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2.1 Physical Characteristics
O
Organic
•
Considered separate from the mineral soil, as it is
made up of plant and animal matter, called collectively
organic matter, accumulated from the surface
•
Organic material deeper in the O horizon will be
progressively more decomposed
A
Surface
•
Considered the top layer of the mineral soil, formed
from weathering of the bedrock
•
Often darker in color due to the presence of organic
matter that has been deposited from above
•
Can have a coarse texture due to water displacement
of finer materials to the horizons below
B
Subsoil
•
Anchor for plant roots, especially for
deep-rooted/tap-rooted plants and crops
•
Zone of accumulation of nutrients and finer
particles from the above horizons
•
Layer where calcium carbonate can accumulate,
potentially forming a restrictive layer
C
Substratum
•
Mixture of soil and bedrock, which maintains
some of the structural features of the bedrock
•
Zone of active soil formation where the
bedrock is being weathered
R
Bedrock
•
The rock from which the soil continues to
form as the effect of the five soil forming factors
•
This is the source of many of the inherent physical
and chemical characteristics of the soil
Arid Soil
Profile
A
Surface
B
Subsoil
C
Substratum
R
Bedrock
Horizons
Photo: USDA NRCS