We often call it earth, but the soil under our feet, in our gardens, and under the plants of forests and fields is much more than gravel and mud. It is made up of a complex mixture of living and non-living elements that create the basis of all living things.
Soil scientists and anyone who likes to dig deep (which I highly recommend) know that soil is made up of layers or horizons. Layers vary in depth and composition, depending on where the soil is located.
The top layer, known as the O horizon, is made up almost entirely of organic matter. When leaves, twigs, grasses, and other plant residue fall to the ground, they begin to decompose, delivering materials to the soil that have helped plants grow.
Below this “duff” is the A horizon or topsoil which is a combination of decaying organic matter from the O horizon, mixing with minerals from below. Roots are usually abundant in this layer because it is the most favorable layer for plant growth. In much of New Hampshire, this dark brown or black layer is only a few inches thick, but in the midwest where prairies were the dominant landscape, the A horizon can be 2 feet thick or more. No wonder our ancestors went to farming in the West!
In most soils, the subsoil, or B horizon is next and this is where organic matter, nutrients, and other chemicals have leached out and accumulate from the O and the TO. Roots are common in this yellowish-brown layer, but not as profuse as in the A horizon.
Horizon C is the lowest level. Also known as the substrate or parent material, it contains little organic matter, little or no roots, and is mostly made up of minerals from the bedrock on which it sits. It is usually the thickest section and is usually brownish gray in color.
In the layers, especially the upper ones, there is abundant life. It is said that a single teaspoon of soil can contain up to 1 billion bacteria, several meters of fungal filaments, thousands of protozoa and dozens of nematodes. These are just things too small to be seen. Add worms, centipedes, sow bugs and other invertebrates and a clod of earth is like a miniature town.
Keeping all of these organisms alive and doing their jobs is essential for maintaining healthy soil, which in turn is essential for the growth of vigorous plants. This happens regularly in uncultivated environments. Minimizing disturbance and mimicking natural systems is a great way to improve soil health. However, in our gardens and yards, the soil has probably been disturbed enough to require some amendments to improve plant growth.
The best way to determine if the soil needs help and thereby increase the chances of successful plant growth is to do a soil test. Testing the soil is much easier than most people think. The UNH Extension website has all the information you need: extension.unh.edu/agriculture-gardens/pest-disease-growing-tools/soil-testing-services. Fall is actually a great time to do a soil test, as the lab is not as busy as it was in the spring, so your turnaround time to get your results will likely be shorter (only about 2-3 weeks). ). By getting results now, you can use this information to plan any changes you might need to your garden or landscape in the spring.
Another thing you can do this time of year that will help your soil and the organisms that live in it is to minimize cleaning up your yard and garden. Leaving the soil covered (such as in the forest or field) is one way to minimize soil loss due to wind or water erosion. It also provides shelter for native insects and insulation for plant roots. Most of us are drawn to raking, blowing, or removing leaves and plant debris at this time of year. But leaving at least some of that “litter” adds to the health of the soil and the overall richness of the ecosystem under and around our feet. So remember, it’s not just dirt out there, and treating the soil with care is important for plants and the planet.