Agriscience 102
Applied Agricultural Science and Technology
Components and Properties of Soil
8382
Class Notes Key
TEKS: (c)(2)(A)
ORIGIN OF SOIL
Parent material is the unconsolidated mass of rock and mineral from which soil is formed. Soil
formation occurs where parent material is exposed at the earth’s surface. Productive soils develop from
parent material that supplies the essential elements needed to support plant life.
Parent material may be classified as residual, transported, or cumulose. Residual materials develop from
the underlying bedrock materials. Transported materials are moved to their final location by water,
wind, ice, or gravity. Cumulose materials develop through the decay of plant residues in shallow lakes.
Parent material is constantly transformed into soil as rocks and minerals are weathered by both physical
and chemical processes.
SOIL COMPOSITION
The four major components for soil are mineral matter, organic matter, water and air. Based on volume,
an average soil might consist of 45% mineral matter, 5% organic matter 25% air and 25% water. The
volume of each of these components varies from one soil to another. Also, the ratio of water to air
within a soil may vary greatly due to changing climatic conditions.
Soil minerals are inorganic materials made up of approximately 64 elements. Among these elements,
sixteen are considered essential for plant growth and development.
Organic matter consists of decomposed plant and animal matter. It provides nutrients to plants and other
soil organisms, holds water in the soil, minimizes the leaching of nutrients, and improves soil structure
and porosity.
Both water and air are necessary for the growth of plants and other organisms. The amount of water in
the soil varies with climatic conditions and drainage. During rainfall, air in the soil is displaced by
water. A certain amount of water is adsorbed by the soil particles, while the rest drains from the soil
once the rainfall stops.
PROPERTIES OF SOIL
-1-
While soils have the same general components, they differ in their properties, or characteristics. These
properties affect soil management. Over time, soil management can also affect certain soil properties.
Soil Color
Topsoil color is often a good indicator of organic matter content and texture. Subsoil color is usually a
good indicator of soil drainage.
Soil Texture
Soil texture refers to the sizes of individual particles in the soil. Most of the physical and chemical
processes in soils involve reactions at the soil particle surface. Therefore, particles with more surface
area are more reactive than those with less surface area. This reactivity determines the capacity of a soil
to retain moisture and nutrients, as well as to maintain good structure.
Soil particles are classified as sand, silt, or clay. Sand, the largest particles, can be seen with the naked
eye. Silt particles are smaller than sand and larger than clay particles. Clay, the smallest of the soil
particles, is microscopic in size.
Most soils contain mixtures of the three soil particle sizes. The proportion of each type of particle in a
soil sample determines which of the twelve textural classes to which that soil belongs.
A mechanical process of separating particle sizes determines the percentage of each particle contained in
a soil sample in the laboratory. With practice, the ribbon test, where soil is moistened and squeezed
between you thumb and forefinger to form a ribbon, can be used to determine soil texture in the field.
In general, sandy soils are relatively infertile, dry out quickly, and erode easily. Silty soil tends to run
together after a rain, forming a surface crust. Silty soils erode easily. Clay soils dry out slowly and
become cloddy unless properly managed.
Soil Structure
Soil structure refers to the arrangement of individual soil particles into aggregates. Very fine soil
particles are drawn into loose clusters in a process called flocculation. Organic polymers then cement
these clusters into aggregates.
Removal of plant cover from the soil reduces the organic matter in the soil. The reduction in organic
matter reduces the organic polymers in the soil and, therefore, leads to a break down of the soil
structure.
-2-
A distinct contrast in soil structure can often be observed by comparing the structure of the soil in field
that has been in continuous cultivation with the structure of the soil in an adjoining fencerow. Loss of
organic matter and physical manipulation in the cultivated field degrades the soil structure.
A well-defined soil structure can alter the effects of soil texture on water and air infiltration and
movement, as well as root penetration. Soil structure may be described in terms of type, class, and
grade.
Types of soil structure refer to the shape of the aggregates. There are four basic types of soil structure:
granular, blocky, platy, and prismatic. Additional conditions include single-grained, where sand will not
form multi-particle aggregates, and massive, where soils with high clay content do not form any definite
or orderly arrangement of lines of weakness.
Classes of soil structure refer to the size of the aggregates. In general the classes are fine, medium, and
course, but classes vary from one type to another.
Grades of soil structure refer to strength of the aggregates. The grades include:

Structureless - the single-grained or massive conditions discussed previously

Weak - indistinct aggregates even in undisturbed soil

Moderate - distinct aggregates that tend to break down when soil is disturbed

Strong - distinct durable aggregates that separate when the soil is disturbed.
Soil Consistence
Soil consistence refers to soil cohesiveness, or resistance to rupture.
Consistence is described in
different terms depending upon the moisture content of the soil. Wet soil is described as plastic,
nonplastic, sticky, or nonsticky. Moist soil is described as loose, friable or firm. Dry soil is described as
loose, soft, or hard.
Soil Fertility
Soil fertility is the ability of the soil to provide nutrients for plant growth. Regardless of present fertility
levels, plant nutrients must be returned to the soil if it is to remain fertile.
Soil Productivity
Soil productivity is the present capability of the soil to produce a specified plant or sequence of plants
under a certain set of management practices. The productivity of a soil under a particular management
-3-
system is dependent upon such factors as soil fertility, climatic conditions, organic matter content,
texture, and structure.
-4-