Living SoilEvery few years a new product promises to transform your soil. Right now two of the loudest are compost tea and biochar. You may have seen brewing kits with bubbling air pumps, or bags of black biochar.
Here is a question worth asking before you buy either one: what does the research actually show? Not the testimonials, not the before and after photos. The controlled studies.
Compost tea
Compost tea is made by steeping compost in water and applying the liquid to plants. Aerated tea has air bubbled through it while it brews, which raises the oxygen in the brew and shifts which microbes grow in it.
The research is unusually direct about it:
- Few controlled, replicable studies support claims of compost tea's effectiveness. As one extension page put it, anecdotal evidence abounds, but controlled experiments do not.
- Aerated compost teas have no scientifically documented effect as disease suppressors.
- In one study on cherry brown rot, the tea did not reduce disease rates, and in some varieties it made infection worse.
There are safety cautions too. In some cases compost tea harms plants, and, of greater concern, it may harbor human pathogens. Overuse and runoff could also add to water pollution.
To be complete: one home composting guide from an extension service does recommend compost tea and describes benefits to plants and soil. So references disagree. But the pages that weigh the research, including that same university's guide for organic farms, find it unproven.
Biochar
Biochar is a charcoal like product, made by slowly burning plant material without oxygen. It works on soil much like organic matter does, through its physical structure. It contains no nutrients itself.
The trials are mixed:
- One set of vegetable trials found no statistically significant increase in yield from biochar.
- Where yields do rise, the gain may be marginal and may not outweigh the cost of the biochar.
- Excessive biochar can reduce yields by tying up nutrients so plants cannot use them.
- Applying raw biochar alone in poor soil has shown little benefit and may slow plant growth for at least 6 months.
Even the reviews disagree with each other. One extension page summarizes trials as showing mixed results; another page from the same university describes mostly positive results. And for home gardeners, there are not yet solid application rates at all.
So what should you do instead?
Notice what both products are trying to supply: microbes and organic matter. The soil food web already supplies the microbes, for free, as long as it is fed. And soil microbes need a constant supply of organic matter, or their numbers decline. Finished compost spread on the surface, mulch and cover crops deliver exactly that, with a long research record behind them.
That is not a rule against experimenting. If you want to try biochar or tea in one bed and compare, go ahead. Just know that you are running an experiment, not following proven advice. For the proven version, start with how to make living soil.