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Compost teas – do they work?

James Beattie

James Beattie

December 9, 2011

A couple of nights ago I had a discussion with a friend of mine who was extolling the virtues of using compost teas on the garden. Applications of said tea included soil drenching and foliar applications, the latter supposedly playing a role in disease treatment and prevention. It sounds great, I know, but do compost teas have any efficacy on plant health at all? Are the claims made by their proponents, who have included Australia’s premier horticultural TV program, Gardening Australia, true?

Firstly, what exactly is compost tea? It is what it says it is – a dark tea that is essentially compost leachate. It is made by soaking compost or any other form of organic matter such as animal manures in water to leach the nutritional goodies out of said matter. The liquid made from these teas is then applied via irrigation or foliar applications to plants and gardens. It is supposed that not only nutrients are leached out of the organic matter but microflora such as fungi and other beneficial microbes are too. When applied to plants and soils such constituent nutrients are said to have a whole raft of positive effects on plants and soils, from nutritional benefits that boost plant performance to an increased number of microbial and fungal species present in soil profiles. First I’ll deal with the process of making compost tea and talk about the liquid that is gained from following this method, then I’ll deal with its two main modes of application – soil applications and foliar applications.

For the sake of this article I’ll assume that we’re making compost tea out of a batch of perfect hot compost, ripe for applying straight onto the garden, or in our case, making a compost tea. The compost is mainly humus – originally plant matter that has now broken down into material that cannot break down any further. It has a dark brown or black colour and a spongy texture that can hold almost its entire weight in water. Compost will vary in its NPK values – this depends entirely on which constituent ingredients went in to making it. However, NPK values in properly broken-down, hot compost are typically low, below 1-1-1. It is the humus in compost that is of most benefit to gardeners. So we have been leaching our bag of compost in a bucket of water for some time. Our water is now a dark colour, almost black and ready to be used as a compost tea on our gardens – but what does such a brew contain?
Contrary to popular belief, the nutritional value of such a brew to plants is low. Lower still is the presence of beneficial soil microflora and fungi – the reason for this is that since our bag of compost has been soaking in a bucket of water it has become anaerobic. That is, what was once a mix of lovely oxygen-rich humus, replete with healthy populations of microflora and fungi, has since been exposed to conditions in which oxygen exchange is typically very low (still water), and our beneficial microbes’ populations have taken a bit of a beating. Some nutrients have been leached from the compost, but nowhere near enough to consider the brew an alternate form of fertiliser for your plants.

I will acknowledge here that some proponents of compost teas will actually aerate their leachate before applying it to the garden. Aeration is typically achieved with the aid of electrical pumps that either agitates the water by moving it through a series of troughs or simply bubbles oxygen through the water. This effort is made because it is thought to make a once anaerobic brew aerobic again, which is exactly what aeration of such brews achieves. However, what aerobic life has been breathed into the mix (which is a negligible mount anyway) is soon made anaerobic again when the oxygenating pump is switched off. Using compost teas is also billed as a way to supplement your existing fertilising regime, which is a good use of it, but the overall benefit of applying such teas is not proven to result in better plant growth – so why do people go to the trouble?

Applying compost directly to the soil is such an easier method, and far more timely than going through the process of making a tea with it. Compost teas just don’t have any extra benefits on either plant or soil health that compost has when applied on its own. Once you apply the compost, every time you irrigate or it rains, a ‘tea’ is made as the water percolates through the soil profile. This is, of course, if you top dress with compost instead of digging it in – which is what I do in my home garden. I then mulch over the topping of compost and the worms do the rest. There is an abundance of evidence out there proving that applying compost to soil regularly results in higher soil water retention, increases soil nutrition, improves soil structure, and can alleviate compaction – all things that result in the healthy, strong growth of plants that are better able to defend themselves again insect predation and pathogen infection. Compost performs better on its own as opposed to a tea because there isn’t a stage at which it goes anaerobic, so the slow release of the humus into the soil always occurs under conditions in which there is plenty of oxygen being exchanged between the microscopic goodies and the wider atmosphere – so the compost never has a chance to go anaerobic. I suspect that applying compost directly on the garden results in far better soil microbial diversity than making a tea with the same compost.

There has been some evidence to suggest that using compost tea as a foliar spray can overcome fungal problems, although this has only been found to be true with powdery mildew on grapes – but only then in greenhouse conditions (i.e. experiments), and has not been able to be replicated in the field.

So the short of it is, compost teas are at best an unnecessary supplement to your existing fertilising regime, and at worst a waste of your time and effort. It is much easier to dump a load of compost on a garden bed then mulch over the top of it than it is to make a tea – and far better for your soil and plants’ health too.

Happy gardening!

+ Compost image by normanack source

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tasos peris
tasos peris
10 years ago

what is the ratio compost tea and water for foliage spray or root application and how often we can do this application?
Warm Regards
Tasos Peris

James Beattie
James Beattie
10 years ago

Dear Tasos,

Thank your for taking an interest in Garden Drum!

Unfortunately I’m not able to help you with your inquiry – if you reread the article you’ll find I’m not an advocate of using compost teas as there is no evidence to suggest they are of any use in the garden at all.

Bill Watts
Bill Watts
9 years ago

Hi James,

While it is true that the stuff you refer to is sometimes still referred to as compost tea, discussion of compost tea these days tends to refer to “actively aerated compost tea (AACT)”, which is a very different thing. While the process maybe similar, AACT contains a variety of sources of microbes, food (preferably a variety of foods) for the microbes, and is aerated throughout the whole process, from the very beginning until it is used. Aeration is of vital importance, and requires something more than a small aquariium pump. Disolved oxygen levels must be kept above 6ppm. Done properly, the process results in a huge increase in microbe numbers, and it is these microbes that provide the beneficial effects. It is not intended to be a fertiliser in itself.
Kind regards, Bill Watts

8 years ago

Bill Watts has done an exemplary job at explaining the value of the scientifically proven benefits of compost teas by clearly distinguishing the difference between steeped and aerated teas. When we examine them under the microscope, they are like chalk and cheese. In any case, the results in the garden or farm speak for themselves.