Jeff Gillman: Benefits? Maybe. Problems? Likely.

Over the years, all kinds of fantastic elixirs have been formulated for the good of our plants, including humic acids, seaweed extracts, vitamin B1, and many other miracle cures. One of the elixirs currently getting a lot of attention is compost tea. What is compost tea?

Well, different people have different ideas, but in its simplest form compost tea includes water and compost. The compost is mixed into the water, allowed to sit for some period of time (perhaps even less than a day) and applied to plants. Air may or may not be bubbled through (aerated compost tea) and sugar of one sort or another may or may not be added.

All compost teas are not created equal, and I'm sure that Jeff L. will provide some insight into the different teas, but for the sake of brevity I'm just going to bash the whole lot of them.

The presumed benefits of compost teas revolve primarily around their ability to supply nutrients and, more importantly, provide beneficial microorganisms to the soil where the tea is applied —these microorganisms may help the plant acquire appropriate nutrition or protect it from disease.

The reason that I am opposed to the use of compost tea is very simple. There have been few benefits proven and there is some risk involved with their use. I'll give a summary here, but if you want a full-blown review you can see the National Organic Standards Board Compost Tea Task Force Report.

This task force recommends compost teas that are produced in particular ways (they list ten procedures that should be followed for compost tea to be acceptable; these are not simple "aerate and it will be OK" recommendations). While I certainly condone the methodology that they recommend for compost tea production IF you're going to use compost tea, I honestly don't see why this stuff is used at all.

Certainly adding compost to water and applying it will provide some nutrients. However, the idea that applying compost tea to the soil supplies beneficial microorganisms is misleading. I can't deny that it's true, but it's also misleading. You see, the earth around us is packed with microorganisms. Every spot of soil that you see that has not been purposely sterilized has these microorganisms in it (To find out about them read Jeff L.'s book—he and I may disagree about compost tea, but there is no better, easier and fun to-read book about soil life than Teaming with Microbes). Research over the years has shown us that, unless we do something physically to the earth, such as add organic matter, or nutrients, we will not be able to fundamentally change the microorganism makeup of that soil for the better for any length of time. We tend to find, almost invariably, that soils have the microorganisms that they need to support plant growth, unless we overdo it with pesticides or fertilizers, in which case the gardener would do well to back off of these inputs, add some compost (not compost tea) to their soil and wait a year to plant anything. As a side note it usually takes anywhere from a few weeks to a year for soil to reacquire appropriate microorganisms if it has been sterilized or disturbed.

The only evidence that we have right now for the benefits of compost teas is composed of testimonials. Good research needs to be done in this area. As a person who has reviewed countless testimonials on gardening products and practices over the last six years let me tell you how valuable they are—they're worthless. On the flip side, compost tea's ability to help control plant disease is a reasonably well researched. There is some evidence that compost tea may be effective against certain plant diseases, but there's more evidence that it isn't. (This is definitely an area that needs more research. See
Linda Chalker's Horicultural Myths)

Current evidence that compost tea may contain pathogens is pretty solid. We know that even small amounts of sugars in the tea increases the likelihood of these pathogens. We know that small, undetectable amounts of pathogens in the compost may mushroom into potentially dangerous levels in a compost tea brewer if small amounts of sugars are present. And we also know that aeration does not necessarily inhibit the growth of pathogens in the compost tea.

So, in short, my conclusion is that the benefits simply don't outweigh the potential problems of compost tea right now. This could change with more research and I'll be more than happy to eat my words. But without this research we're relying on faith that compost tea is beneficial while knowing that it may be harmful.