A further complication is that the basic "bag in a bucket" method produces anaerobic compost tea; that is, whatever happens between the compost and the water, it happens largely in the absence of oxygen. If an effort is made to add oxygen, the result is aerated compost tea, which (probably) has rather different properties. The simplest way to do this is frequent, vigorous stirring, but the process can be automated. Symbio's most basic kit includes a bucket, an air pump, pipework and an air diffuser, rather like the ones you see in tropical fish tanks.
So once you've made your compost tea, what does it do? The best piece of research I've seen is a major project completed a few years ago by the UK Horticultural Development Council.
It used four different commercially available compost brewers to turn compost (based on green waste, so very similar to home-made compost) into compost tea.
They then used the compost tea as a foliar spray and checked its effect on growth, health and presence/absence of pests and diseases in a number of popular garden plants.
What did they find? In a glasshouse study on lavender and choisya, none of the compost teas had any effect on growth or on susceptibility to botrytis (grey mould) or spider mites. The results of a much larger trial, at four different nurseries and using a wider range of plants (the original two, plus cordyline, phygelius and a rose cultivar) were more complicated.
Sometimes plants treated with compost tea were taller, sometimes they were shorter, sometimes there was no difference. Sometimes plants treated with compost tea were of higher quality, but often they weren't, and treated choisya plants at one nursery were significantly worse. Generally, there were few pests and diseases anyway, but only rarely was there any difference between plants treated with compost tea and those treated with plain water. In short, the effects of compost teas were "extremely inconsistent".
These findings agree with other work, much of it in America: occasionally compost teas do something, but mostly they don't. Which? Gardening has also trialled compost tea (made using the Symbio kit) on potatoes, and found no effect on blight, or on yield or quality of tubers.
Given that compost tea, whatever else it may be, is certainly a fertiliser, I'm genuinely surprised by how often it seems to do nothing at all. So if you like the idea, go ahead and use compost tea. It's unlikely to do any harm, but almost equally unlikely to do any good.
Ken Thompson is a plant biologist with a keen interest in the science of gardening. He writes and lectures extensively and has written five gardening books, including Compost and No Nettles Required. His most recent book is Where do Camels Belong? The Story and Science of Invasive Species
Source : http://telegraph.feedsportal.com/c/32726/f/564649/s/3f81a94b/sc/36/l/0L0Stelegraph0O0Cgardening0Cgardeningadvice0C111212880CCompost0Etea0Edoes0Eit0Ereally0Ework0Bhtml/story01.htm