Trancegemini emailed to tell me you folks were having some fun with leachates, and invited me over to join in the discussion. I hope you don't mind since I am neither in nor from Australia (though I would LOVE to have an accent as cool as the one you folks have), but I have spent several years working with worms, composting tea and, yes, leachate and always enjoy these threads.

Some of the confusion regarding whether or not to use leachate results from the current popularity of so called "compost teas" and "castings teas". To ensure we are all on the same page, "teas" are defined as a steepage generated by placing finished, stable compost and/or worm castings in water; some folks agitate the water, some don't; some folks aerate the water, some don't; some add nutrient solutions to amplify the biology, some don't. Regardless, "teas" all use as the substrate from which the initial biological community, nutrient and other chemistry are extracted, finished, stable material.

Leachate, on the other hand, is liquid that drains from an actively decomposing mass of organic matter, thus it is liquid that is in contact with undecomposed material in the system. It is this factor, coupled with the cause for liquid to be draining in sufficient quantity that it can be collected, that creates concern.

Composting and vermicomposting are processes understood to destroy or significantly reduce pathogenic organisms, typically rendering their numbers suficiently low to pose little threat to human health. Because leachate drains through material that has not been thoroughly processed, however, pathogens like e-coli and salmonella associated with household organic waste and even yard debris can potentially be picked up in the leachate and contaminate the surfaces of fruits and veggies to which it is applied. It's important to understand that the concern is not that plants might take these pathogens up through their systems, but is a concern over surface contamination of garden produce. Those who chose to use leachate on edible plants are encouraged to wait several weeks after application before harvesting to ensure pathogens have died off before produce is eaten.

Water is generated by the composting and vermicomposting processes as well as being released from OM as it is broken down. In most healthy worm bins and compost piles the released moisture is absorbed by the the surrounding OM, thus little water drains freely from the system. A healthy worm bin, for instance, generally produces little more than a few ml of liquid per week. A system from which liquid is draining freely is typically a system that is saturated, thus, it can hold no more water. This is of concern because the water saturating that system is displacing oxygen, setting up conditions favorable to anaerobic microorganisms. Even if there are no significant odors, very wet compost piles and worm bins tend to have significant levels of anaerobic activity. The by-products of anaerobic decomposition, phenols, terpenes and alcohols, are water soluble and are toxic to plant roots, thus leachates often contain significant concentrations of these phytotoxins though there is no outward evidence of such. These leachates applied to plants often cause damage and can even kill sensitive plant species or plants treated with leachate with significant levels of phytoxins. To be sure, many people use leachate with excellent effect, but for every leachate advocate whose garden sees tremendous benefit I can introduce you to someone who killed or weakened their garden with leachate use. It's a risky thing using leachate. Many people will dilute the leachate (the general recommendation is 10 parts clean water to one part leachate) which often, if not usually, does remediate problems, but dilution also tends to dilute the benefits, rendering the stuff little more benefical than plain water.

So, bottom line; leachate is generally discouraged for garden use, particularly on food crops.

The link below will take you to an excellent article synopsizing the research on "teas" and leachates. I think you'll find it helpful in determining whether or not you want to give leachate a try in your garden!

Kelly S