The soil organic matter composition in your garden means all the biological stuff, living and dead. Tiny bacteria work beside earthworms in the dirt. Dead leaves break down next to dark crumbs that formed long ago. All of this counts as organic matter in your soil.
I spent years confused about this topic until I dug through my compost pile at different depths. Near the top sat fresh kitchen scraps shaped like banana peels and eggshells. Deeper down, the material looked nothing like the food scraps that went in. That dark stuff that crumbled in my hands was organic matter in its most processed form.
My neighbor showed me another way to spot the difference last spring. She pulled up a forkful of soil from her neglected garden bed. The dirt looked pale gray and fell apart like sand. Then she grabbed soil from her raised bed where she adds compost each year. That handful was dark brown and held its shape. The color difference showed how much organic matter each bed contained.
FAO research breaks down the organic matter components into four groups. Living organisms make up about 5% of the total. Fresh plant and animal residues account for about 10% of the mix. Material breaking down right now represents 33-50% of organic matter. The remaining 33-50% consists of stable humus that took many decades to form.
The living fraction includes creatures you can see like earthworms and beetles working through the soil. Billions of tiny organisms also call your dirt home. Bacteria and fungi break down dead material. They cycle nutrients back to plants and process tons of organic material per acre each year.
Fresh residues come from fallen leaves, dead roots, and plant debris on the soil surface. You can still tell what these materials used to be. A maple leaf stays shaped like a leaf for weeks after it lands. Root bits from last year's tomatoes remain visible until soil organisms eat them up.
The decomposing fraction means material being processed by soil life right now. This stuff looks brown or black and feels soft when you squeeze it. You might see bits of plant fiber or wood. But the shape has broken down. Most compost you buy falls into this group.
So what is humus then? This stable fraction represents the final product after full breakdown. True humus binds tight to soil minerals and resists further decay for hundreds to thousands of years. It gives healthy soil that rich, dark color gardeners love. You can't make humus in a compost bin since it forms through long chemical changes between decayed material and soil particles.
Check your own garden soil to spot these organic matter components at work. Dig a small hole and look for dark coloring through the soil. Squeeze a handful to see if it holds together in a loose ball. Give it a sniff. Healthy organic matter smells earthy, not sour or rotten. Finding earthworms and seeing root channels means your soil biology is doing its job well.
Read the full article: Soil Organic Matter: The Essential Guide
