Common terminology and practices associated with compost teas.
Compost Leachate
Compost windrow leachate — the dark-colored solution that leaches out of the bottom of the compost pile — most likely will be rich in soluble nutrients; but, in the early stage of composting it may also contain pathogens. It would be viewed as a pollution source if allowed to run off-site. Compost leachate needs further bioremediation and is not suitable or recommended as a foliar spray.
Compost Extract
Compost watery extract — made from compost suspended in a barrel of water for 7 to 14 days, usually soaking in a burlap sack — a centuries-old technique. The primary benefit of the extract will be a supply of soluble nutrients, which can be used as a liquid fertilizer.
Compost Tea
Compost tea, in modern terminology, is a compost extract brewed with a microbial food source — molasses, kelp, rock dust, humic-fulvic acids. The compost-tea brewing technique, an aerobic process, extracts and grows populations of beneficial microorganisms.
Summary
Compost teas are distinguished from compost extracts both in method of production and in the way they are used. Teas are actively brewed with microbial food and catalyst sources added to the solution, and a sump pump bubbles and aerates the solution, supplying plenty of much-needed oxygen. The aim of the brewing process is to extract beneficial microbes from the compost itself, followed by growing these populations of microbes during the 24- to 36-hour brew period. The compost provides the source of microbes, and the microbial food and catalyst amendments promote the growth and multiplication of microbes in the tea. Some examples of microbial food sources: molasses, kelp powder, and fish powder. Some examples of microbial catalysts: humic acid, yucca extract, and rock dust.
Source Document: Notes on Compost Teas: A Supplement to the ATTRA Publication Compost Teas for Plant Disease Control