Soil texture is a qualitative classification tool used in both the
field and laboratory to determine classes for agricultural soils based on
their physical texture. The classes are distinguished in the field by the
"textural feel" which can be further clarified by separating the relative
proportions of sand, silt and clay using grading sieves: The Particle-size
distribution. The class is then used to determine crop suitability and to
approximate the soils responses to environmental and management conditions
such as drought or calcium requirements. A qualitative rather than a quantitative
tool it is a fast, simple and effective means to assess a soil's physical
characteristics. Although the U.S.D.A. system uses 12 classes while the UK-ADAS
uses just 11, the systems are mutually compatible as shown in the combined soil
textural triangle below. Field assessment using a hand texture
chart Hand analysis, while an arbitrary
technique, is an extremely simple and effective means to rapidly assess and
classify a soil's physical condition. Correctly executed, the procedure allows
for rapid and frequent assessment of soil characteristics with little or no
equipment. It is thus an extremely useful tool for identifying spatial
variation both within and between plots as well as identifying progressive
changes and boundaries between soil classes and orders.
The method involves taking a small sample of soil, sufficient to roll into
a ball of approx 2.5 cm diameter, from just below the surface. Using a small
drop of water or 'spit' the sample is then moistened to the sticky point. The
ball is then molded to determine its workability and its class according to
the steps in the chart opposite. Soil separates
Soil separates are specific ranges of particle sizes. In the United States,
the smallest particles are clay particles and are classified by the USDA
as having diameters of less than 0.002 mm. The next smallest particles are silt
particles and have diameters between 0.002 mm and 0.05 mm. The largest
particles are sand particles and are larger than 0.05 mm in diameter.
Furthermore, large sand particles can be described as coarse, intermediate as
medium, and the smaller as fine. Other countries have their own particle size
classifications. Soil texture classification
Soil textures are classified by the fractions of each soil separate present
in a soil. Classifications are typically named for the primary constituent
particle size or a combination of the most abundant particles sizes, e.g.
"sandy clay" or "silty clay". A fourth term, loam, is used to describe a
roughly equal concentration of sand, silt, and clay, and lends to the naming
of even more classifications, e.g. "clay loam" or "silt loam".
In the United States, twelve major soil texture classifications are defined by
the USDA. Determining the soil textures is often
aided with the use of a soil texture triangle.
History of classification The first classification, the
International system, was first proposed by Albert Atterberg, and was based on
his studies in southern Sweden. Atterberg chose 20 μm for the upper
limit of silt fraction because particles smaller than that size were not visible
to the naked eye, the suspension could be coagulated by salts, capillary rise
within 24 hours was most rapid in this fraction, and the pores between
compacted particles were so small as to prevent the entry of root hairs.
Commission One of the International Society of Soil Science recommended its
use at the first International Congress of Soil Science in Washington in 1927.
Australia adopted this system and according to Marshall its equal
logarithmic intervals are an attractive feature worth maintaining. The USDA
adopted its own system in 1938, and the FAO used the USDA system in the
FAO-UNESCO world soil map and recommended its use.
See also Soil color
Soil horizon Soil type
References Soil Texture, by R. B. Brown, University
of Florida, Institute of Food and Agricultural Sciences.
Atterberg A Die rationalle Klassifikation der Sande und Kiese.
Chemiker Zeitung 29, 195-198. Davis ROE, Bennett HH "Grouping of soils
on the basis of mechanical analysis." United States Department of Agriculture
Departmental Circulation No. 419. Marshall TJ "Mechanical composition of
soil in relation to field descriptions of texture." Council for Scientific and
Industrial Research, Bulletin No. 224, Melbourne.
Prescott JA, Taylor JK, Marshall TJ "The relationship between the mechanical
composition of the soil and the estimate of texture in the field." Transactions
of the First Commission of the International Society of Soil Science 1,
143–153. Rowell D Soil Science; Methods and
Application, Longman Scientific & Technical, 350 pages [1]
Toogood JA "A simplified textural classification diagram." Canadian
Journal of Soil Science 38, 54–55. Whitney M "The use of soils east of the
Great Plains region." United States Department of Agriculture Bureau of
Soils Bulletin No. 78.