Translations:Adventist Youth Honors Answer Book/Nature/Soils/5/en

4. Define the following terms:

Soil Horizons

a. A-Horizon

The A Horizon is the top layer of the soil horizon. The technical definition of an A Horizon may vary, but it is most commonly described in terms relative to deeper layers. A Horizons may be darker in color than deeper layers and contain more organic material, or they may be lighter but contain less clay or sesquioxides. The A is a surface horizon, and as such is also known as the zone in which most biological activity occurs. Soil organisms such as worms, nematodes, fungi, and many species of bacteria is concentrated here, often in close association with plant roots. Thus the A-horizon may be referred to as the "biomantle". However, since biological activity extends far deeper into the soil, it cannot be used as a chief distinguishing feature of an A Horizon.

b. B-Horizon

B Horizons are commonly referred to as ‘subsoil’, and consist of mineral layers which may contain concentrations of clay or minerals such as iron or aluminium, or organic material. In addition, they are defined by having a distinctly different structure or consistence to the A horizon above and the horizons below. They may also have ‘stronger’ colours (i.e. higher chroma) than the A horizon.

c. C-Horizon

C Horizons are simply named so because they come ‘after’ A and B within the soil profile. These layers are little affected by soil forming processes, and their lack of pedological development is one of their defining attributes. C Horizons may contain lumps of unweathered rock, rather than being comprised solely of small fragments as in the solum. ‘Ghost’ rock structure may be present within these horizons.

d. Organic layer

The upper soil horizons containing mostly organic matter.

e. Mineral layer

The lower soil horizons containing little organic matter.

f. Clay

Soil composed primarily of fine-grained minerals, which show plasticity through a variable range of water content, and which can be hardened when dried and/or fired.

g. Leaching

Leaching is the loss of mineral and organic solutes due to percolation. It is a mechanism of soil formation.

h. Silica

Silica is the chemical compound silicon dioxide, SiO₂. It is most commonly found in nature as sand or quartz.

i. Humus

Humus refers to any organic matter which has reached a point of stability, where it will break down no further and might, if conditions do not change, remain essentially as it is for centuries, if not millennia

j. Soil Profile

The different soil horizons stacked one atop the other makes up the soil profile.

k. Parent Material

The rock from which mineral soil components was formed.

l. Pedologist

A pedologist is a scientist who studies soil in its natural environment.