The "A-horizon" is the surface layer of soil that contains organic matter and nutrients required for plant growth. Synonyms for the A-horizon include topsoil, surface soil, humus, earth, loam, and clay. Topsoil is composed of organic matter such as fallen leaves, decomposed plants, and animal waste. Surface soil refers to the top layer that has been exposed to weather conditions and has most of the nutrients. Humus is decomposed organic matter that is added to soil to increase fertility. Earth or soil is a mixture of minerals, organic matter, and air. Loam is a soil type that has equal amounts of sand, silt, and clay. Clay soil is composed of fine particles that are packed together.