Massachusetts Soil Evaluator Practice Test

The layer known as the A horizon is the surface mineral layer that contains most of the soil’s organic matter and roots. It typically ranges from about 3 to 10 inches thick and is usually darker in color because of humus, signaling rich biological activity and good plant support. This is the layer plants primarily interact with, where most root activity occurs and nutrients are readily available to vegetation. In soils that have an eluviation layer above or below, you might see an E horizon as a lighter, leached zone, but the topsoil itself is defined by the A horizon. The B horizon lies below it and is the subsoil where materials like clay accumulate, and the R horizon is the bedrock layer.