Yes you can build good soil by growing plants, the roots do most of the work for you in this method. It takes a minimum of 3 years up to 10 years to get a red clay based soil to become dark, fertile loam soil.
This means, how many years do you have is the real question. Using the plants for chemical analysis is a good idea but there are other factors that have a lot of impact on what your findings would possibly be.
John misinterprets the way exudates work in the soil and that leads to a misinterpretation of where and how the nutrients become available to the plant roots. Either that or he has over simplified the processes.
His work is worth reading but I would not simply read his work and go blindly forth, I would read all I could find on the subjects and then come to my own conclusions to give a try.
Dr. Elaine Ingham is one of the leaders when it comes to soil improvements done quickly and she has had many great successes.
Mark Shepard is a leader in soil building, along with restoration agriculture and water management.
There is a huge list of people that are considered leaders in these fields and they are fairly easy to get access to via the internet.
My issue with John is how he describes the way soil works.
What happens is; plants issue exudates to the soil, bacteria begin to process the minerals they eat, fungi and other members of the microorganism soil environment eat the bacteria and poop out excess minerals, plant roots suck up the pooped out excess minerals, bring them up into the stem tissues where they go to whatever part of the plant that needs those minerals. The process is repeated for as long as the plant is alive and the microorganism world is alive. Minerals are not only made available when bacteria die, the minerals become available when they are pooped out by the organisms, the organisms do not have to die, even though many are eaten by other, "higher" life forms in the micro world of soil, it is the excess minerals that are pooped out that are what the plant eats. If a bacteria died, it also has to decompose before the minerals inside it would become available.
Exudates are sugars, simple sugars to be exact and there are several different ones that plants make use of as exudates, each exudate is tailored to give a specific set of messages to the organisms of the micro world of soil.
Plant sap is the blood of life for that plant, just like our blood, testing it can tell us things, but it can't tell us everything we want to know about health of the sampled organisms be it plant or human.
There is more to plant immune systems that just complex carbohydrates, they are a part of the mechanism but not the whole mechanism just as lipid production is a part of fungal pathogen resistance.
Foliar feeding can clog the stoma of plant leaves, stoma are the organs through which plants inhale and exhale, they also get rid of excess moisture through these stoma.
Ask yourself this, how do I like to breathe water into my lungs? What happens to me when this occurs? Now you may have an idea of why I do not make use of foliar feeding.
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Farmers know to never drive a tractor near a honey locust tree. But a tiny ad is okay: