Geoscience Reference
In-Depth Information
plow. In spite of Greek philosophers' intentions to devote their studies on soils to
the abstract level, they often compared a soil to a woman giving birth to a new life.
Theophrastus (about 2,300 years ago) proposed the fi rst classifi cation of soils
according to fragments of his scripts on plants.
The practical Romans took over and extended the Greek knowledge on agricul-
ture related to observation of soil properties. In the second century BC, Cato recom-
mended use of animal manure and green manure to improve soil fertility. Marcus
Terentius Varro, a close friend of Cicero, described and classifi ed various soils in his
Rerum Rusticarum Libri Tres
and warned his contemporaries to avoid swamps and
marshland that serve as sources of disease. To his merit we have information on the
Greek Theophrastus.
With the decline and eventual end of the Roman Empire, all earlier scripts on soil
were lost and forgotten until the arrival of Renaissance. However, this comeback
was not so easy and obvious. It all started with the new role of money in the society
where the fi rst economists did not call themselves so, but who recognized that soil
is a tool in production of foodstuffs and that this tool should be taxed instead of
harvests.
The fi rst soil taxation attempts appeared at and after the end of the 30-year war
in Europe, when the great majority of population was very impoverished, including
the lower noblemen and the landlords who distributed lands to themselves if their
rulers were winning battles. In the fi rst rustic law, soils were divided into three
“bonity” (meaning quality) classes: good, medium, and bad bonity. The developing
tax system required a fundamental criterion based on objective details. Using mod-
ern terminology, that criterion was “soil class” according to percentages of sand,
silt, and clay particles in each soil. Today, we also speak about it as soil texture. As
methods for determining soil texture advanced, the more objective was the charac-
terization and class of a soil. We should not forget that it was the tax system that
required an objective soil classifi cation system in order that individual governments
or countries could have or expand their own system of taxation that imposed taxes
on land or soil.
We fi nd now the following terms in the US system: sand, loamy sand, sandy
loam, sandy clay loam, sandy clay, loam, silt loam, silt, silty clay loam, clay loam,
silty clay, and clay. The classifi cation depends upon the content of three groups of
particles characterized by their sizes: sand (2-0.05 mm) and silt (0.05-0.002 mm or
50-2
m). In some classifi cation systems, the silt is in ranges 0.02-0.002 mm. Then
there the sand reaches up to 0.02 mm. The size of clay is in all systems below
0.002 mm, or < 2
μ
m. If there are particles bigger than 2 mm, they are denoted as
gravel or skeleton and their amount is expressed in %, but the amount of sand, silt,
and clay is determined in the laboratory after gravel particles are separated by siev-
ing. Therefore, the percentage of sand, silt, and clay is measured and calculated as
if there was no gravel and as if particle shape is nearly spherical (Fig.
5.10
).
The group of sandy soil, loamy sand, and sandy clay loam is sometimes called
(mainly in verbal communication) as light soils, while clays and soils near to clays
are denoted as heavy soils. Do the scientists as well as laymen wrongly refer to
“light” versus “heavy” soils when they are talking about coarse-textured versus
μ
