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What are compost teas - do they work?

Tony Callaghan 24/06/2022

What are compost teas - do they work?

What are compost teas?

Liquid plant feeds made using microbial digestion (decomposition) of plant material.

How are Compost teas made?

Compost teas are usually made at home using one of these starting materials/methods:

1) Compost (or worm) leachate which is then diluted 

2) Anaerobic digestion of green leaves such as comfrey or nettles to create a liquid digestate

3) Using a solution made from finished compost and 'brewing' it up in the liquid form

Compost (and or wormery) leachates

Simply collect the liquid oozing out of your compost bin, dilute it (10:1) and spread it on the soil. Usually only a wormery or hot compost system (e.g. the HOTBIN) processing “wetter” food waste will generate enough leachate to collect. You need to take care to only collect the dark brown liquid. Ensure it has no odour.

If it smells putrid/pungent it has gone anaerobic. We suggest you leave for a few days to neutralise.

If it smells sweet and fragrant, more likely you have washed out the organic acids produced by microbes early in the composting process - again leave it to brew for a few days.

Anaerobic digestion of green leaves 

There are lots of recipes using comfrey or nettle leaves to make compost teas. The leaves are placed in a sealed container (often just black plastic bags tied off) to prevent air (oxygen) from getting in. Anaerobic microbes break down the leaves to leave a black liquid/sludge which is then diluted and spread on the soil. The liquid often has a very strong putrid odour.

I’m not a fan of spreading anaerobic digestion materials straight onto soil.  We know that soils need to be aerobic as roots need oxygen. That’s the gardening psychology for you. The science says aerobic or anaerobic – eventually, the nutrients in the leaves get recycled back into the soil.

Brewing compost teas

Here you take FINISHED mature compost and put it into a filter bag (usually an old pair of tights, but you can use nut-milk filter bags available on eBay). The filter bag (with compost) is added to a bucket of water along with a small (sometimes precise!) amount of sugar (e.g. molasses) is added as microbial food. Air is bubbled into the water using fish/tank pumps/stones to add oxygen. There is a rapid expansion of certain microbes as they eat the sugar. The philosophy is that these bacteria with the nutrients from the compost “jump-start” the soil microbiology.

Which compost tea is best?

Most of the online reports on the benefits of compost teas are anecdotal rather than peer-reviewed scientific papers. Home users often report their recipes and method achieve magnificent results. Peer-reviewed papers seem few and far between. We will keep searching. 

If we look at the basics: What do plants need?

Plants need nutrients. Plants can only take up nutrients when they are of a specific type of chemical molecule/ion. Example: plants need nitrogen, but can only take up nitrogen as the nitrate ion (NO3-) or ammonium ion (NH4+). Don’t confuse chemical ions with the chemical industry making fertilisers. The plant cannot distinguish between biologically derived (natural) nitrate ions and man-made nitrate ions from fertilisers.

Where do the nutrient ions come from?

There is no ability for planet earth or mankind to make new nitrogen atoms. The nitrogen we have (and always have had) just gets recycled via the nitrogen cycle. Within the cycle, ‘microbes’  decompose plant matter to release the nitrogen back into the soil as nitrate or ammonium ions. The plants then absorb and use the nitrogen to make new amino acids and growth - the cycle starts again.

Estimates indicate about 80% of plant species co-operate with soil microbes to increase the uptake of nutrients. This is termed symbiosis and is often summarised using the mycorrhizal fungi–root relationship where the plants supply sugar to the fungi via the root hairs in exchange for the fungi collecting and providing phosphorus to the root. This implies that healthy plants (in soil) need a symbiotic root-microbial system.  The microbe-root interaction is important but we also need to keep in mind that many plants can grow

i) without soil e.g. most of the lettuce you buy today is grown hydroponically, i.e. in nutrient-rich water

ii) in soils with very few microbes e.g. crops grown in long-term heavily fertilised soils have very poor ‘microbial’ populations.

The discussion around natural soil systems (organic) and agricultural methods in terms of what is best for food security, £cost/Kg, human health and the planet’s health will continue.

All compost teas and brews are attempting to add either nutrients or beneficial soil microbes or both to your soil.

Nutrients from liquid (compost teas) versus solid (compost)

Liquid teas disperse and make dissolved nutrients available faster than solid compost with nutrients still in solid form (e.g. amino acids in old plenty matter). The same is true for man-made fertilisers - liquid fertilisers act faster than solid fertilisers,. 

If you start with 1Kg of the plant material which has for example 3g of nitrogen, whether you compost or brew a compost tea, the maximum amount of nitrogen in the final product is 3g. Any change in performance between the methods is more likely caused by changes in the concentration/dilution of the solution you use.

Our advice is to follow the compost tea recipes and use them sparingly until you are happy that they work on your soil with your plants.

Microbes from liquid compost teas versus solid compost

Things are a bit more complex where the goal of adding compost tea is to add microbes to the soil. Healthy soil is a very complex ecosystem of microbes (bacteria, fungi, protozoa, nematodes, etc). Building up this healthy soil-food web can be a complex task. You need to understand where you are starting from and what the end point looks like and the steps in between.

If we assume the compost tea is rich in good microbes and we add these microbes to the soil... 

If your soil is already lacking in microbes, then it follows that it lacks an environment that supports healthy microbes. Adding microbes (Inoculating) poor soil might not work as the new microbes have nothing to live on and so might die off. To continue to grow more bacteria in the next stage,  you need more food for the bacteria.

For me, the arguments for teas get a little hazy from here. Almost everyone agrees a healthy soil starts with a good dollop of organic matter – i.e. compost. This supports and builds microbes which then support the bigger soil-food web. If we need compost for the microbes to use, then inoculating without also adding compost seems at odds. 

Right now this suggests (to me) that adding a layer of compost to the soil followed by heavy watering would be the best combination.

When you make a compost tea using compost, does the tea ‘wash out’ other things from compost?

What do compost teas leave behind?

When you add a spade full of compost into the water and slush it around, you do not only wash out nutrients and microbes into the solution, but you also wash all the small particles that will fit through the sieve (often the sieve is old tights!). This means you wash the humus into the liquid. Humus is hugely beneficial to soil structure and performance. I see this as good news.

When we do experiments with washing compost, what is usually retained on the sieve is bits of wood and partially degraded organic matter.  It is highly debatable how valuable they are. The fine humus is the most valuable. The partially degraded organic matter will decay and eventually release nutrients (good). The bits of wood will affect soil structure (tilth and aggregation - this can be good (heavy clays) and bad (sandy soil). Woody bits will only decay if you have fungi with lignin-degrading enzymes. These are not always abundant in flower beds/grass meadows soils.

We have asked Eddie over at Rhizophyilla (expert in the Soil Food Web methodology) to compare a brew made from SF60 with just a solution flushed out from just SF60.  Eddie has years of compost tea brewing and the microscopes to look investigate what is really going on.  We will update the blog with when we have the results.

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