There are a number of theoretical benefits of Actively Aerated Compost Tea (ACT) over compost tea "brewed" simply by steeping compost in water. - however it is important to note there are a number of studies that find no benefit to its use.
Aerated Compost Tea is created by putting high quality compost into water and adding nutrients to increase the multiplication of microorganisms present in the compost. Common additives include: molasses, proprietary microbial nutrients, yeast extract and whey blends, kelp meal, blood meal, bone and feather meal, cottonseed meal, fish emulsion, and humic acids. During the brewing process, the solution is aerated with the claim that the environment created favors the growth of beneficial bacteria and fungi at the exclusion of pathogenic microorganisms.
The key idea behind ACT is that soil microbiology is one of the drivers of soil fertility and disease resistance.
Some other benefits include:
- Bubbling/Aeration produces agitation of the compost which can liberate microorganisms from the compost substrate.
- The nutrients used in the solution can alter the fungal/bacterial
balance of the final product. Turf is said to benefit from a more
bacterial rich tea, while shrubs and trees benefit from more fungal
teas. In addition, by using decomposing leaf litter from natural areas, one can tailor compost and tea to support the mycorrhizal conditions types of plants. My favorite example of this is collecting organic matter from an area in which wild blueberries thrive. This litter can be used to inoculate compost, which can then be used to produce ACT. IN THEORY, the microbiology, especially symbiotic mycorhizal fungi, that helps the wild blueberries thrive in a given location can be multiplied and delivered to blueberry plants in the garden to give them a leg up.
- Aeration of the nutrient rich compost tea leads to an explosive growth of the microorganisms. I actually have experience with this and the transformation is quite impressive. I brewed a number of 60 gallon batches in which the initial formulation of compost, water, molasses, fish emulsion, and humates was quite stinky and almost oily. After 8hrs or so of vigorous aeration a beige foam began to form in great quantities - presumably the result of microbial activity. After 24hrs (in temps around 70 deg. F.) of aeration, the solution no longer produced new foam, and it smelled like the most incredible organic rich soil - no hint of fish emulsion or molasses. I'll confess that it smelled so pleasant that I was tempted to drink it.
- The process is like a compost multiplier in that with a small amount of compost (say 5-10 lbs per 60 gallons) it is possible to produce numbers of microorganisms equal to that found in many yards of compost. You can deliver these organisms to plants/lawns in ways that would be impossible using just compost. For example it would be impossible to apply compost as a foliar spray. It is also impractical, and in some cases inadvisable due to the potential for applying too much nitrogen, to apply compost at the rates required to deliver the same numbers of microorganisms to a lawn or garden.
As a biologist and ecologist, I personally like the theory behind ACT. However, as I mentioned before, it should be noted that the scientific evidence to support the claims of many ACT proponents is disputed:
For example, in this writeup of a study by the North Central Sustainable Agriculture Research and Education (SARE) project:
There were no significant effects on soil health indicators or soil food web organisms from the application of aerobic compost tea. However, there were significant changes for several parameters of soil health and organisms when comparing all plots between the baseline samples of 2003 and the ending results from 2005. This result is even more surprising given that soil moisture levels were considerably lower and soil temperatures higher during the 2005 sampling. Baseline data from 2003 showed that all field plots were lacking in beneficial levels of fungi. Both beneficial and fungal properties of the soil greatly improved during the two year study. In addition, aggregate stability, infiltration rates, and cation exchange capacity of the soil all significantly increased when comparing baseline results from 2003 with final results in 2005. We believe this positive result is due to the increased attention we each gave to soil health by reducing or eliminating tillage and salt-based fertilizers and pesticides, while increasing cover cropping and the time that the soil had living plants growing.
That said, there are some very smart people who claim to demonstrate its utility. The SoilGuy has a nice piece here on the work of Dr. Elaine Ingham, considered to be "inventor" of Aerated, Activated Compost Tea (AACT). This is too long to copy/paste here, but it illustrates where some other studies may have gone wrong in tea prep.