WHAT IS SOIL MADE OF

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CHAPTER 2 -  CALL IT SOIL...

    ...the dirt's outside in the yard.  Soil, or potting mix, or potting medium, is the stuff in the pots, and it is truly the next key, after watering, to having healthy and beautiful plants.  In fact the next secret of the green thumb is
                          HEALTHY PLANTS
                                  REQUIRE
                                 HEALTHY SOIL

When all the  parts of the soil are working together in harmony,  the result is a beautiful plant.  When you see a beautiful plant,  it's partly because the soil is healthy.  And because soil so important, we're going to talk about it next, before we get to the parts of the plant.

Soil is as much a part of the plant as the roots, stems, and leaves.  Soil acts as bone and muscle - it supports the roots and thus the plant body.  It acts as part of the circulatory and excretory system -  it provides and drains away water and excreted compounds.  It has a role is a kind of nervous system as well.   It functions as lungs - bringing air to the roots.  And, amazingly enough, it also acts as grocery store, kitchen, dinner table, and digestive system in one - it turns minerals into usable forms and carries them to where the roots can absorb them for the rest of the plant.

As animals evolved movement, they had to develop ways to carry all these processes around with them.  Plants have it all for them, right there in the soil.   Truly wondrous stuff.

                                     WHAT IS SOIL AND WHAT  DOES IT DO
   
Soil, indeed all kinds of potting media, basically consists of particles and spaces.  The particles may be organic or inorganic, the spaces may be air or water.
                                   
THE PARTICLES...

Organic means the material was once living; these are things like bark, charcoal, sawdust, bits of sphagnum moss or peat, coir (coconut shell fibers), rice hulls, partially decomposed  leaf, pine needles, compost, etc.

The inorganics are made from minerals, things that were never alive.  These include crushed rock or brick, coarse sand, pumice, clay and mineral components, or processed forms of minerals such as perlite (volcanic glass expanded by heat), vermiculite (heat-expanded silicate materials), or kiln dried granulated clay products.

The particles, whether organic or inorganic, are of various sizes.  These large materials are broken down into smaller and smaller bits by geologic erosion, weathering, machines, and even  inside the plant pot through the actions of watering, chemical activity, organic decomposition, and acids secreted by the roots.  As the particles become smaller, they get more and more chemically active - that means they can contribute to the chemical processes that go on in the soil.  The very smallest particles are as small as 8/100,000 of an inch (0.002 mm.)

At this stage, the  organics mentioned above are now completely broken down into a substance called humus.  Also in the organic column there are micro-organisms - fungi, bacteria, microscopic animals and plants that live in the soil, along with the by-products of their biological action.

As for the inorganic particles, the sand, stone, artificially produced components (like perlite) continue to be broken down also. The very smallest are clay particles, along with the mineral compounds formed in the soil by microbial and other chemical activity.

(Excerpt from The Color of Your Thumb Has Nothing To Do With It)
                                                                                                                   by  Marlie Graves