed results, particularly in terms of the vital microbial life the liquid contains. My first introduction to the horrors of what can happen when manures are allowed to ferment. A large number of the organisms in the ferment were human disease-causing organisms, or pathogens.

Animal manures contain large amounts of E.coli, but even when compost rather than manure is used to make tea, the bucket method can be problematic. When bacteria grow too quickly they use up all the oxygen, creating anaerobic conditions that favour pathogens and cause good microbes to go dormant.

If the compost is not mature and there are lots of available foods [for bacteria] in the compost it can cause anaerobic conditions, If the compost is mature, it is not as likely to result in something detrimental to your plant... rapid microbial growth won’t occur.

Weed teas generally turn anaerobic in the first week, the best thing to do is leave them for weeks or even months. When the food runs out, the bacteria stop growing, oxygen moves back into the tea and as it becomes aerobic the good microbes emerge from dormancy, growing slowly on the wastes that the pathogens produced.


Abundant life

To quickly produce a compost tea packed with good microbial life (bacteria, fungi, protozoa and nematodes), a different method is needed. These days a number of farmers and backyard growers are “brewing” compost tea using a constant, substantial supply of oxygen. This allows good microorganisms to multiply rapidly without turning the brew anaerobic. The result is actively aerated compost tea (AACT). It contains an abundance of biological life: the essence of organic farming.

Compost tea is becoming widely adopted by organic farmers and is used on a wide range of crops including vegetables, fruit, vines, cotton and cereals. It is used on trees, grass and gardens in public parks, on golf courses and bowling greens, and in the remediation of mining sites and saline and acidic soils.

Good compost tea requires good compost. To make this, assemble a minimum one-cubic-metre heap of balanced and diverse materials (50 per cent brown, such as twigs, dried grass and shredded paper; 40 per cent green, such as fresh grass and soft stems; and 10 per cent high-nitrogen, such as manure or legumes) and keep it damp but not wet.

Monitor with a temperature probe and turn the heap each time its temperature rises above 55°C and starts to drop, or if it reaches 65°. When you turn the heap and the temperature does not rise again, it is ready for tea brewing.  

If you suspend this compost in water and connect a pump, the turbulence knocks microbes off the compost and into the water. Give the microbes food, such as kelp, fish or molasses, and they breed prolifically.

Of course, you could introduce these microbes to your garden simply by applying compost. But spreading compost over large areas can be back-breaking and expensive. Compost tea is easier – a 20-litre brew made from a handful of compost can inoculate an acre (almost half a hectare) with good microbes. Compost tea can also be applied as a foliar spray, with potential pest and disease-fighting benefits (see below).

If you are not certain about the quality of your compost, author and soil expert Tim Marshall suggests enlivening it with recently collected liquid from a worm farm. “It is free and easily obtained and will definitely contain viable organisms,” he says.


Soil biology

Plants attract microbes to their root zones by giving off “exudates” of carbohydrates. Microbes feed on the exudates and each other, producing their own wastes and exudates, which becomes food for plants.

Any nutrients that plants don’t use are locked up again by other microbes, which means that – unlike chemical fertilisers – they won’t be washed away. Food is constantly being cycled, and made available for plants to take up whenever they need it.

As well as improving nutrient availability, a healthy soil food web will:

improve soil structure and reduce compaction: bacterial exudates are sticky and make soil particles stick together, while fungi, earthworms and arthropods move through the soil, creating pathways;  control disease: fungi and bacteria form protective nets around roots, outcompete pathogens or consume them;  decompose chemical residues and toxic materials.

In short, a balanced soil food web is what organic growers should  aim for. Chemical fertilisers, pesticides, land clearing and tillage destroy this biology. Compost and compost teas are ways of putting it back.


To buy or make a brewer?

Instead of the bucket or bin brew method, better results come from brewers that pump oxygen into the mix. Commercially made compost tea brewers vary in size from 20 litres to thousands of litres. Home-made brewers are cheaper and can be easy to put together. If you follow the right plan they can also be effective.

Home-made brewers can be as good as mass-produced brewers, the sticking point is aeration and good cleaning. 


How to use compost tea

Compost teas are used in a couple of ways. Sprayed (or watered using a watering can) on soil, they add soluble nutrients and biological life to that soil. Applied as a foliar spray, they allow these goodies to occupy the leaf surface so that baddies such as powdery mildew have nowhere to get a foothold.  Exactly how efficacious compost teas are in preventing disease has varied greatly in different trials. This may be because there are so many variables in tea-making. 

Compost tea used as a foliar spray will stick to leaves without adding a surfactant. Actively growing organisms make glue layers around their bodies in order to hold themselves on surfaces. To add microbial life that will help control soil pathogens and pests, Dr Ingham recommends an annual application of compost (a minimum of 3kg of compost added to a 1m seed row), or several applications of compost tea to soil each year.

If using a backpack sprayer to apply compost tea, it’s important not to kill the good microbes in the process. Nozzle apertures should be no finer than 450 microns. Fan jet nozzles are best, and pressure should not exceed 65 PSI (about 4.5 bar). It is best to spray during cooler parts of the day, when evaporation and UV levels are low. Dusk is ideal. Microbes take 20 minutes to adhere to leaves, so apply foliar sprays at least 20 minutes before rain. Soil sprays can be applied during rain.



Fruit trees grow best in fungal-dominant soils, whereas vegetables prefer bacterially dominated soils. Brewing the right compost tea will help give the plants the conditions they like best.

Commercial growers rely on soil tests to brew teas specific to their existing soil life and crops. For backyard growers these are a good place to start. Each recipe makes 20 litres of tea, which will cover one acre (0.4 hectare), but can be used on a smaller area: you can’t apply too much.


General purpose tea

(balanced fungal-bacterial brew)

20 litres water

80ml fish hydrolysate*

40ml liquid kelp

200g compost

Add water to bin. If using town water, turn on pump and aerate water for 30 minutes to get rid of chlorine. Add food (fish and kelp) to water. Put compost in ‘tea’ bag. Brew for 24-30 hours.

Vegie patch tea (bacterial brew)

20 litres water

30ml fish hydrolysate*

60ml liquid kelp

10ml blackstrap molasses

200g compost

Three days before brewing, feed compost by mixing 10ml fish hydrolysate with 20ml of water and sprinkling over the compost. Keep compost in a cardboard box (the cardboard will absorb any excess moisture and allow compost to breathe) and keep in a warm spot (20-30°C). After 2-3 days the compost will have a ‘fuzz’ growing over it – this will be laden with fungal spores. After the compost has ‘fuzzed’ you are ready to brew. Brew as per general purpose tea, above.

* Fish hydrolysate is made from whole fish, broken down by enzymes rather than heat. It contains fish oils that are good food for fungi. The more commonly found fish emulsion is less effective at growing fungi but will suffice as a substitute if need be.

Note: In hot weather reduce food and brewing time. With overnight temperatures above 20°C, brewing will take only 18 hours and food should be halved. Teas should be used within four hours of brewing, before oxygen and microbe levels drop. A quick alternative to brewing compost tea is making compost extract. The same equipment is involved, but only compost is brewed, with no added food. It can be used after just 20-30 minutes of brewing. It will be lower in microbial life but still effective. Food for the microbes can be added to the tea just before it is sprayed.





  • 1 x 60-litre wheelie bin
  • 1 x 90-watt aquarium blower (minimum
  •  200 litres a minute)
  • 1 x 450-micron tea bag
  • 2m 20mm electrical conduit
  • 2m 20mm corrugated electrical conduit
  • 1 x 20mm elbow
  • 2 x 20mm joiners
  • 2 x snap-clips
  • 1 x rubber chair leg tip
  • 20cm clothesline or other plastic cord
  • Drill bits: 3mm and 25mm

Step 1. Drill a 25mm diameter hole in the front left corner of the lid, in front of the handle. This is where the vertical aerator pipe will sit. Using strong scissors or pruners, cut the lip out to open up the hole. This enables the lid to be opened with the pipe in place.


Step 2. Cut a piece of the 20mm electrical conduit so that it sits in the bottom left front corner of the bin, and sticks up above the lid about 150mm.


Step 3. Put a 20mm elbow on the bottom of the vertical pipe. Measure from the front of the elbow diagonally across to the back right bottom corner of the bin. This is where the horizontal aerator pipe will sit. The aerator pipe will go into the elbow about 20mm, and in the other corner will have a rubber chair leg tip on the end. Cut long and be prepared to trim, to make a snug fit.


Step 4. Drill a hole in the left rear corner of the bin, just below the lip, and the right front corner just below the lip. Thread clothesline plastic cord through the holes and tie knots to hold in place. Use snap-clips to hang tea bag from clothesline. The top of the bag should sit well above water level and be reasonably taut.


Step 5. Connect about 1.5m of corrugated electrical conduit to the vertical aerator pipe using a conduit joiner. Use another joiner to connect the other end to the pump outlet. (The pump outlet may need tape wrapped around it to ensure a snug fit.) Step 6. Drill two rows of 3mm holes in the horizontal aerator pipe. The holes need to be 20-25mm apart, and located at the 5 and 7 o’clock positions (if the pipe is sitting on the bottom at the 6 o’clock position.)

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last year I used a nettle tea in my backyard

because it is quite cold and plants grow slowly,

good results for tomatoes.

now I am trying with my little white strawberry

and it seems she likes it.

I made it on july so it should be ok,

I made same more in september but it is still fresh

and I am not positive I can use it

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