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SOILS AND FERTILIZERS: SOILS
Ch. 2, pp. 4 - 8
The physical properties of a soil are those
characteristics which can be seen with the eye or felt between the
thumb and fingers. They are the result of soil parent materials
being acted upon by climatic factors (such as rainfall and
temperature), and affected by topography (slope and direction, or
aspect) and life forms (kind and amount, such as forest, grass, or
soil animals) over a period of time. A change in any one of these
influences usually results in a difference in the type of soil
formed. Important physical properties of a soil are color,
texture, structure, drainage, depth, and surface features
(stoniness, slope, and erosion).
The physical properties and chemical composition
largely determine the suitability of a soil for its planned use
and the management requirements to keep it most productive. To a
limited extent, the fertility of a soil determines its possible
uses, and to a larger extent, its yields. However, fertility level
alone is not indicative of its productive capacity, since soil
physical properties usually control the suitability of the soil as
growth medium. Fertility is more easily changed than soil physical
When soil is examined, color is one of the first things noticed. It
indicates extremely important soil conditions. In general, color
is determined by: (1) organic matter content, (2) drainage
conditions, and (3) degree of oxidation (extent of weathering).
- Organic Matter
- Drainage Condition
- Degree of Oxidation
Surface soil colors vary from almost white, through
shades of brown and gray, to black. Light colors indicate a low
organic matter content and dark colors can indicate a high
content. Light or pale colors in the surface soil are frequently
associated with relatively coarse texture, highly leached
conditions, and high annual temperatures. Dark colors may result
from high water table conditions (poor drainage), low annual
temperatures, or other conditions that induce high organic matter
content and, at the same time, slow the oxidation of organic
materials. However, soil coloration may be due to the colors
imparted by the parent material. Shades of red or yellow,
particularly where associated with relatively fine textures,
usually indicate that subsoil material has been incorporated in
the surface layer.
Subsoil colors, in general, are indications of air,
water, and soil relationships and the degree of oxidation of
certain minerals in the soil. Red and brown subsoil colors
indicate relatively free movement of air and water allowed by the
soil. If these or other bright colors persist throughout the
subsoil, aeration is favorable. Some subsoils that are mottled
(have mixed colors), especially in shades of red and brown, are
Yellow-colored subsoils usually indicate some drainage
impediment. Most mottled subsoils, especially those where gray
predominates, have too much water and too little air (oxygen) much
of the time. The red-to-brown color of subsoils comes from iron
coatings under well-aerated conditions. In wet soils with low
oxygen levels, the iron coatings are chemically and biologically
removed, and the gray color of background soil minerals shows.
Texture refers to the relative amounts of differently sized soil
particles, or the fineness/coarseness of the mineral particles in
the soil. Soil texture depends on the relative amounts of sand,
silt, and clay. In each texture class, there is a range in the
amount of sand, silt, and clay that class contains.
The coarser mineral particles of the soil are called sand. These
particles vary in size. Most sand particles can be seen without a
magnifying glass. Sand particles feel gritty when rubbed between
the thumb and fingers. Relatively fine soil particles that feel
smooth and floury are called silt. When wet, silt feels
smooth but is not slick or sticky. When dry, it is smooth, and if
pressed between the thumb and finger, will retain the imprint.
Silt particles are so fine that they cannot usually be seen by the
unaided eye and are best seen with a microscope. Clays are
the finest soil particles. Clay particles can be seen only with
the aid of a very powerful (electron) microscope. They feel
extremely smooth when dry, and become slick and sticky when wet.
Clay will hold the form into which it is molded. Soils high in
clay content often show pronounced surface cracking when dried.
Soil textural classes take their names from the
particle size categories (sand, silt, and clay) and also from the
category called loam. Loam is a textural class of soil
that has moderate amounts of sand, silt, and clay. Loam contains
approximately 7% to 27% clay, 28% to 50 % silt, and 23% to 53%
sand. It is smooth to the touch when dry, but when moist, it
becomes somewhat slick/sticky.
Most surface soils fall into five general textural
classes. Each class name indicates the size of the mineral
particles that are dominant in the soil. Intermediate texture
soils are called loams. Texture is determined in the field by
rubbing moist-to-wet soil between the thumb and fingers. These
observations can be checked in the laboratory by mechanical
analysis which separates particles into clay, silt, and
various-sized sand groups.
Estimating Soil Texture
Soils can be classified by their texture or particle size distribution.
Practically speaking, a soil particle must pass through a 2mm
sieve (No. 10, US) to be called a soil particle.
Soil particles are either organic or inorganic. Texture
is used only to describe the distribution of the inorganic
fraction. In the laboratory, the organic matter is either removed
prior to mechanical analysis. In the field, organic matter is
disregarded when the texture is determined.
An estimation of texture can be made in the field by
using the following method:
- Place about a tablespoon of soil in the palm of your hand.
- Mix it with water and form a moist ball. The soil is at the
correct consistency when the ball does not leave soil on the
palm of your hands when it is rolled around (the consistency
of modeling clay.)
- Press the moistened soil ball between the thumb and
forefinger in an attempt to form a ribbon with the soil. As
the thumb and forefinger are pressed together the soil will
extrude forming the ribbon. The motion should be repeated
several times to test the cohesiveness of the ribbon,
attempting to form a continuous ribbon.
- Ribbons can be classed into three broad categories:
- Good Ribbon The ribbon does not break and
has very little cracking along the sides.
- Medium Ribbon The sides of the ribbon
crack deeply and eventually the ribbon will break and fall
- Poor Ribbon No ribbon formed (no
cohesiveness) or the ribbon breaks with the first applied
pressure and does not cohere.
- The sample is then further wetted and mashed between the
thumb and forefinger. This is to determine the amount of
grittyness or smoothness. The soil should be wet enough to
feel individual particles. If the soil feels like sand, then
it is called "gritty." If it feels like flour then
it is called "smooth." There is a category between
the two that is both smooth and gritty but no descriptive term
Students will be able to:
- determine soil composition
- identify the 3 soil particle sizes.
- Collect soil from 2 different areas in the outdoor
- Fill two quart jars 2/3 full of water. Label them
soil 1 and soil 2.
- Pour 1 cup of finely crushed soil from area 1 into
the jar labeled soil 1.
- Pour 1 cup of finely crushed soil from area 2 into
the jar labeled soil 2.
- Add 3 tablespoons of non-sudsing detergent (dish
washer detergent) to each jar.
- Cover the jars tightly and shake hard, at intervals,
for at least 5 to 10 minutes or until the soil particles
are broken apart.
- Place the jars where they will not be disturbed for
- After 24 hours of settling, place an index card
alongside each jar and carefully make a mark to show the
thickness of each layer of settled soil. Label each
layer of soil as illustrated. Coarsest particles (sand)
will settle first: finest particles (clay) will settle
- Which jar has the clearest water?
- What is the approximate thickness of the layer of
sand from the area 1 soil sample?
- What is the approximate thickness of the layer of
sand from the area 2 soil sample?
- Which soils have the most sand? silt? clay?
- What are the percentages sand, silt, and clay for
- Can you explain why some soils have different
proportions of sand, silt, and clay than the others?