A Brief Primer on Compost Tea

By Jim Toler

What is Compost Tea?
A liquid form of compost with concentrated active and
dormant forms of microbes plus organic plant growth promoting substances.

How do you use it? Spray it on the soil and plant surfaces or inject it into rooting areas. It
may also be used to treat seed before or during planting and to control insects.

How does it help? It improves soil structure and the nutrient cycling capability of the soil.
Also, it improves pest and disease resistance of plants.

How does it improve soil structure and why is that good? Microbes create pore spaces in
soil through their ability to stick fine particles together, aggregate larger clumps with fungal
branches and tunnel through the aggregates. As fine structure builds, larger organisms can
inhabit the soil, creating even more pore space. This allows more oxygen to enter the soil so
even more biological activity can occur. It also allows the soil to act like a sponge and soak
up rain water, storing it for the plants to use and preventing erosion. Soil with good structure
is easier for plants to root into.

How does compost tea help nutrient cycling and why is that good? Plant nutrition is
mediated by the microbes and higher organisms in the soil. Through their digestion of
organic matter, they pass a steady stream of mineral nutrients to the plant roots. Healthy
soils, containing high numbers of diverse microbes have the capability of passing a full
spectrum of mineral nutrients, hormones, amino acids and vitamins to plants. By enhancing
soil biology, compost tea builds the capacity of the soil to pass nutrients to the plants.

What are the drawbacks of compost tea? It is of little value when microbial biomass or
diversity is low.
It does not have the same long term ability as compost to improve soil.

What are the advantages of compost tea? It is inexpensive.
It has demonstrated the ability to protect plants from
It has demonstrated the ability to improve forage
quality in pastures.
It has demonstrated the ability to improve plant stress
It can be applied directly to plant foliage, stems,
limbs, etc.

How is compost tea made? Modern compost tea “brewers” use either moving water or
air to agitate compost in a water bath to dislodge the bacteria, fungi, protozoa, nematodes

Phone (503) 585-9320

Willamette Organics
12390 Sunnyview Rd. NE
Salem, Oregon 97317
Fax (503) 585-9321 Web Site www.willametteorganics.com

and other small soil organisms, as well as compounds that promote plant growth. Microbial
foods are then added to cause the microbes to grow and multiply while the entire batch is
aerated to keep the oxygen level high enough to avoid fermentative conditions (anaerobic
activity). Most tea brewers are designed to produce a finished compost tea in about 20-24

How is compost tea quality measured? Early work with compost teas demonstrated their
effectiveness in controlling some plant diseases if the microbial concentrations were high
enough. Many laboratories will perform direct count bio-assays on compost tea to evaluate
its quality. Laboratories usually measure bacteria and fungi, and sometimes protozoa and
nematodes in compost tea.
What is required to make a high quality compost tea? There are two controlling factors
in the production of high quality compost tea. First, extraction grade compost must be used.
Secondly, controlled conditions in the brewer are essential.

What is extraction grade compost? Composts made with a wide variety of materials
are best for compost tea brewing because they tend to have a greater diversity of
microorganisms. For example, manure based composts are typically low in saprophytic fungi
but high in bacteria, while wood waste compost is usually high in fungi and comparatively
lower in bacteria. Protozoa usually do not have time enough to proliferate in a 24 hour
brewing cycle but if the brewing cycle is extended, they usually increase in numbers.
Nematodes will not increase in the brewer, so they need to be supplied by the compost. The
best compost for compost tea production is usually a blend of bacterial compost and fungal
compost that is activated by the addition of minerals and complex carbohydrates.

What production controls are necessary? There are four parameters (other than compost
quality) that control the quality of compost tea:
§ Water temperature – higher temperatures increase the rate of multiplication of bacteria
and growth of fungi, which can speed up the brewing process. At the same time, warmer
water holds less oxygen. Water that is too warm therefore, can cause the available
(dissolved) oxygen to be used up too fast, which will cause an anaerobic condition.
(Anaerobic conditions result in the production of plant toxic compounds.)
§ Water quality – the optimal range of pH for compost tea production is between 6.5 and
7.5. Also, water treated with chlorine or chloramines should be de-chlorinated before use
in the brewer.
§ Amount and types of microbial foods added to the brewer – if too much nutrient,
especially nutrients high in sucrose (sugar, molasses, fruit syrups, etc.) are added to the
brewer, microbial growth will outstrip the supply of oxygen, again causing an anaerobic
condition in the compost tea. If too little nutrient is added, a ‘weak’ compost tea will be
the result.
§ Brewing time – there is an optimum time period for brewing compost tea. Too short
a brew time will result in a weak tea containing unused microbial foods. Too long
a brewing time will usually result in a loss of bacterial and fungal biomass, a loss in
nematodes and frequently an increase in protozoa (protozoa feed on the bacteria and

Phone (503) 585-9320

Willamette Organics
12390 Sunnyview Rd. NE
Salem, Oregon 97317
Fax (503) 585-9321 Web Site www.willametteorganics.com

How should compost tea be handled and applied? Compost tea is a living substance.
It should be applied shortly after the end of the brewing cycle. It may be sprayed on all
above ground parts of plants for foliar feeding and disease protection. Addition of a sticker
spreader is very useful for foliar applications. Once applied to the foliage, the active
microbes in compost tea begin to attach themselves within 20 minutes and will not usually
be dislodged by a rain event. Compost tea may also be applied to the ground, including
turf, mulched beds or bare soil. When applied to the ground, it is almost always beneficial
to water it in well. To improve plant rooting in poor soils, root injections can be very

How much compost tea is needed? In agriculture, as little as 5 gallons per acre of compost
tea are often beneficial. This is of course added to enough water to evenly apply it, and
frequently added to the irrigation water. For landscape applications however, it is not
untypical to apply 10 gallons per 1,000 square foot area. Much depends on the purpose of
the application. When applying compost tea as a foliar spray to help protect plants from
disease, it is recommended that top and bottom leaf surfaces be sprayed. About a cup of
compost tea should be enough for an average rose plant.

What to expect from compost tea? While compost tea is always beneficial to soil and
plants, often the effects of one application are too subtle to notice. Occasionally, the plants
treated will receive an essential limiting nutrient or nutrients from the action of the compost
tea and very noticeable changes are seen. The truest benefit of a program of compost tea
application are the long term building of soil quality and the resultant hardiness of plants
resulting from it. Plants well rooted in healthy soil look better, resist diseases and withstand
stress better.

Is compost tea safe? When brewed properly, compost tea is safe for application to plants,
soil and harmless to people and animals. However, because the process of brewing compost
tea involves propagation of microbes, control of the brewing conditions is critical to safety.
It is possible to create a toxic batch of tea. This is usually the result of failure to maintain
high oxygen levels in the brewing process. When dissolved oxygen dips too low, organisms
that produce plant toxins multiply. The only way to ensure this does not happen is by
monitoring oxygen levels during the brewing process. Commercial compost tea brewers
usually come with a brewers guide to help avoid safety problems.

Beyond compost tea. Addition of soluble and chelated nutrients, as well as specific
microbial agents to compost tea can have a dramatic effect on plants. From simple
performance boost to plant rescue, enhanced compost tea is an important new tool in
agriculture and general land care. Addition of enhancing products is usually done just before
the application of the compost tea.