Forest Soils of Mississippi
People often take for granted the importance of soil in their lives. Soils provide the foundation for our homes, cities, and roads. Soils are the medium for bountiful crops and forests. Soils store and filter the groundwater that nourishes our lives. Indeed, soils provide for the diversity of plants and wildlife upon which Mississippians depend for commerce and recreation.
Landowners interested in making the most of their forest need a basic understanding of soils. Forest productivity and wildlife habitat begin with the quality of the soil. Knowing proper soil management techniques is crucial to conserving the land and protecting watersheds from soil erosion.
Soils form through the interactions of physical, chemical, and biological mechanisms on the geological materials exposed to the earth’s surface. Essentially, soil is the product of weathering by climate and organisms, which are acting upon a geological parent material. These processes are affected by local topography over very long periods of time. Soil takes hundreds to thousands of years to develop in the landscape. Consequently, soil is not a renewable resource, even though the crops and forests grown on the soil are renewable.
Geologic formations that are exposed to the surface of the earth provide the parent material for the production of soil. In Mississippi, the geology for most of the state is dominated by the Mississippi embayment. This large, inland sea formed as the North American continent was taking shape. The Mississippi embayment covered most of what is now Mississippi and Louisiana. Marine and alluvial sediments that were deposited over hundreds of millions of years dominate the geology of Mississippi. The oldest deposits (some more than 250 million years ago) are in the northeast corner of the state as part of the Appalachian foothills, while the youngest soils are constantly being formed by annual flooding in river bottomlands across the state.
More recent soils of the Mississippi River floodplain formed during the Ice Age up to 1 million years ago. The Mississippi River drained immense glacial lakes that formed as the great ice sheets melted. Eolian (wind-blown) deposits of this glacial outwash formed the Loess Hills bordering the Delta region to the east. Altogether, sedimentary deposits are relatively easier to weather than rock. The most recent alluvial sediments in the Delta are very fertile and are the basis for the success of Mississippi’s agricultural industry.
Mississippi has a warm, humid climate. Weather patterns are dominated by the continent to the north and the Gulf of Mexico to the south. During the summer, average daytime temperatures are typically in the 80s or 90s. Annual average winter temperatures are in the 40s in the north and 50s along the coast.
Average annual precipitation increases moving southward across the state, with around 50 inches falling in north Mississippi and 67 inches at the coast. The frequency of thunderstorms also increases moving southward across the state, from about 55 days per year in the north to 75 days at the coast.
Tropical storms are common for coastal counties. On average, the Mississippi coast experiences a hurricane once every 12 years. Although wind damage is limited to the coastal region, heavy rainfall and flooding can occur throughout the state during these storms. Mississippi is also in the path of tornadoes that move across the country with advancing cold fronts in the spring and fall.
This warm, humid weather accelerates the weathering of parent materials that make up the geology of the state. Weathering involves the biological processes and chemical reactions that transform parent material into soil, which occur faster as the temperature rises. Water is the universal solvent and provides the means for weathering. Rainwater is slightly acidic due to dissolved carbonic acid, which hastens the weathering of minerals from these sediments.
Consequently, the soils of the Coastal Plain are more weathered than those in river bottomlands. The soils of the river bottoms, including the Mississippi River floodplains, are much more variable than Coastal Plain soils in terms of fertility, degree of weathering, and development. This has implications for agriculture, forestry, and wildlife management.
Mississippi has a great variety of vegetation from north to south and east to west, forming major eco-regions across the state. These include the Mississippi Delta, which is dominated by agriculture and bottomland hardwood forests of oak-gum-cypress. Forests in the Delta include numerous bottomland species, such as bald cypress, willow oak, swamp chestnut oak, cottonwood, black willow, green ash, sugarberry, and swamp hickory.
To the east of the Delta are the Loess Hills. The vegetation in the Loess Hills region is dominated by cherrybark oak, yellow poplar, water oak, mockernut hickory, and sweetgum. Also, these soils have some of the highest productivity for loblolly pine in the state.
The northern section of the state is within the Upper Gulf Coastal Plain. Vegetation is dominated by oak-pine and oak-hickory forests. Common species here include shortleaf and loblolly pine, upland oaks (such as southern red, white, post, scarlet, black, and blackjack), hickory, and sweetgum.
The Blackland Prairie region has soils with relatively high alkalinity. Vegetation in this region must be tolerant of higher soil pH, such as Eastern redcedar, post oak, blackjack oak, shumard oak, and hickory.
The Lower Coastal Plain and Northern Gulf of Mexico regions are dominated by upland forests of longleaf pine, slash pine, and upland oaks (such as live, southern red, and turkey). Longleaf pine is well adapted to the frequent, low-intensity fires that burned across this landscape. Slash pine, while tolerant of fire as a mature tree, is less tolerant as a seedling or sapling and needs to be kept in wetter sites. Live oak is very hardy and tolerates the high winds of tropical storms and salt spray along the coast. Bottomland species in this region include American elm, bald cypress, green ash, sugarberry, water oak, and red maple.
Pine and oak vegetation tend to produce organic acids upon decomposition due to their oleoresins and tannins, respectively. Their decomposition will tend to acidify soils, which further accelerates mineral weathering of geologic sediments.
Prehistoric humans also influenced the Mississippi landscape through the use of fire. As Native American cultures developed agriculture, humans used fire more extensively. This tool was used to clear land for farming and to keep hunting grounds open. Consequently, pre-Columbian civilizations created a mosaic of villages, agricultural fields, and extensive woodland-savannas in Mississippi. Therefore, the native vegetation adapted to very frequent burning.
The effects of topography on soil formation occur on a smaller, local scale. First, the local relief of the land affects the internal drainage of water through the soil. A common observation is that soils on higher positions in the landscape, such as ridges, are dry, whereas those in lower positions, such as ravines and wide floodplains, are wet. These differences in drainage will affect the types of chemical reactions involved in the weathering of parent material into soil.
Secondly, available moisture in the soil has a tremendous influence on the kinds of vegetation growing across the landscape. This, in turn, affects the animals living in these areas. Altogether, these differences in the plant and animal communities from one area to the next will also influence the weathering of parent materials into soil.
The floodplains are the youngest, most fertile, and most highly variable soils. Otherwise, the soils found in Mississippi tend to be very old. These old soils are highly weathered and lower in nutrients than river bottomlands.
Certainly, prehistoric vegetation was very different than today’s. However, it still responded to changes in soils and climate. As the last Ice Age was ending, 10,000 to 12,000 years ago, beech, hornbeam, oak, white pine, and hemlock dominated Mississippi forests. As vegetation adapted to cooler climates, weathering processes also changed and slowed.
Yet, as vegetation developed on bare ground, organic matter was gradually incorporated into the soil-forming processes. As the climate warmed, the forest composition in Mississippi gradually developed to that which is seen today, with the southern pines, oaks, red maple, and sweetgum. Further, the organic acids from decaying pine needles and oak leaves accelerated the weathering process.
Describing Soil Layers
Over time, soil-forming processes transform geological parent material into soil. Intensive weathering of the topsoil occurs near the soil surface. This weathering dissolves and transports minerals downward through the soil. As the minerals move downward, the chemical and physical interactions in the soil change, with new minerals created and deposited. This results in the formation of layers, or horizons, in the soil. These layers are characteristic of the geology, hydrology, chemistry, and vegetation on a site. Horizons and their properties are used to describe the soil (Figure 1). Horizons are called O, A, E, B, C, and R.
The O horizon is on the soil surface and consists of leaves, branches, and other organic materials in varying stages of decomposition. In forests, this horizon is divided into the litter and duff layers. While the O horizon is not a part of mineral soil, it is very important. It is in the litter and duff layers that organic material is recycled, with nutrients being returned to the soil through decomposition. These layers also help prevent wind and water erosion of soil particles.
The A horizon is called topsoil. It is a combination of mineral soil and organic matter that has been incorporated from the litter layer. The topsoil is a zone of leaching, as water moving through the soil washes minerals off soil particles.
Occasionally, there is a layer beneath the topsoil called the E horizon, which is also a zone of intense leaching. Consequently, this layer is comprised predominantly of washed sand grains. This layer is commonly found in the highly weathered soils of the Lower Coastal Plain and Coastal Flatwoods in southern Mississippi.
The B horizon is the beginning of the subsoil. This is a zone of accumulation, usually of clay minerals or compounds of iron, aluminum, and organic complexes. For many Mississippi soils, it is generally the next most fertile horizon after the A horizon.
The C horizon is usually deeper subsoil, considered to be the residual parent material from which the soil formed. It bears the effects of weathering but with relatively little influence from soil organisms, as compared to the O, A, E, and B horizons.
The R horizon is found beneath the C horizon and is the original bedrock. In Mississippi, the R horizon is found in the soils of the Appalachian foothills, in the northeastern-most corner of the state.
There are several soil chemical and physical properties that are important to forest management. They are organic matter, texture, porosity, drainage, and soil pH. These physical and chemical properties of the soil interact to determine its fertility. This concept is known as “carrying capacity.” Essentially, a given unit of land can produce a set amount of biomass, measured as yield. Below are more detail descriptions of these key soil properties that are so important to soil fertility.
Soil Organic Matter
Organic matter is a vital component to soil fertility. Organic matter helps retain plant nutrients and moisture. It also helps the development of soil structure, which is how individual soil particles bind together. Good soil structure leads to more and larger pore space, which is necessary for internal drainage and adequate aeration for plant roots. Organic matter decomposes into acidic materials, which further enhances chemical decomposition and weathering and influences soil acidity.
Plants and animals are the sources for organic matter. Since the natural vegetation for most of Mississippi is forest, organic matter often accumulates in litter and duff layers atop the mineral soil. Soil organisms such as springtails and millipedes eat and break down the litter layer into finer material that constitutes the duff layer. Other soil organisms, including fungi, microbes, and earthworms continue decomposition of the duff and further transform the organic matter into humus. Eventually, some of the humus is mixed with the mineral soil to form the A horizon.
Soil texture is the proportion of different particle sizes of soil material. Sand has the largest particles, followed by silt, then clay. Texture can be determined by two methods: separation of the size fractions in the lab, and by feel in the hand.
Texture by feel in the hand is determined by how well moist soil sticks together. Sandy soils have individual grains that feel gritty. If the soil holds its shape (known as a cast), it is loamy. If the cast can be rolled into thin ribbons between the fingers, that indicates clay.
Traditionally, clay soils were referred to as “heavy” because the soil stuck to the plow and made it heavy. By contrast, sandy soils were known as “light” soils because the sand grains did not stick to the plow.
Soil is not a solid mass. The spaces between soil particles are called pores, which allow for the movement of water and air through the soil. Porosity is measured as the proportion of void space in a given volume of soil. Ideally, nearly half the volume of soil should be pore space.
Large pores allow internal drainage of water through the soil and aeration for roots. Small pores are essential for retaining water for root absorption.
If the soil is too dense, plants cannot absorb sufficient water and air from the soil. Insufficient pore space, therefore, may restrict root growth or even lead to plant death. Further, roots cannot penetrate highly compacted soil where pore space has been removed.
The position of the soil in the landscape determines the depth to the seasonal water table. Wet soils (very poor, poor, and somewhat poorly drained) will have a high water table for some time during the growing season. Root growth may be limited due to excessive moisture and limited aeration for respiration. On the other hand, drier soils (moderately well, well, and excessively drained) have a deeper water table. As drainage improves on drier sites, moisture may become limited during the growing season.
Soil pH is a measure of acidity or alkalinity, which is the concentration of hydronium ions in the soil solution on a negative logarithmic scale. A soil pH of 7 is neutral, a pH value less than 7 is acidic, and a pH value greater than 7 is alkaline. The soil pH influences nutrient uptake and tree growth.
Many soil nutrients change chemical form as a result of reactions in the soil that are largely controlled by soil pH. Soils with a pH between 6.0 and 6.5 generally have the best growing conditions with regard to soil chemistry. At these pH levels, most nutrients are readily available. Nevertheless, the vast majority of commercially important tree species can live in a broad range of soil pH values as long as the proper balance of required nutrients is maintained.
Soil pH values at the extremes (less than 4.0 or greater than 8.5) can make some nutrients toxic and others unavailable to plants. At lower pH levels (less than 4.5), aluminum, iron, and manganese are very available for plant uptake, while at high pH levels (greater than 8.5), calcium and potassium are overabundant. In these situations, many plants will take up too much of some nutrients and not enough of others. This is what causes toxic conditions.
Major Soil Resource Areas
In Mississippi, there are several major soil resource areas (Figure 2). These predominant soils and composite vegetation define major ecoregions across the state. These are the Delta, Loess Hills, Upper Coastal Plain, Blackland Prairie, Lower Coastal Plain, and Flatwoods.
The Delta region of western Mississippi is part of the Mississippi River floodplain. Soils here are moist to wet, with medium to heavy texture. Most of this area is in agricultural production, with some in bottomland hardwoods and a little in pasture. Soybeans, cotton, wheat, and rice are the main crops.
Adjacent to the Delta to the east are the Loess Hills, formed from wind-blown glacial outwash some 10,000 to 12,000 years ago. These silty upland soils are deep, well drained, and fertile. Forest production predominates, although agricultural production of cotton, corn, soybeans, and wheat is important.
Upper Coastal Plain and Interior Flatwoods
The Upper Coastal Plain and Interior Flatwoods major soil regions are in the northeastern section of the state. The soils in these regions are older and more highly weathered, so they are less fertile than those to the west. These soils display advanced soil development, with well-defined topsoil and subsoil horizons. Mixed pine-oak forests predominate, with loblolly and shortleaf pines and a variety of upland and bottomland oaks. Major crops include soybeans, corn, peanuts, and vegetables. Pastures are a minor crop.
Interspersed within the Coastal Plains is the Blackland Prairie ecoregion. The Prairie soils bisect the northern and central portions of the state. Having developed from more chalky sediments, soils in this region are fine-textured with shrink-swell clays. While forestry is significant, there is a greater amount of pasture and cropland in this region.
Lower Coastal Plain and Coastal Flatwoods
In the southeastern corner of the state are the Lower Coastal Plain and the Coastal Flatwoods along the Gulf of Mexico. Predominant soils are sandy and wet with low native fertility. Forests cover nearly 90 percent of the area. The proximity of the coast caused natural vegetation—including longleaf and slash pine and live oak forests—to adapt to tropical storms.
Proper soil management requires a long-term commitment by landowners to sustain key soil properties. They must:
- protect the soil from erosion,
- maintain good physical condition of the soil,
- maintain proper balance of chemical characteristics, and
- maintain and enhance the organic components of the soil.
Management techniques will vary widely depending upon land use, soil region, specific location, and topography of a given property.
Landowners are strongly encouraged to obtain a detailed soil map of their property from the Natural Resources Conservation Service. These are available in the local NRCS office or online at https://websoilsurvey.sc.egov.usda.gov/App/HomePage.htm. These manuals provide a great deal of useful information, including drainage, fertility, best tree species to plant or manage, and development ratings.
Site-level soil properties can be determined by the Mississippi State University Extension Service soil testing laboratory for a modest fee. Forms and soil sample boxes are available at the local county Extension office.
Routine soil testing includes soil pH, lime requirements, and the amounts of available nutrients. If you specify a crop, fertilizer recommendations will be included, as well. Many helpful publications on interpreting soil test results for timber, horticulture, agriculture, and wildlife food plots are available at http://extension.msstate.edu. Search under the Publications link.
For More Information
Chapman, S. S., G. E. Griffith, J. M. Omernik, J. A. Comstock, M. C. Beiser, and D. Johnson. 2004. Ecoregions of Mississippi. U.S. Geological Survey. Reston, VA.
Crouse, K. and W. McCarty. 2016. Soil Testing for the Farmer, Information Sheet 346. Mississippi State University Extension Service, MS. 4p.
Eyre, F. H., ed. 1980. Forest Cover Types of the United States and Canada. Society of American Foresters, Washington, DC. 149 p.
Fickle, J. E. 2001. Mississippi Forests and Forestry. University Press of Mississippi, Jackson, MS. 347 p.
Fisher, R. F. and D. Binkley. 2000. Ecology and Management of Forest Soils. John Wiley and Sons, Inc. New York. 500p.
Publication 2822 (POD-07-17)
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