Compost Tea

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There are several forms of liquid fertilisers that most people are familiar with. The commonest are liquid fertilisers such as the commercial fish emulsions or soluble fertilisers. Alternatively, there are manure or compost ‘teas’ which are made from materials such as manure soaked in water. Herbal teas, such as those made from comfrey, are also useful for providing nutrients to plants. Most such liquids can be applied as a foliar spray or directly on the soil around plants.

Brewed compost tea, however, is completely different to these products. It is brewed aerobically, and this process grows millions of beneficial microorganisms to feed the soil and help combat disease. This type of compost tea is increasingly being used in permaculture and on organic farms as a way of improving soil fertility naturally to produce strong and vigorous disease-resistant plants. It is quite easy to make your own compost tea at home to try out its properties for yourself, and there are also commercial products available.

There are many articles and websites about making brewed compost tea. An article by Elaine R. Ingham, who is the president and director of research at Soil Foodweb, Inc. and a professor at Oregon State University in Corvallis claims that compost tea is more beneficial than simply applying compost to the soil in the traditional way. She states that compost tea “makes the benefits of compost go farther, helps suppress foliar diseases, increases the amount of nutrients available to the plant, and speeds the breakdown of toxins”.

There is sound science to support these claims. As Ingham points out, the soil is full of both ‘good’ and ‘bad’ microorganisms. The goal of gardeners is to enhance the beneficial microorganisms in this soil foodweb. The ‘bad’ bacterial decomposers and the plant-toxic products they make are enhanced by anaerobic, or reduced-oxygen, conditions. However, compost tea is made through an oxygenated and highly aerobic process, which grows the ‘good’ bacteria and other microorganisms. Good bacteria work against the detrimental ones in several ways: They consume the bad ones, compete for nutrients, and they compete for space.

The brewing process in compost tea production extracts microbes from compost and then multiplies them. Beneficial microorganisms include bacteria; fungi and protozoa. Ideally the compost tea will contain both a vast number, and a great diversity of types of organism.

Brewed compost tea can be used in two main ways. Used as a soil drench it provides food and energy for microbes in the soil. As a foliar spray it combat disease and acts as a natural fungicide – for example for controlling downy and powdery mildew on grapes. When sprayed on leaf surfaces, the beneficial organisms in the compost tea eat up the exudates from leaves that would otherwise feed harmful organisms.

Making your own compost tea

To make good compost tea, you need good quality compost which has been made aerobically. That means turning the compost heap and ensuring it heats adequately so weed seeds and pathogens are killed.

Ingham describes how compost can be dominated by either bacteria or by fungi. For compost tea to be used as a foliar spray, it must be bacteria-dominated, whatever the plant. Bacteria-dominated compost is also best for applying to the soil before growing vegetables and herbs. Research has shown that a foliar spray of bacteria-dominated compost tea is extremely useful to prevent the foliar diseases that plague most gardens. Therefore the bacteria-dominated compost tea is the most useful for home gardeners although fungi-dominated does have some applications.

Ingham’s main points for making good bacteria-dominated compost are summarised below:

  • compost should be made from a preponderance of green materials – a mix of 25 percent high-nitrogen ingredients, 45 percent green ingredients, and 30 percent woody material. High-nitrogen materials include manure and legumes, such as alfalfa, pea, clover, or bean plant residues. Green material includes any green plant debris and kitchen scraps. Woody material includes wood chips, sawdust, paper plates and towels, and shredded newspaper
  • mix a whole pile at a time
  • make sure you have a mass that measures at least one cubic metre.
  • Moisten the pile as you make it so that it is damp but not wet.
  • The more you turn the pile, the more the compost tends to become bacterial.
  • after six to eight weeks, the centre of the pile is cool or barely warm to the touch. The compost is now ready.

Brewing and using the tea

The requirements for making brewed compost tea are easily obtained. There are many different recipes for making the brew – Ingham’s simple method is summarised below, but you may want to research other possibilities on the Internet. You will need:

  • some of your well-prepared compost
  • clean water (rain water, bore water, or town water that has been left to stand in the sun for a day to lose its chlorine)
  • a 25 litre plastic bucket
  • an aquarium pump that can run three bubblers (air stones), three bubblers. and a three-way gang valve to distribute the air to the three bubblers
  • a couple of metres of tubing
  • unsulphured molasses (preferably organic)
  • something to use to strain the tea (e.g. old nylon stocking)

Tea made with this method brews for two or three days and must then be used immediately. So plan your batch to suit your needs, such as making it on a Wednesday for use on the weekend.


Fill the empty bucket loosely half full of compost. Set up your pump and tubing so that the pump delivers air to the three bubblers at the bottom of the bucket. Hang the gang valve on the lip of the bucket and bury the bubblers at the bottom under the compost. Fill the bucket to within 7cm of the rim with water, and start the pump. When it’s going, add 30g of molasses, then stir vigorously. After stirring, you’ll need to check the bubblers to make sure they are on the bottom and well spaced apart. Stir the tea at least a few times a day. A vigorous mixing with the stick shakes more organisms loose and into the tea. Every time you stir, be sure to reposition the bubblers.

After three days, turn off the pump and remove the equipment. If you leave the tea aerating longer than three days, you must add more molasses or the good organisms will start going to sleep because they don’t have enough food to stay active. Let the brew sit until the compost is pretty much settled out, 10 to 20 minutes, then strain it into the other bucket or directly into your sprayer. You’ll have about 12 litres of tea. Use the tea immediately, within the hour if possible.

The solids can be put back into the compost pile or the garden soil, as there will still be good bacterial and fungal foods left in them.

Anaerobic decomposition

The most important thing in making brewed compost tea is for it to have sufficient oxygen, otherwise the tea will start to smell and become anaerobic, which can harm your plants. Ingham points out that “bad smells mean bad business”. Properly oxygenated compost and compost tea smells sweet and earthy. Anaerobic compost tea should not be used on your plants, as a product of anaerobic decomposition is alcohol, and even a very small amount destroys the plant’s cell walls. Ingham recommends that if compost tea smells bad, you can add another pump with more bubblers, and stir it more often, to aerate it until the smell goes away.

Using the tea

In a healthy your garden, spraying plants once in spring may be adequate . The beneficial insects in the garden should spread the compost tea organisms around the garden, preventing pest problems for the rest of the season. If you don’t have good levels of beneficial insects in your garden, you can spray at least once a month, or as often as every two weeks.

To control damping-off, spray the soil with full-strength tea as soon as you plant. On trees and shrubs, spray two weeks before bud break, then every 10 to 14 days.


‘Brewing Compost Tea: Tap your compost pile to make a potion that is both fertilizer and disease prevention’ by Elaine R. Ingham. From Kitchen Gardener issue 29, pp. 16-19

‘Notes on Compost Tea’ by Steve Diver, Appropriate Technology Transfer for Rural Areas, 2002.