Soil-forming Factors

• Time
• Climate
• Parent Material
• Biology
• Topography

   Time 

Soils change over time as they weather. Weathering in the soil takes place over thousands of years. Just like people, soils can be mature or youthful. Each has its particular characteristics based not so much on their age as on their level of development.

 

Slightly developed	Highly developed

   Climate 

Climate refers to the amount of precipitation (rain, snow, humidity) and the temperature in a given locale. The hotter and more humid a climate, the faster and more completely big rocks are broken into smaller rocks, then into pebbles, then into tiny particles that make up the loose stuff we call soil. If a climate is cool and dry, this "weathering" process proceeds very slowly.

 

   Parent Material 

Soils are formed from a wide variety of raw materials. We call these "parent" materials, because they came first and were the materials that developed into soils. These include many types of rock, vegetation, and animal life. For example, the soil of some Pacific islands are formed largely from coral. Rock varies in its mineral makeup and other qualities. Some rocks like granite and sandstone are acidic and tend to form more acidic soils than rocks like limestone.

 

   Biology 

Biology refers to vegetation, microorganisms, and animals.

A. Vegetation

"Vegetation" refers to any type of growing plant, from small grasses to large trees. Different types of vegetation impart different qualities to the soil in which it grows; soils formed under grassland differ from those formed under forest. For example, soils formed under pine trees tend to be more acidic than those formed under other tree species.

 

Grassland soil	Pine forest soil	Oak forest soil

B. Microorganisms

These soil organisms are responsible for decomposing all the organic matter which enters the soil (such as leaves falling on the surface, roots dying underneath the surface) and recycling the nutrients contained in it for further plant production. Larger soil animals chew the organic material into smaller pieces and make burrows, which aerates the soil and provides channels for water movement. Tiny microorganisms are responsible for most of the decomposition of organic materials and also produce special glues which stick soil particles together, making the soil less prone to wind erosion. Some microorganisms can even produce plant growth hormones which speed up the growth of plants. Without these millions of creatures, the soil is dead and produces nothing.

The soil is a thriving biological laboratory, and a teaspoon easily contains a billion microorganisms such as fungi and bacteria. Some cause plant diseases but most are beneficial. Current estimates of the number of species of some groups include;

• bacteria (30,000)
• fungi (1,500,000)
• algae (60,000)
• protozoa (100,000)
• nematodes (500,000)
• earthworms (3,000)

 

Microorganisms do some "dirty" jobs:

• Many kinds of soil bacteria and fungi decompose organic matter into crumbly humus that does all those great things for the soil. The compounds produced by decomposition are also beneficial.
• Most of the nitrogen, phosphorus, and sulfur in fresh plant residues is tied up in the unavailable organic form which plants cannot use. Soil microbes change these tied-up nutrients into available inorganic (mineral) forms that plants can use.
• Several kinds of bacteria "fix," or capture, nitrogen from the air and convert it to a form that plants can use. The most important type are rhizobia bacteria that live in small nodules on the roots of legumes. Legumes are plants that produce their seeds in pods, such as beans, peas, and peanuts. The rhizobia have a symbiotic, or mutually beneficial, relationship with legumes. The bacteria live off sugars provided by the plant and supply their host with nitrogen.
• Mychorrhizae are a kind of mushroom fungi commonly found in most soils and infest the roots of many plants and trees. They cause no harm. They enhance the host's uptake of plant nutrients, especially phosphorus. They also improve water uptake, lessen the toxicity of salinity, and stimulate the growth of other beneficial microbes like rhizobia.

  Here are a few examples of microorganisms:

This is a picture of an Amoeba trying to engulf a fungal spore	The long rods in this picture are Bacteria that live in the soil.	This is a Ciliate, a beneficial soil organism.
The long, white strands here are fungal mychorrhizae.	This is a close up view of a soil mite.	Here are nodules on roots that contain the rhizobia bacteria.

C. Animals 

"Animals" refers to worms, insects, gophers, humans and any other non-plant living thing that affects soil development.

Humans are factors in soil formation, especially considering farming practices like land clearing, tillage, and cropping. Other human impacts include road construction, forest fires, or swamp draining. These affect erosion, soil acidity (pH), organic matter content, etc.

Here are a few examples of soil organisms that you can see without magnification:

This is a close-up picture of an Ant.	This isn't an ant, but a wood-eating Termite.	This multi-legged insect is a Centipede.
This insect has even more legs than a centipede, and is called a Millipede.	This Collembola, or Springtail, can leap several feet with a flick of its tail.	Many insects start out their life-cycles as larvae in the soil, such as this fly Larva.
This is a picture of a small worm called a Flatworm, due to its shape.	This worm-like organism is called a Nematode, and can be very numerous in the soil.	This is a common Earthworm.

   Topography 

Topography refers to the shape of the land with regard to elevation. Some terms that help explain topography include flat versus hilly, or gently-sloping versus sheer cliff.

Land surface characteristics exert a big influence on erosion and drainage. For example, if a slope is very steep, rain water will not be able to soak into the soil before it runs off down the slope, possibly carrying precious topsoil off down the slope with it (erosion), and limiting the amount of water that actually reaches the plant roots.

A general example of the influence that topography has on soil formation is with regards to soil color. The topography will dictate how much water reaches a particular spot on the landscape and how long the water stays at that spot. We explore soil color in more detail in the next section, Physical Properties of Soil.

 

Here are some examples of erosion and its effect:
 

This gully is the result of erosion by water flowing over a surface that lost its protective vegetative cover.	How would you like to be in this dust storm?! Wind is a strong erosive force that can devastate an exposed soil.