What Is Subsoil Mostly Made Up Of?

Subsoil, or the B horizon soil layer, consists mostly of inorganic minerals that leach down from the topsoil. Subsoil is between the topsoil and bedrock.

Formation

Minerals accumulate over thousands of years to form subsoil. The minerals may include iron, aluminum, carbonates, gypsum, silicon and silicate clays. Small amounts of decayed organic matter from the topsoil may also be present.

Color

The color of the subsoil indicates how much water or air is present and also indicates the condition of the subsoil's minerals. Brighter reds and browns are a sign of good water and air movement through the layer. Impaired drainage can cause yellow subsoils. Low oxygen and poor drainage can cause subsoil to become gray.

Erosion

Erosion can wear away topsoil to expose subsoil, which does not contain enough organic matter to grow plants. The conversion of subsoil to topsoil can be accelerated by adding amendments such as humus, compost, other organic materials and fertilizer.

References

• Arizona Cooperative Extension: Physical Properties
• Arizona Cooperative Extension: Depth
• Argonne National Laboratory: Subsoil