How is organic matter classified in soil?

Organic matter in soil is classified according to the percentage of organic material present, particularly in the top 12 inches of soil, as this layer is crucial for plant growth, soil structure, and overall soil health. In this context, organic matter includes decomposed plant and animal residues, which significantly influence soil fertility and its ability to retain moisture and nutrients.

The specific focus on the top 12 inches is important because this is typically where the majority of root activity occurs and where organic matter plays a vital role in supporting plant life. Understanding the percentage of organic matter in this layer helps in managing soils for agricultural and horticultural practices, enabling more effective practices such as fertilization and irrigation.

Other methods of classification, like the type of minerals present or soil texture, do not directly pertain to organic matter itself but rather to the mineral composition and granulation of the soil. While color and appearance can provide insights into the composition and health of the soil, they do not offer a quantitative measure necessary for evaluating organic matter content. Thus, measuring the percentage of organic matter in the top layer provides a direct and practical way to assess soil quality.