Good organic matter content in topsoil helps retain water.

Which Layer in a Soil Profile Contains the Most Organic Matter?

by Joan Norton

Topsoil is the uppermost 2- to 8-inch layer of ordinary garden soil, and it is where all the important growth activity takes place. The topsoil layer is easily eroded, so many gardeners practice the “no bare soil” rule to keep it safely in place. The topsoil layer has the highest concentration of the organic matter and beneficial microorganisms that give plants their daily nutrients.

1. What's Soil Made Of?

Good garden soil is half solid material and half open space. Organic materials contribute less than 5 percent of the total layer, but without it, soil is not fertile and can easily be washed away by water or wind. Organic matter helps bind the other materials together as well as supply nutrients. Microscopic to large size chunks of minerals make up about 45 to 50 percent of soil content. The remainder of healthy soil content is about 25 percent each water and air. Interspersed within this mix are billions of microorganisms that perform functions that keep soil healthy.

2. Organic Matter

Organic matter in the productive topsoil layer is made up of everything that has ever lived and died in it, plus plant and animal material that is constantly being added. The materials are at various stages of decomposition, providing food for tiny organisms that can’t be seen without a high-powered microscope. Decomposing organic matter also holds soil particles together, creating its structure. Organic matter makes soil less clay-like and sticky, or it provides structure for sandy soils to form into usable garden loam. The organic matter content in soil is capable of transmuting environmental toxins, such as pesticides and heavy metals, to make it safer for growing food.

3. Life in the Soil

A teaspoon of healthy, productive soil with good organic matter content contains between 100 million and 1 billion bacteria, as well as fungi, earthworms, insects and other tiny organisms. The bacteria help transport water and nutrients to plant root systems, as well as help suppress disease. The enormous mass of bacteria in the topsoil layer converts the organic matter content into forms that are useful to the rest of the living organisms in the soil. Fungi in topsoil are tiny cells usually formed into strands. They have the important function of converting dead organic matter into carbon dioxide and organic acids that become food for other creatures. Fungi also bring nutrients in the soil to growing plants.

4. How to Tell Your Soil is Healthy

The simplest way to determine if you have enough organic matter in the topsoil layer is with your eyes. Dark brown soil is full of organic matter and water. If areas of the garden having puddling or standing water, the organic matter content is low. Rub some soil between your fingers and look for it to form crumbs and fall easily from your hand. Lastly, use your nose. Healthy soil with lots of organic matter smells good.

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