I analyzed the field and garden soil from the previous post, and it’s clear under the microscope that the structure is quite different between them. This is probably why there was a big difference in clarity when the samples had settled.
A single soil sample can vary quite a bit under the microscope, so I included two photos from each one to give a general idea of how they looked. I just held my iPhone up to the eyepiece on the microscope to take the pictures, so they are not the greatest quality. All photos are taken at 400x magnification.
What causes this difference?
The answer is probably humus. Humus is essentially the end of the line for living organisms. When something dies (a plant or animal), it is decomposed by a progression of smaller and smaller organisms. This process breaks large organic molecules down into smaller ones. Then, the material goes through mineralization and humification, both done by microorganisms. The microbes will transform organic matter into inorganic components (mineralization) and humic substances (humification). After humification, the material is considered stable (to varying extents), which means it has reduced or lost its potential to be broken down or mineralized further.
Humus benefits the soil in a number of ways. I might write a more in depth post on humus in the future, if there is any interest in that. For now, I’m just interested in the binding ability, since this is what most likely created the difference in clarity between the above samples. Humus causes particles in the soil to become organized into clumps, which improve the soil’s porosity and ability to hold water. You can see that the field soil, which has less organic matter, is just a dense carpet of small particles, while the garden soil is significantly less cluttered.
Microscopic organisms that live in the soil actually live in water. They survive in the thin film of water that surrounds soil particles, so despite living in soil, they actually swim to get around. Imagine a creature like the ciliate pictured below swimming in the soil on the right, compared to the soil on the left. Even if the amount of food available for it in each soil was the same, it would probably survive better in the “cleaner” soil, simply because it would be easier to get around and hunt. It would be like the difference between running on sand versus a paved surface.
So even with this single factor considered, it would make sense that only a dense population of bacteria and a few tiny flagellates were found in the field soil. There is probably a lack of predation from creatures higher on the food chain (such as protozoa which prey on bacteria), which means there may be less biodiversity in this soil ecosystem. The garden soil had many different kinds of bacteria, but fewer of them overall. I found almost no protozoa in the field soil, but many different kinds in the garden soil, including both flagellates and ciliates. I have been looking at agricultural soil samples under the microscope for two years now, and this kind of result is consistent with almost every field we have sampled from.
Going back to the original post, which showed these two jars next to each other…
… I think we now have a better idea of why one was so clear and the other was so cloudy. The field soil is full of tiny particles and vast populations of bacteria, whereas the garden soil had more structured soil with greater biodiversity.
So what does this mean for plant growth?
Here was the result of a sprouting test using the two different soils. Without looking at the labels, can you guess which one was which?
You probably figured out that the left is the garden soil. Even a hardy, fast sprouting plant like cress struggled to survive at all, but flourished in the garden soil. Why might this be?
An even better question: why is a farm producing soil like this, and why is this considered normal?