According to one of the definitions provided by the United States Department of Agriculture, soil is the unconsolidated organic or mineral material that covers the Earth’s surface and acts as the medium on which plants grow. Soil has a complex structure and composition that varies with depth and geographic location. Vertically, soil has three main layers starting with topsoil on the surface, followed by subsoil and bedrock at the bottom.
As the name suggests, topsoil is the topmost layer of the soil. It is this layer of the soil that we walk on and cultivate our crops. Topsoil is made of fine, loose soil particles. Agents of erosion like wind, water, temperature fluctuations and glaciers wear out the bedrock to form the topsoil. The topsoil is porous. It has air between its particles and water easily drains through it.
This layer lies just beneath the topsoil. It is more compact than the topsoil. Hence, the subsoil is not as well aerated as the topsoil. It is lighter in colour and stickier than the topsoil. Therefore, it is difficult to handle, compared to the topsoil.
According to the Forest Preserve District of Cook County (Illinois), subsoil is mainly clay, muck or loess. It is rich in minerals. It has a few elements that are important plant growth. However, these elements need to be converted into forms that can be absorbed by plants. As the topsoil is made from erosion of bedrock, it is rich in nutrients that can be easily used by plants for their growth. Besides this, the topsoil is also rich in organic matter.
Topsoil is more fertile than subsoil. It is because the topsoil is rich in organic matter. As plants and animals die, they fall on the topsoil. Here they disintegrate to make the soil fertile. This is called the humus. Humus cannot reach down to the subsoil. Hence, it is not conducive for plant growth.
Being closer to the surface, topsoil is in direct contact with the atmosphere. Its loose particles allow air to pervade the topsoil. This makes it possible for earthworms, millipedes, centipedes, millipedes and other soil dwelling organisms to survive in the topsoil. These organisms further mix and turn the topsoil making it more porous. This enables gaseous exchange that is so important for living things. Since the subsoil is more compact than the topsoil, it does not hold air. Hence, it cannot support life.
Advantages and Disadvantages
Despite its fertility, topsoil also poses problems for gardeners as it harbours weed seeds. Subsoil is free from weed seeds. Hence, gardeners prefer using subsoil to topsoil, for their lawns. Also, by adding nutrients, subsoil can be easily enriched.