Sub-soil is the layer of soil beneath the topsoil, which contains less organic matter and nutrients. The antonyms for the word sub-soils can be the following words: topsoil, humus, fertile soil, rich soil, loam, or surface soil. The topsoil is the uppermost layer of soil, which contains a high concentration of nutrients, organic matter, and microorganisms. Humus is the decomposed organic matter that enriches the soil and promotes plant growth. Fertile soil refers to the soil that has the necessary nutrients and pH level to support plant growth. Rich soil is highly nutritive due to its mineral content. Loam is a fertile mixture of sand, silt, and clay soil. Surface soil is the uppermost layer of soil, where plants grow and thrive.