Tristan Aoki's herb garden in Fushimi, in southern Kyoto, sits on alluvial soil that has been worked for vegetables for at least two hundred years. The neighbours' grandfathers grew daikon on the same ground in the 1950s. The soil is forgiving, but it is not, by any measure, rich.
In the spring of 2024 a colleague at the Kyoto Botanic Garden gave Aoki a small five-gallon brewer for actively aerated compost tea, a contraption of a bucket, an aquarium pump, a length of tubing, and a mesh bag for the compost. The colleague was a believer. Aoki was not, exactly, a skeptic, but he had been gardening for long enough to want to see the evidence on his own ground.
He decided to run the brewer for two full seasons and to take notes.
Actively aerated compost tea, in its standard formulation, is a brew of finished compost, water, and a small amount of unsulfured molasses, kept moving and oxygenated for twenty-four to thirty-six hours. The aeration multiplies the aerobic microbial populations in the compost, and the resulting liquid is meant to be applied within a few hours of brewing, before the microbes settle out and the brew turns anaerobic.
The claims made for it are large. Better plant health, faster nutrient cycling, suppression of foliar diseases, improved root development. The peer-reviewed literature is, depending on which review one reads, either supportive of some of these claims or skeptical of most of them.
Aoki set up two parallel beds in his garden in April 2024. Both were planted with the same varieties of basil, shiso, and a bush tomato called Stupice. Both received the same compost amendment in the spring, the same mulch, the same watering schedule, the same weeding.
The only difference was that Bed A received a weekly drench of actively aerated compost tea, brewed from his own kitchen-and-garden compost, applied at the rate of one gallon per ten square feet. Bed B received an equivalent volume of plain rainwater.
He ran this comparison from April through October 2024, then repeated it in 2025 with a different pair of beds and different crops — bush beans, cucumbers, and a Japanese leaf called mizuna.
The results across two seasons were modest. The compost-tea beds did not, by his observation, produce noticeably more food than the control beds. The first-season basil in Bed A was perhaps slightly larger by mid-July; by harvest the two beds were indistinguishable. The tomatoes were a wash.
In the second season the compost-tea beans were marginally ahead at the four-leaf stage and not ahead at harvest. The cucumbers produced about the same. The mizuna was, if anything, slightly worse in the tea-treated bed, which he attributes to chance.
What he did notice, and this is where the report becomes harder to dismiss, was that the soil in the tea-treated beds looked and smelled different by the end of each season. It was darker in the top inch, slightly more crumbly, and had a stronger fungal smell when lifted. The earthworm count, when he sampled with a hand fork in October, was higher in the tea beds — averaging eight worms per spade compared with five.
Whether this difference in soil biology will translate, over years, into a difference in plant performance is a question he cannot answer from two seasons.
He is going to keep brewing, on a reduced schedule. Once a month rather than once a week, and with less attention to the specific claims for plant health and more interest in what the tea may or may not be doing to the soil itself.
He is also more cautious now about recommending it to other gardeners. Compost tea is not a miracle. It is, at best, a way of distributing the microbial life of a good compost more widely across a garden than the compost itself can be spread. Whether that is worth the bucket, the pump, and the molasses depends on the gardener.
There are, he will admit, easier ways to add microbial life to a soil. Mulching with finished compost, top-dressing twice a year, leaving the soil undisturbed. These do most of what the tea is meant to do, more slowly and with less equipment.
Aoki keeps the brewer on a shelf in the tool shed now, between a stack of plastic seed trays and a bag of fish meal he is using up. He brews a batch on the first Sunday of each month from April to October. It takes about an hour of his attention, spread over a day, and he applies the result to whichever bed needs it most.
He does not expect dramatic results. He has not, over two seasons, had dramatic results.
He has had, perhaps, slightly better soil. That, in a garden, is usually enough.