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Introduction to Soils

There may be as many definitions of soil as there are people who worry about it. One way to think about it is to define what it is not. Dirt is not soil. It may have once been soil, but once it was removed from its place in the landscape, it probably ceased being soil. It can become soil again in the right circumstances.

Potting soils are not soils, they are growth media. Pure sand or water can serve as media, and neither can be confused with soils. This discussion will concentrate on soils. Media properties are covered elsewhere.

For wholistic purposes, soil should be thought of as a three-dimensional living, dynamic resource that supports plant life. It has constantly changing biological, chemical, and physical properties that affect its ability to function in the environment.

Let us look at soils in the reductionistic (old science) manner. If we break a soil down into its components we will find different sized mineral particles, organic matter (things that once were alive), some things that are currently alive, and some air and water.

The different size mineral particles will break out into three distinct classes: The biggest ones are called sand. They will have a diameter of 0.05 to 2.0 mm. You can pick most sand particles out with the naked eye. Intermediate sized particles are called silt. It requires 10x to 50x magnification to distinguish them by visual examination. The diameter ranges from 0.002 to 0.05 mm. The smallest particles are called clay. They will have diameters of less than 0.002 mm and required magnification greater than 50x to see. Note that clay particles have diameters ranging from 25 times less than the smallest sand or largest silt to 1000 times less than the largest sand particles.

Organic matter is the residue left after multitudes of soil microbe generations have used the carbon and other easily usable constituents of once-living flora and fauna that ended up in the soil matrix. True organic matter, or humic substances, are relatively stable and resist any more ërapid' breakdown, although there will continue to be some activity on it.

There are astonishing numbers of living organisms in most soils (as many as two billion per cubic inch). They are continuously recycling nutrients in the soil released from the minerals, added plant material, added water, and previous generations of the organisms.

How do soils develop?

Soils over time develop from parent materials lying at a topographical location with vegetation growing under the influence of the climate. The material is undergoing additions, subtractions, and in-place transformations.

Over time, the combination of factors and processes results in an unique soil. When similar conditions occur in various places, the soils will develop somewhat alike, but rarely will be exactly the same.

The overall side view of a soil is called the profile. The long-term process of soil development results in the formation of distinctive layers called horizons visible in the profile. Horizons reflect the cumulative effect of all the soil formation factors undergoing slow genesis processes (generically called weathering).

The top layers, often referred to as topsoil, is often two different horizons. If there is a layer of partially decomposed plant material, it is named the O horizon. Below it, there is usually a dark layer several inches thick called the A horizon. If the soil has ever been manipulated by humans (plowed, bulldozed, shoveled, raked etc), a subscript is added, i.e. Ap. This plow layer is the primary rooting zone and nutrient supply area.

Lower horizons show different degrees of development (soils weather more near the surface) and have other designations by soil scientists. One feature in much of Mississippi hill sections is the presence of a restrictive layer some 15 to 24 inches below the surface called a fragipan. Roots cannot elongate through these special horizons, nor can water penetrate easily. Growing plants on raised beds on these soils increases the effective root zone and improves water relations.

What does soil do?

There are many functions of soil. Some are obvious to gardeners, others are not.

Soil provides a physical matrix, chemical environment, and biological setting for water, nutrient, air, and heat exchange for organisms living totally within the soil, as well as the roots and tubers of plants.

Soil regulates water distribution to runoff, infiltration, or storage. This affects movement of soluble materials including pesticides and nitrate nitrogen.

Another function soil performs is the regulation of biological activity and molecular shifts between solid, liquid, and gaseous phases. This affects the cycling of nutrients in the soil and plant growth.

Physical functions of soil include serving as a filter to protect environmental quality, and as support for buildings.

What is soil texture?

Simple answer: content of sand, silt, clay, and organic matter of a particular soil.

If there's a simple answer, it must mean there is much more to the story. The relative size of the mineral particles was discussed above. The relative proportions of each in a soil determine its texture. Texture is a very important property which affects:

  • soil water: infiltration rates and water holding capacity
  • soil structure
  • soil consistence
  • nutrient holding capacity
  • ease of cultivation

Most people are familiar with the expressions loam, clay, silt loam and so forth. These are the names of soil textures. Professional soil scientists perform exacting laboratory procedures to determine the exact proportions of the size classes in a soil. The results are projected on a textural triangle and the soil texture is determined. However, ribbon tests can give reasonably good guesses.

The ribbon test is performed by working in the hand a very wet clump of soil slightly larger than a golf ball. The soil should be kneaded until it has fairly uniform wetness throughout. Ribbons are formed by working the ball of soil in the palm up through the thumb and forefinger.

A) If a ribbon is formed:

  • If the ribbon is long, flexible, and can sustain its own weight: clay
  • If ribbon is weak, breaks readily, but forms a ball which withstands much handling: clay loam
  • If the clay loam is soapy or slippery feeling, and appears powderly when dry: silty clay loam
  • If the clay loam has lots of visible sand: sandy clay loam

B) If soil will not form a ribbon, instead giving a broken appearance:

  • If wet soil is friable, somewhat gritty and sticky, the ball handles without breaking: loam
  • If the dry soil feels soft and floury, wet feels slippery, ball does not break: silt loam
  • If sand can be seen, and the ball cannot withstand handling without breaking: sandy loam

C) Soil is loose and single grained.

  • Individual grains can be easily seen, and the aggregate crumbles when touched: sand

What is soil structure?

Soil structure refers to the way the soil aggregates, particles, and pore space are arranged with respect to each other. Soil scientists use shape, size, and strength to define it. Platy, block, granular, subangular blocky are some terms associated with soil structure.

Structure, or even lack of structure, has several possible ways of affecting plant growth. It affects air exchange with the atmosphere, the speed of water movement into and through the soil, it influences the quantity and size of pores, and the rate and extent of root growth. Structure may be altered through management.

Organic matter and clay minerals are the primary agents for binding soil particles together to form aggregates. Sandy, low organic matter soils often lack structure.

Crusting often occurs when bare soils are subjected to intense rainfall. The intense rain destroys the surface structure by washing away the clay particles and organic material holding the soil particles together. Without the binding agents, the particles clog up the natural drainage of the surface layer and make it impervious to water coming in and gases going out.

Tell me more about soil organic matter.

Organic matter is the remains of plants and animals in the soil. Bacteria, fungi, insects, and earthworms (among other creatures) use fresh organic matter as food. Their digestive processes convert the fresh residue in humic substances and nutrients. The nutrients in fresh organic residue are largely unavailable to plants before this conversion.

The organic matter ranges from very simple to very complex chemical compounds. These includes sugars, starches, carbohydrates, nucleic acids, etc. Living organisms are also included in organic matter considerations.

Positive results of increased organic matter in soils:

  • enhanced development of soil aggregates
  • increased pore space
  • increased infiltration and percolation rates
  • more high water holding capacity
  • greater capacity to hold and release nutrients
  • nutrient storage
  • improved cultivation ease (tilth)
  • promotes further biological activity

Organic matter content of soil is a very dynamic property. For example, excessive cultivation speeds decomposition and thus lowers organic matter. On the other side, addition of compost is an effective way of rapidly increasing organic matter. However, to maintain higher levels of organic additions probably have to made each year.

There is incredible biological activity going on right under our feet!

What about soil acidity?

The most important property of soils in Mississippi for growing plants is pH which measures soil acidity. Soil acidity controls the availability to plants of practically all micro-nutrients, and also affects nitrogen, phosphorus, sulfur, calcium, and magnesium bio-availability. The pH itself, or material added to affect it, does not really affect plants per se.

Chemically, this all important variable is the negative log of the hydrogen ion activity in the soil solution. Simply, when there are lots of hydrogen ions floating around, a soil is acid. If there are not many floating around, the soil is neutral or may be alkaline.

The pH is measured on a scale of 0 to 14, however for the vast majority of soils in natural environments, pH ranges from 4.0 to 8.3. A pH of 7 is considered neutral, below 7 is acid, and above 7 is alkaline. The lower the pH value, the more acid the soil. Acidity is measured on a log scale, so a change of pH of 1 unit (from 6 to 5 for example) is a 10x change in hydrogen ion activity. A change of 2 units (from 6 to 4) is a 100x change.

Soil acidity is adjusted upward by adding liming materials to the soil. Some horticultural crops prefer more acid conditions, and some materials can be added to reduce pH. Therefore, plant knowledge is crucial in horticultural situations.

Application of liming materials should be based on the results of a soil test. Lime recommendations from the Mississippi State Extension Soil Testing Laboratory are based on a buffering capacity analysis different than the water pH analysis on the soil test report. The buffer pH varies with soil mineralogy and organic matter among other things, thus it depends on the unique nature of a particular soil.

Terms defined

  • Acidity: the relative quantity of hydrogen ions in soil solution, when pH is below 7 soils are acid
  • Aggregate: groups of soil particles that bind to each other more strongly than to adjacent particles, the space between aggregates provides pore space for water and air movement
  • Clay: the smallest of the three primary soil mineral particles; also a soil texture
  • Fragipan: natural occurring layer in soils which restricts root growth and water percolation
  • Horizon: identifiable layer of soil in a profile
  • Infiltration: the movement of water into soil
  • Lime: material added to soil to increase pH (reduce acidity)
  • Percolation: the movement of water within soil
  • Profile: soils as viewed from the side, usually from a hole or other excavation such as road cuts
  • Sand: the largest of the three primary soil mineral particles
  • Silt: intermediate sized soil mineral particles
  • Structure: the arrangement of soil aggregates, particles, and pore space with respect to each other
  • Texture: the relative proportions of sand, silt, and clay in a soil; identified by names such as silty clay loam or silt loam
  • Topsoil: the soil at the surface


Extension Soils/Fertilization publications
Soil Fertility
Nitrogen in Mississippi Soils
Phosphorus in Mississippi Soils
Soil Testing for the Farmer


Other Soil Information

Dirt Land! Enjoy a visit with other subterrainian citizens.
The Composting Page offers lots of composting information.

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Publication Number: IS372
Publication Number: P1466
Publication Number: IS894
Publication Number: IS0871


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