Mythology in Gardening: Compost Tea

Today, I happened across an article about how to make aerated compost tea.  You get a bucket, an air pump, some compost, water… blah blah blah, and pretty soon you’ve got this lovely bucket of disease-preventing, nutrient-rich foliar spray!  That’s awesome, I thought!  I’ll do that!

As is my style, though, I thought I’d do a little reading on the topic before I launched into yet another project.  This reminded me of my experience with seaweed extracts.  They’re great, everyone says!  They’re super expensive, but so natural!  Seaweed!  That’s like having an indigenous person echo the calls of humpbacks on sustainably harvested conch shells in your backyard, right?

The thing is, though, that there’s no real evidence that they do anything worth talking about. Aside from people saying “that stuff’s awesome,” there’s no conclusive scientific evidence that it does bupkis.  Still, science-schmience, I thought, and I bought me a couple bottles of Maxi-Crop and sprayed it all over my half of a few different crops for a season.  Bupkis.  I couldn’t detect any difference in them.  Growth regulators, my hiney.

Moreoever, you have to figure that they’re harvesting kelp from the ocean (where the humpbacks can sing to it), grinding it up, packing it in plastic containers, and shipping it across the globe.  Hey, that’s not… earth-friendly at all!  That’s downright irresponsible!  Could it be that gardening companies are actually just like every other marketing machine?  That they’ll say whatever, and encourage misinformation if it means selling more units?  Wow.  Dude.  So uncool.

OK, back to compost tea.  Now, let’s just assume that we’re going to leave out the financial interests of the companies that make aerated compost brewers– they aren’t cheap– because you can make the stuff yourself.  What’s more, let’s narrow down our concern to the value of foliar application of the final product since I’d think that soil application of it is probably not unlike applying compost to the soil in that it provides innoculants of beneficial organisms and nutrients.

So, is it a *good* idea to apply compost tea to the foliage of my plants?

Here’s what a study by the USDA had to say:

“Results clearly show that nutrient and other supplements support growth of human pathogens in both aerated and non-aerated CT and should be avoided when CT is used on fresh produce.”

And this…

“The compost Tea Task Force, formed by the National Organic Standards Board. took this research into consideration when framing new guidelines.

‘Use of supplemental nutrients and other additives to produce compost tea gives even a few pathogenic bacteria a growth boost, so testing of the final tea before spraying may be necessary to ensure the absence of human pathogens,’ said Ingram.”

I.e., if you have any Salmonella or E. Coli in your compost– and how sure are you that you absolutely do not?– and you feed your compost tea with anything other than compost, you’ll basically breed a big bucket of human pathogens that you will then spray all over your plants.  Super.  My two year old will love that when he picks peas off the vine to eat them.

OK, add to that that foliar sprays, if they are absorbed at all, don’t do anything to rectify systemic (e.g., soil, siting, and variety) problems.  So, now we’re hoping that this potentially infectious soup will prevent… something.  I don’t have much in the way of disease problems, but maybe I can use this to stave off powdery mildew in the fall?

Well, as it turns out, there’s a crotchety, old mythbuster, Dr. Linda Chalker-Scott as WSU Puyallup (N.B., for all I know, Linda’s neither crotchety nor old) who’s written an entire paper on the subject.  The conclusion:

“Because WSU Master Gardeners are volunteer educators who rely on science-based information, they cannot recommend a practice or product that lacks a legitimate scientific basis.”

In other words, there’s no reproducible evidence that compost tea does much of anything (besides spread potentially infectious nasties).  In addition, it’s pretty clear that my compost is not your compost.  For that matter, this batch is not that batch.  What I put into my compost varies greatly throughout the year.  Sometimes it gets really hot.  Sometimes it gets kinda hot.  Sometimes, like in winter, it’s mostly worms.

I know I’ll probably hear from a bunch of people who bury their cow manure in a bull’s horn under a full moon that compost tea is awesome, and that I’m a tool of The Man in encouraging chemical pesticides, and so forth.  Maybe.  But what I’m really suggesting is that we don’t *need* all that stuff.  Compost is super-duper when used in the soil.  Proper moisture, planting times, siting, and rotation generally takes care of most things.

Making up more stuff to spray on your plants because someone is pretty sure it’s a good idea– even if all the evidence points to the contrary and basic logic would suggest that it’s not even the same stuff from spray to spray– just doesn’t sit with me.  Nor does harvesting kelp forests to package and ship across the world for dubious benefits to the home gardner seem like anything other than boredom in action or the need for a quick fix.

So, no, I’m afraid it’s back to wandering around looking at my plants and picking off slugs when I see them.  Ah, well.

Maybe bokashi is the answer!!!