Topsoil Horizons

To be diagnostic, a horizon must occur wholly, or in part, within 1.5m of the soil surface. There are five surface horizons in the topsoil horizon: organic, humic, vertic, melanic and orthic.

Organic 0 Horizon

This form the upper part of the soil and consists of decomposed organic matter accumulated under marshy conditions. It is non-arable, and best used for water storage.

Humic A Horizon

This contains more than 1.8% organic carbon and is dark in colour. It does not overlie an E horizon, hard and soft plinthic, prismacutanic or red structured horizons. The soil is productive but highly susceptible to erosion by wind and water when exposed.

Vertic A Horizon

This has a strong, blocky structure, slickensides, swell-shrink potential and is black or very dark in colour. It is fertile but difficult to cultivate due to mechanical limitations.

Melanic A Horizon

This is a black to dark colour, has a blocky structure, lacks slickensides and self-mulching properties, and cracks less than vertic horizon. It occurs mainly in dry areas. It is fertile and responds well to irrigation.

Orthic A Horizon

This is a surface horizon that does not qualify as organic, humic, vertic or melanic. The potential depends on the nature of the underlying horizon.