To Tea or Not To Tea    

There are many different opinions, and not many scientific facts regarding the use of compost tea.  While anecdotal evidence and claims can be exciting, it’s important to step back at times, to unearth what we really know and understand.

There have been many excellent scientific trials testing the effects of compost, which provides a good base for understanding compost tea. 

What we DO know about compost is:

Compost is a great soil amendment.  It:

  • Improves soil structure, porosity & density—provides better root environment
  • Loosens up clay soils for air & water
  • Helps sandy soils retain water & nutrients
  • Helps prevent runoff.  Only a 5% increase in organic material quadruples soils water holding capacity.
  • Makes any soil easier to work

We also know compost:

  • Adds essential nutrients & soil microorganisms
  • May reduce incidence of plant diseases & other harmful organisms.

Compost is comprised of a very large & diverse community of microbes, humic acids, chemicals, which varies wildly from compost to compost. Different feedstocks, different methods (thermophillic vs. mesophillic vs. vermiculture) and different climates are just three variables that change the organic and chemical outcome of compost.  Compost brings and feeds diverse life in the soil.  We know that these bacteria, fungi, insects, worms, chemicals, humic acids and more support healthy plant growth.

Although not a fertilizer, compost does have a positive, cumulative effect on soil nutrient levels.  Whatever nutrients are present in the soil become more bioavailable to plants growing in soils with added compost.

There have been many studies that support all of these benefits of compost.  There is also recognition that different composts can affect different crops in very different ways.

Compost tea research is more recent.  Again—it has been documented that compost tea has a very large & diverse community of organisms, depending on the compost, method of brewing and temperature among other factors. 

While there have been some scientific studies which show a positive effect on certain crops, there are also some studies which show a negative or no effect on certain crops.  Some of these studies suggest that the effects are nutritive; others suggest that there may be some disease suppressive qualities in certain compost teas.  On the compost website (, links to some of these studies and discussions are under the “Compost Uses” section.  These include a link to an Organic Farming Research foundation Information bulletin which includes a review of recent literature along with several controlled experiments, a review of compost tea trials on the Appropriate Technology Transfer for Rural Areas (ATTRA) site, and an article by Linda Chalker-Scott of Washington State University, "Compost Tea: Examining the science behind the claims." In the “Research and Publication” section, a published study designed by IPM Manager, Todd Murray in cooperation with Cascade Cuts Nursery, “Brewing up Solutions to Pest problems” describes the trials and outcomes of compost tea on basil.

Some people have tried inoculating compost teas with specific beneficial organisms.  While this may help, it is still hard to be consistent, since those beneficial organisms need the right environment to maintain high populations in the brew. In general, the effects of inoculants are very short lived as the native, adapted organisms quickly reclaim their niches within the system.

There is also a controversy over whether compost teas should be produced with or without aeration.  Much of the literature refers to this as “anaerobic” vs. “aerobic” teas.  Teas without aeration are made with placing compost in a permeable “bag” soaking it in water for 24 to 48 hours.  Teas with aeration are made with air bubbling through the compost tea mix continually, and take less time to brew.  Again, although many people support the “aerated” approach, saying that those active systems push the beneficial microbes out of the feedstock & into the tea, a number of studies and researchers suggest that anaerobic teas may actually have greater disease suppressive capabilities.

The USDA has some concern over human health issues.  They do not encourage people who are growing uncooked edible foods to spray with compost tea, due to the possibility of inoculating food with e-coli, or other harmful pathogens possibly living in the compost tea mixture.

Confused?  So are most of the rest of us!  What we DO know is:

Right now, the community dynamics of compost tea is not understood or currently controllable.  Thus using it as a reliable amendment is difficult since benefits are contributed through many factors (chemical, nutrient, microbiological) instead of known active ingredients.  

Hopefully, as we do more research we can provide this information to folks interested in the technology so they can make good choices.

So despite all they hype, compost tea is not the silver bullet everyone is looking for.  Unfortunately, nature is not that simplistic.  But it’s fun to experiment.  (I do lots!) If you do experiment, and come up with a formula that you think works—try replicating it in a scientific way, and let us know the results! 



Contact us: e-mail | (360) 778-5800
Whatcom County Master Composter Recycler Program, 1000 N. Forest Street, Suite 201, Bellingham, WA, 98225-5594 USA