Larry, I thought the two links were mostly disproving strawman arguments. Hormones? Enzymes? Hunnh? I never heard those claimed as benefits of compost tea.
If you are trying to establish a more-varied microbial population, AND THEN HOPE THAT the new microbes help digest OM by excreting enzymes, sure.
If you are trying to establish a more-varied microbial population, some of which have evolved to "work with" the roots of plants you are growing, AND HOPE THAT the new microbes exchange signaling molecules with the roots and each other and with plant pathogens ... and you call those "hormones" ... sure.
But those fall under the category of trying to establish a more-varied microbial population and hoping that more beneficial ones multiply and colonize the roots and root zone, perhaps through cooperation with the plant. I don't recall those links actually saying there was ANY scientific evidence to DIS-prove that theory.
Nutrients? I GUESS ... if you expect to extract a significant amount of any nutrient from the small amount of compost, manure or molasses you add to a 5 gallon pail. I'd call that another easy-to-refute straw man or red herring argument that I didn't know anyone was proposing seriously.
>> In summary, there is little scientific evidence to support the idea that compost tea solves disease problems.
That should be taken more as a limitation of science than of compost tea.
I'm pretty sure there are few studies spending millions of dollars and man-years of effort to bash or support the theory that compost tea CAN confer benefits. More likely a few teams had a few small plots and a few months to attempt to get a publishable paper.
"Little evidence to support a theory" doesn't mean the theory is wrong, just that it's hard to PROVE. Even though I have a strong loyalty to the scientific method, I know its shortcomings. It works best in very simple systems, like physics and inorganic chemistry.
Science can take deep breath, try really hard, and do a great job of studying organic chemistry and understanding it so well that they can make numeric predictions that are pretty accurate - to maybe one decimal place.
Spectroscopy might be different and more accurate - the only variables in spectroscopy are the atoms in the molecule, the electron orbitals, magnetic fields, temperature and sometimes the solvent system. Much simpler than a plant leaf!
Science does a fair job of studying biochemistry and medicine, but they don't so much "prove" numerically accurate things as "discover" things, which is more like descriptive science than analytical science. In medicine, something like 50% of the patients express 50% of the "expected" symptoms.
Psychiatry, social "science", plant physiology and ecology? We do CALL those sciences because practitioners TRY to be evidence-based, but they ain't "science" in the "PROOF" sense, like "measure it to three significant digits and then PREDICT it to two-and-a-half significant digits".
Maybe plant physiology is a simple enough system that science can be numerically accurate to one or two digits, by ignoring complicated interactions or trying too average them out of consideration. I'm thinking of a few corn plants in a growth chamber where they measure the ever-living doo-dah out of atmospheric gases, air movement, temperature and insolation, ... then come to the SOIL and opt for something "reproducible" rather than representative. They probably get reproducible results that are useful for some purposes, but ...
Absence of PROOF is not any kind of proof of absence.
"PROOF" is very hard. The first thing you have to do is control EVERYTHING so you can get repeatable results or your colleagues will mock and shun you and vote against your papers when they come up for peer review. That right there rules out or makes very difficult to test something that occurs in complex hard-to-recreate situations.
The easiest thing in the world is to simplify and "control" a complex situation enough to prevent anything complicated from happening at all!
For example, prove this: "Giving flowers and chocolates improve a guy's chances of encouraging a woman to sleep with him". A scientist would have to find a source of standardized men and women (hundreds of each), standardized flowers and standardized chocolate, standardize the setting, and then run them through tests in a sterile, white-walled environment surrounded by video cameras and microphones and multiple white-lab-coated introverts with clipboards.
I shudder to think what a scientist would consider a "reproducible" source of standardized men and women! The very first thing they would try to rule out is "contamination" of results where men that bring flowers are better gardeners and hence more attractive, and men who bring candy tend to be thoughtful and considerate but dumb, since not many women want to be fattened up.
They would surely NOT find any measurable increase in hanky-panky attributable to flowers and chocolates! In fact they would probably conclude that men and women never have sex (in a controlled, reproducible laboratory environment, but who remembers to stress that in the paper they want to publish?)
It's not as bad as "consider a spherical plant", but you could easily see one result in a controlled, laboratory situation designed to maximize repeatability, and different results in 100 real-world, representative gardens.
If 100 gardeners think they get improvements good enough to swear by a method, then I would assign the scientists to KEEP LOOKING until they found the conditions or factors that gave some good real-world results. THEN, if practical, try to PROVE something about the factors they think they saw in play, in controlled and repeatable conditions.
The links did a good job of saying "I didn't find proof yet", but that doesn't debunk anything. Once the people in the field have a theory based on their observations and experience (or based on eager optimism and love of compost) , the burden is on the scientists to DIS-prove it before they mock it. I'm sure gardeners could do a more scientific job of testing their theory, like spraying only one corner of a field and then photographing growth and measuring yield per square meter.
(Scientists are more likely to be impressed by "per square meter" than by "per square yard". if you use "yards", you must not be one of the club, and hence your beliefs are suspect.)
I didn't see anything in the article to rule out the following speculative scenario:
Plant root hairs (and probably leaves) have co-evolved with microbes.
They "encourage" beneficial microbes to colonize around and in some cases inside root hairs (and maybe leaves). Some of that has been seen to occur in labs.
There are interactions among microbes in soil, including competition and "signaling" with excreted chemicals.
There are some observed, proven cases where certain "beneficial" microbe species decreases the population of, and the harmful effects of, certain plant pathogens.
Considering that we can't even CULTURE more than 10% of all soil organisms, the are probably many more interactions than have been proven to exist or observed tentatively.
Co-evolution would tend to favor interactions that increase plant productivity, since that is the source of food for the entire soil web.
Co-evolution ought to favor interactions among roots, "beneficial" microbes and "harmful" microbes that tend to increase plant productivity and soil fertility, since those are the energy sources and living space for the entire soil ecosystem.
Adding a wide variety of microbes to the root zone (or phytosphere) ought to enable the plant to find and encourage more beneficial species and varieties of microbes than they had access to before the compost tea (or compost) was added.
