The Soils of Mississippi

Note: This material is a composite of various articles on soils, mostly excerpted and modified from MS extension service, NRCS and other agricultural literature - Samuel Faulkner

Introduction to Soils
There are many definitions of soil. One way to think about soil is to define what it is not. Soil is not dirt. While it is possible that "dirt" may have once been soil, once it has been removed from its place in the landscape, it ceases being soil. It can become soil again under the right circumstances. Potting "soils" are technically not soils, they are “growing media”. Pure sand, rock wool or even fertilzers dissolved in water in "hydroponic" growth systems, can also serve as media, and none of these should be confused with soils. Our discussions here will concentrate on true soils.

In an ecological context, soil should be thought of as a three-dimensional living, dynamic resource that supports plant life. In most courses on soil science, soil is defined as a "natural body" derived from weathering and decomposition processes, and capable of supporting plant life and other organisms. Soil is an interesting feature of the natural landscape in that it has constantly changing biological, chemical, and physical properties that affect its ability to function in the environment.

Functions of Soils in the Environment
There are many functions of soil. Some are obvious, 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.

Soil Development (Pedogenesis) – Soils are an end product of the rock weathering cycle and effects of the biota. Jenny in the 1940s noted that soil formation is a function of five major factors: 1) parent materials (the regolith); 2) climate (temperature, precipitation, etc.); 3) biotic factors (mostly effects of plants); 4) topography (high slope areas change quicker than flat places); and 5) time (soil formation takes a long time). Of course, other factors are at work as well.

The Soil Profile – As they weather, and precipitation percolates through a soil, distinct layers called horizons, will form. Very young soils (inceptisols) will not have profiles at all. The entisols (very common in the Delta – why?), will have faint horizons. The alfisols (brown forest soils) have well-defined horizons, as do most others. In a well-developed soil, the first (top) layer we’ll see is the humus layer or O horizon. The thickness of this dark layer will depend on many factors – vegetation type, climate, etc. It’s fairly thin in Delta soils, but can be more than several cm thick in hilly areas. Next, we’ll see the “topsoil” or A horizon. If this layer has been disturbed (as most have), it’s called an Ap horizon. The A horizon will vary in thickness from place to place. The A transitions into the B horizon. This transition is called the E horizon. It’s usually lighter in color than the A or B. The B is commonly called “subsoil” and because of clay-percolation and accumulation, this layer is often clayey in texture. The B arises from percolation processes from the A, and weathering of the C horizon (if present). The C represents partially weathered parent material. Since Delta soils have arisen from flood deposition processes, the C is poorly defined or even non-existent. Below the C is the bedrock or parent material. Since the silty soils of the Loess Bluffs are of recent wind (aeolian) origin, what would you suppose the profiles there are like?

Soil Taxonomy – Like the biota, there is a classification system for soils. The system used in the US is The Seventh Approximation. At the highest level, soils are divided into 12 orders:

entisols – very young soils that show little weathering;
inceptitisols – young soils that show very faint horizonation;
vertisols – soils with shrink-swell clays (common in Delta);
alfisols – brown forest soils (a few in the Delta);
mollisols – dark, rich soils of mostly grasslands (we have a few in the Delta);
histosols – organic soils (mostly in marshy or montane areas);
oxisols – highly weathered soils of the tropics;
ultisols – highly weathered soils of mostly temperate areas (“red dirt” in MS);
spodosols – ashy soils of wet, sandy areas;
aridosols – arid-region soils;
andisols – dark soils formed from volcanic activity; and
gelisols – frozen soils of tundra areas.

See more details on taxonomy below, as well as discussions on land use, etc.

Components of Soil
If one breaks a soil down into its various components, one finds a complex mixture of weathered rock materials - i.e., different sized mineral particles, organic matter, a soil biota of bacteria, fungi and various invertebrates, and soil atmosphere ("air") and water with dissolved substances.

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