SOIL BASICS I MACROSCALE FEATURES - Introduction to Soil Chemistry - page 8

Table 1.1. Soil Parent Material Transporting Agents, Including Name of Material
and Geomorphic Features They Form a

Agent Name Applied to Material Geomorphic Features

Gravity Colluvium Toe slope of hills

Air Loess Loess and loess cap

Dune sand Sand dunes

Volcanic ash Volcanic ash layers

Water Alluvium Floodplains

Deltas and alluvial fans

Lacustrine Lake sediments

Outwash Terraces, outwash planes, kames,

and esker

Ice Glacial till Till plains

Ocean water Marine sediments Costal plains

a

This is not an exhaustive list.

morphic features they form is given in Table 1.1. Once deposited, these mate-
rials become the parent material from which soil will develop.
It is logical to assume that the surface horizons in a soil will be made up of
material derived from the parent material and plants growing on the soil. In
most, if not all, cases this is not true. Many soils have continuous additions of
both inorganic and organic material deposited from both water and wind. In
Ohio (USA) many of the soils develop from an underlying glacial till and an
overlying silt loess, wind-transported silt, cap. In other areas occasional or
regular flooding may deposit material on the soil surface, which then becomes
part of the soil surface horizons . Even in areas where there would appear to
be little wind- or water-transported material, small amounts of both inorganic
and organic compounds will be added to soil from the atmosphere.
The first horizon develops on the surface of the soil and is called the A
horizon. Because it has organic matter deposited on and in it and is the first
to have salts dissolved and eluviated, it is higher in organic matter and lower
in salts than the lower horizons. Clay is eluviated out of this horizon and is
deposited lower in the profile. It also has finer granular and crumb structure.
This is the horizon, which is plowed in preparation for planting crops 2 and is

the one most commonly sampled for analysis.
The A horizons are slightly different from other horizons because they
are plowed and planted. In many cases a soil may have an Ap horizon, indi-
cating that it is or has been plowed. The plowing need not have been done
recently for the horizon to be described as Ap. Even a soil so newly develop-
ing that it has no horizons will have an Ap horizon designated if it is plowed.

2 Soils may be worked in a number of different ways other than plowing. In addition, they may
not be worked at all when no till planting in used. In this case soil disturbance is minimal.