The topsoil is the most biologically active layer that is responsible for soil fertility. Often it is called a turf, or a moistened layer, and its thickness does not exceed 5 cm (if the layer thickness is more than 3 cm – it is isolated in an independent soil horizon).
Sod has a gray or dark gray color, lumpy-grained or dusty structure. It is easily separated from the lower soil layers.
The topsoil is humus-accumulative, i.e. soddy soil-forming processes occur in it: in this layer, the main part of the rhizomes of herbaceous vegetation is fixed, which, when the plants themselves die, decomposes and turns into humus, then deposited in the profile.
Humus colloids glue the soil into lumps, forming its lumpy or granular structure, which is optimal for the development of plant roots due to water resistance.
Soddy process – the main soil-forming process involved in the formation of chernozems, as well as sod-podzolic, chestnut, brown forest and gray soils.
To ensure a high-quality process of humification, the sod is plowed in such a way that the vegetation is covered with earth, thereby ensuring its slow decay and conversion into humus.
Plant residues decomposing in the upper soil layer are also processed by microorganisms into nutrients that dissolve in groundwater and enter plant rhizomes.
To improve fertility, the topsoil needs to be healed from time to time. For this, mineral fertilizers, humus or compost are applied to the soil.
Digging and loosening can also have a beneficial effect: the loose structure of the top layer promotes the propagation of soil insects and earthworms, which in their own way improve aeration and removal of carbon dioxide from the soil.
The ability of cereals to form a dense turf layer, which protects the soil from erosion, allows the use of the topsoil layer to equip lawns (including sports ones) and flower beds and strengthen slopes of highways and railroads by the sodding method.
Sod is grown on specialized farms – “sod nurseries”, it can also be grown independently.