In soil science, humus (coined 1790–1800; from the Latin humus: earth, ground) refers to the fraction of soil organic matter that is amorphous and without the "cellular structure characteristic of plants, micro-organisms or animals." Humus significantly influences the bulk density of soil and contributes to moisture and nutrient retention. Soil formation begins with the weathering of humus. In agriculture, humus is sometimes also used to describe mature, or natural compost extracted from a forest or other spontaneous source for use to amend soil. It is also used to describe a topsoil horizon that contains organic matter (humus type, humus form, humus profile).