Soil Weathering and Soil Formation
Soil formation and development is a dynamic rather than static process. Soils were present when prehistoric animals roamed the Earth and, like those animals, some are no longer present or are preserved only as fossilized soils buried deep beneath our present soil.
Weathering describes the means by which soil, rocks and minerals are changed by physical and chemical processes into other soil components. Weathering is an integral part of soil development. Depending on the soil-forming factors in an area, weathering may proceed rapidly over a decade or slowly over millions of years.
The development of a soil reflects the weathering process associated with the dynamic environment in which it has formed. Five soil-forming factors have been identified that influence the development of a specific soil. Wherever these five factors have been the same on the landscape, the soil will be the same. However, if one or more of the factors differ, the soils will be different. The factors are?
1. Parent material
3. Living organisms
Parent material is made of rock and minerals. When the other four soil-forming factors act on parent material, it is weathered into smaller particles forming soil.
There are many types of parent material with different mineral contents. The Earth is believed to be about three billion years old. Mountains have been created and eroded away and then created again. Seas have covered the land and receded leaving layers of mud, sand and lime carbonate thousands of feet thick. Volcanoes have erupted. Glaciers have formed during long periods of cold weather and melted during long periods of warm weather.
Parent material can be rock formed in place or the remnants of rock that was moved by wind, water, ice or even gravity. A variety of parent material can be found in Nebraska ranging from sand in the Sandhill Region to clays in the Missouri and other river bottoms.
In the Great Plains, especially in the south, parent materials are primarily associated with ancient seas. These seas came into the region and receded several times, leaving sediment behind, which, over time, became sandstone, limestone and shale bedrock formations. Bedrock soil formations are classified as residuum parent materials and can be exposed and broken down to form soil.
In Nebraska, most residuum (e.g., limestone, sandstone and shale bedrock formations) is covered with more recent geological materials such as glacial deposits, windblown minerals or materials moved by water (Fig. 1.1). One or more overlying parent materials may have been deposited in an area throughout time.
Glacial deposits originated from great ice sheets that moved across Canada and the north central United States (Fig. 1.2). Glaciers are believed to have invaded only the eastern portion of Nebraska where they filled the valleys and leveled the hills. Sand and gravel were deposited along with boulders, clay and other sediments as the glaciers melted and retreated. Rivers which had previously flowed full of water were blocked and large amounts of sand and gravel washed from the Rocky Mountains were deposited in central Nebraska. These accumulations of sand and gravel are now the aquifers that provide our abundant supply of groundwater.
Many soils in southeastern Nebraska were formed in parent materials deposited by the glaciers, usually referred to as glacial drift, glacial till or glacial outwash.
Much of the parent material deposited in ancient times has been covered by windblown material. The windblown silty material is called loess. It covers most of Nebraska to varying depths, except in the Sandhills and western portions of the Panhandle. This yellow-brown loess is primarily found in the subsoil zone and may be 700 feet or more deep in the northeast and central areas of the state and only a few feet deep in western and southeast Nebraska. Loess soils are generally very fertile. Some are among the most productive soils in the world.
Windblown sand material is called eolian sand. It predominantly covers residuum in the Sandhills and western portions of the Panhandle. This coarse textured parent material is usually several feet deep and is found in both the surface and subsoil zones. Eolian soils are not very productive because they have very low water-holding capacity, are low in organic matter, and are nutrient deficient as compared to loess soils. Most are used for grass production or natural habitat.
Geologic materials moved from the parent material by water are known as alluvium. Alluvial deposits are found in flood plain areas such as the Platte River and other stream valleys. Since stream beds constantly change over time, alluvial parent materials are highly variable as are the soils that form them.
The physical and chemical weathering processes that change parent material into soil include:
• Temperature changes — freezing and thawing.
• Erosion by water, wind, ice and gravity.
• Roots of plants, burrowing animals, insects and microorganisms.
• Water relations — wetting and drying.
• Changes in chemical composition and volume.
Physical processes primarily result in the breakdown of rocks into smaller and smaller particles. As the particles become smaller, various living organisms begin to have a great impact on soil formation because they contribute organic matter. In addition, the smaller particles speed chemical processes which result in new chemical compounds. All of these processes are greatly influenced by climate, especially temperature and precipitation.
Nebraska’s climate is quite variable and influences soil development. Precipitation, in particular, ranges from an average of 33 inches per year in southeastern Nebraska to 15 inches per year in western Nebraska (Fig. 1.3).
The amount of water entering a soil influences the movement of calcium and other chemical compounds in the soil. Ultimately, if more chemicals are removed, the soils will be deeper and more developed. Precipitation influences vegetation and, therefore, greatly determines the organic matter content of soils. Because of greater precipitation in eastern Nebraska, native vegetation included luxuriant growth of the tallgrass prairie. In western Nebraska where precipitation is about half that in the east, plants of the shortgrass prairies grow much less abundantly. Thus, soil organic matter content is greater in the east than in the west.
Higher temperatures can speed the rate of organic matter decomposition. Temperatures are typically higher in the southern portion of the state than in the northern portion (Fig. 1.3). Because of this trend, organic matter content decreases from north to south. However, the change in organic matter content from north to south due to temperature is minuscule when compared to the change from east to west due to precipitation.
Soils in eastern Nebraska commonly contain 3 percent organic matter as compared to about 1 to 2 percent in the west.
The most abundant living organism in the soil is vegetation. Vegetation influences the kind of soil developed because plants differ in their root systems, size, above ground vegetative volume, nutrient content and life cycle. Soils formed under trees are greatly different from soils formed under grass even though other soil-forming factors are similar. Trees and grass vary considerably in their search for food and water and in the amount of various chemicals taken up by roots and deposited in or on top of the soil when tree leaves and grass blades die.
Soils formed under grass are much higher in organic matter than soils formed under forests because of their massive fibrous root structure and annual senescence of above ground vegetation. Grassland soils tend to be darker, particularly to greater depths, and have a more stable structure than forest soils. Soils developed under grass are generally more fertile and best suited for crop production. Nebraska soils from any parent material are nearly all formed under grass and, with adequate water, can be very productive.
The kind of plant growing influences residue composition. For example, the decay products from conifer tree needles are different from those of hardwood tree leaves. These decay products affect soil formation and development differently when water moves them through the soil.
The kind of vegetation and climate also affects the kind and numbers of other organisms that live in the soil, such as insects, small animals, and microorganisms. Organisms chew, tear and digest plant and animal material, causing it to undergo further biochemical action as it decays. Nondecomposed plant and animal material may be consumed by some organisms while others feed off of organism excrements.
There are a multitude of organisms living in the soil. Included among them are mites, snails, beetles, millipedes, springtails, worms, ground squirrels, gophers, grubs, nematodes, and microorganisms (e.g., bacteria, fungi, actinomycetes and algae). Microorganisms are the most abundant organisms in the soil.
The activity of soil organisms is strongly influenced by soil temperature, acidity and soil-water relations. Their major contributions to soil are improved soil structure, nutrient transformations and fertility, aeration and enhanced productivity.
Under forests, soil microorganisms are more diverse than under grasslands; however, microorganisms under grasslands are more active and have greater mass than under forest conditions. In general, cultivated fields have fewer organisms than virgin areas. A generalized ratio for the mass of organisms under grass/meadow:oak forest:spruce forest is 13:5:1.
Among soil organisms, bacteria are most abundant followed by actinomycete (rod-shaped microorganisms) and earthworms. As much as 4,000 pounds of bacteria can be present per acre-furrow slice (furrow slice = a 6-inch depth of soil). This is more than four times the mass of earthworms that can be present.
Because of the quantity of organisms present in the soil and their ability to accelerate the decay of organic material, they play a major role in soil formation.
Variations in topography affect moisture and temperature relations. While Nebraska is considered to be in the Great Plains, the topography within its borders varies greatly. From a broad perspective the state can be divided into regions encompassing valleys, sandhills, plains, rolling hills, dissected plains, bluffs and escarpments, and valley-side slopes (see Appendix 1: Topographic Regions of Nebraska). Each of these topographic regions has some common features which affect soil formation.
On a local scale, we can compare a nearly level field with one that is hilly. The steeper the slope, the more influence topography has on soil development on hills and steep land. Runoff is accelerated on sloping land, so less water infiltrates the soil. Plants, therefore, tend to have shallower root systems; and less organic matter is produced, as compared to nearly level land. Steep slopes are also subjected to more erosion which removes soil as fast, or faster, than it forms. On nearly level land, water tends to pond on the soil surface. Here, plant growth may be prolific, resulting in the production of large amounts of organic matter.
Slopes with a southern exposure are warmer and drier than slopes with a northern exposure. In fact, topography affects the micro-environment for soil formation in a manner similar to climate’s affect on macro environment for soil formation.
Soils have been referred to as young, mature, and old, depending on the degree of weathering. A mature soil is in equilibrium with its environment and shows full development of layers or horizons in its profile (Fig. 1.4).
Soils probably never reach equilibrium, but they do get older and are weathering all the time. The rate of weathering, however, slows considerably as the soil nears equilibrium with its environment. The longer a parent material has been exposed, the greater the degree of weathering and the more developed the soil. Soils in southeast Kansas, for example, are highly weathered. Parent materials in southeast Kansas have been exposed for about 200 million years. This compares to the loess soils in Nebraska, which are only 10 to 50 thousand years old.