COMPONENTS OF SOIL, physical properties of soil




The mineral components of soil are the three smallest rock particles: sand, silt, and clay. These make up 95% or more of the soil solids. The first two, sand and silt, are generally composed of silicon dioxide (quartz). (There are a few exceptions to this, such as the black mineral sands of Hawaii or the calcium-containing sands of the Caribbean.)   The reason that most sands and silts are quartz is that silicon is the most abundant mineral on earth after oxygen. It is also the most stable of the common minerals found in rocks.   Quartz is more resistant to chemical weathering. Acidic and basic conditions have very little effect on it.   That’s why we use glass, a product made from quartz, as containers for caustic chemicals. (GARDENING: COMPONENTS OF SOIL)

Quartz is also more resistant to physical weathering.   It is one of the hardest minerals, so abrasion has less of an effect on it.   This means that when water and air move these particles around, other minerals are ground up into very fine particles and carried away by these forces.   This leaves the relatively stable quartz particles behind.   Sand ranges in size from 2 to .05 mm.  Silt ranges from .05 to .002 mm.

The third mineral component of soil is clay.   Clay particles are so small individual particles can’t be seen without magnification. The structure and minerals of clay are different from sand and silt. (GARDENING: COMPONENTS OF SOIL)

The genesis of clay (how they came to be) is primarily chemical and biochemical.   The minerals that are weathered out of the parent material are chemically recombined into more complex chemical and physical structures. The more stable of these form the clay.   So, clay is the result of the recombination of the unstable minerals of rocks.   Clay varies in composition and structure depending on the types of minerals available and the amount of weathering.  Clay particles are less than .002 mm in size. (GARDENING: COMPONENTS OF SOIL)

Most clay particles are formed into plate-like structures composed of very thin layers.    This structure allows the clay to hold ions between these “plates” and prevent them from being leached by water moving through the soil. This mechanism of holding is called adsorption. Clay particles can also adsorb water between the plates.   This is what causes swelling of clay soils. insoluble. And quartz is not affected by biological or biochemical weathering. COMPONENTS OF SOIL


The organic component of soil is made up of the soil life and the remains of plant and animal life in various stages of decomposition. All of these together make up the organic matter in soils.   Organic matter content is usually 5% or less of total soil solids.

Organic matter is commonly divided into 3 groups:

  1. Living organisms
  2. Fresh residues
  3. Well-decomposed residues or hummus

Simply stated, these groups are the living, the recently dead, and the “been dead a long time.”

The living part of the soil’s organic matter is made up of a wide variety of creatures of all different sizes.   These include bacteria, protozoa, fungi, and algae. There are also mobile creatures: micro-arthropods, nematodes, flagellates, and others.  The largest of these are earthworms, arthropods, and mites.   It even includes live plant roots. This living portion of the soil makes up about  15% of the soil’s organic matter. These organisms are constantly feeding on other organisms and giving off wastes, or they are feeding on these wastes and the recently dead organisms and giving off wastes.   There is a constant cycle of life, death, and feeding going on in the living part of the soil.

Fresh residues or  “the recently dead” in or on the soil are dead organisms, plant material, crop residues, dead roots, and wastes from organisms.   These are the food for the living soil organisms.   As the fresh residue is consumed, two things happen: many of the nutrients needed by plants are released and hummus is produced.

Humus or the “been dead a long time” is sometimes used to describe the total soil organic matter.    It is more properly a word for the dark, relatively stable portion of the soil organic matter.    Humus is relatively stable because it is made up of complex molecules that aren’t easily broken down by soil organisms and molecules from fresh residue too complex for any organism to break down rapidly.

Humus has a number of characteristics that make it an important part of the soil solids, even though it makes up a smaller portion- 5% or less of the organic matter of most soils.   One of these is the ability of humus to hold onto some plant nutrients, preventing them from being leached.   Humus can buffer some harmful elements in the soil, such as heavy metals, and prevent plant uptake of these.


Minerals and organic matter together make up 100% of the soil solids.   However, when we look at a loam soil in good condition we find that the solids make up only half (50%) of the total volume of the soil.   The other 50% of the volume is pore space. This space is macropores, spaces between soil conglomerates, and micropores, spaces inside soil aggregates. Both of these types of pores are where the soil water is held.

Optimally, water fills 20-30% of the pore space or about one-fourth of the total soil volume. During and just after heavy rains or irrigation, water can fill all of the pore spaces.

This porosity of the soil is vital to plant life. Most nonporous soils do not have the capacity to absorb water and hold it for use by the plants. Also, most plant roots can’t grow through nonporous soil. (GARDENING: COMPONENTS OF SOIL)


Soil air takes up the other 50% of the pore space- all the pore space that doesn’t contain water.    The amount of air in the soil fluctuates depending on the water content.   As water moves into the soil from rainfall or irrigation, it fills the pores and forces the air out.   As the water drains out of the pores and evaporates at the surface, the air is drawn back into the soil. Soil water and air are interrelated.  That is, the amount of water moving into or out of the soil determines the amount of pore space for soil air.


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