All the dirt you need to know on organic soil

Published 3:17 pm Friday, October 11, 2019

There are four components of soil. Two of them fluctuate based on recent weather and compaction of the soil by traffic, be it afoot, tractor or riding mower. Those two are moisture and air. The third soil component is the age-old mineral portion from weathered rocks.

The original mineral remains fairly constant in undisturbed natural areas. We artificially bring it back up to par with fertilizer, where farming, gardening or yearns for greener lawns has reduced that original level.

The fourth component is organic matter. It is by far the smallest component in soil on a percentage basis, but it is a very important one.

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Of the many definitions of organic matter, the one I like is “recent formerly living organisms in various stages of decay.”  In other words, it is dead plants, animals and fungi and other microorganisms that slowly rot in amongst the mineral, water and air. Leaves, grass blades, corn stalks and insects, as well as dead birds, mice and possums all, become organic matter. So do unseen microscopic critters with long taxonomic names.

The reasons why soil organic matter is important are numerous. For one, it helps keep the soil loose enough for earthworms and water to move about and roots to grow freely. Also, organic matter holds on to nutrients such as nitrogen, phosphorus and potassium and slowly lets roots soak those in. Plus organic matter acts kind of like a sponge, soaking up water when there is plenty so plants can drink longer when a dry spell starts up.

Be skeptical when reading generic gardening information that claims an average organic matter level in soil is about 5 percent. You might think with our long growing season and lots of plants and bugs to die and rot the organic matter level in local soil would be at least that average level. Nope, not so; our natural, undisturbed soils have two percent or less organic matter. And the ones we till and mow and baby along are even lower.

The same warm weather over many months that grows so much plant life also speeds up the underground consumption of organic matter by microorganisms.

Significantly higher soil organic matter levels are found in cooler climates that also have moisture aplenty. Think forests or prairies in Minnesota or Canada for high organic matter soil. The rainforests of the tropics with lush growth and no freezes join ours here with low levels of organic matter in soil.

Organic matter loss is sped up when soil is plowed. That’s because living soil bacteria that feed on it are aerobic, meaning they need oxygen to eat and multiply. Plowing opens up the soil and lets lots of oxygen in, resulting in faster aerobic bacteria reproduction.

Conversely, one of the benefits of the move some years back by farmers to reduce tillage was increased organic matter. Along with more erosion protection by crop residue at the surface over the winter and less costs for fuel, less plowing means more organic matter.


Terry Rector writes for the Warren County Water and Soil Conservation District.