Course: BIS 105 Environmental Biology

Unit 8 Learning Module

Ecological and Societal Aspects of Soil

Ecological and Societal Aspects of Soil

As the medium for native plant growth, agriculture, building construction, waste disposal, and a pathway for groundwater infiltration, soil plays an important role for many key activities of our society. Soil scientists, agronomists, foresters, plant biologists, land-use planners, engineers, archeologists, and geologists, among others, all consider soil type (composition, texture, structure, density, etc.) in many aspects of their research or work. Below are some examples of the importance of soils in natural plant growth, in agriculture, and related societal issues. The long-term sustainability of soil is vital to both human ecology, even in modern society, and the ecology of our natural surroundings.

Soil-Plant Relations: Natural Processes

Soil plays a key role in plant growth. Beneficial aspects to plants include providing physical support, heat, water, nutrients, and oxygen. Heat, light, and oxygen are also obtained by the atmosphere, but the roots of many plants also require oxygen. Elemental nutrients, dissolved in soil water solution, are derived from soil minerals and organic material.

Plants mainly obtain nutrients from dissolved soil solutions. Though many aspects of soil are beneficial to plants, excessively high levels of trace metals (either naturally occurring or anthropogenically added) or applied herbicides can be toxic to some plants.

The ratio of solids/water/air in soil is also critically important to plants for proper oxygenation levels and water availability. Too much porosity with air space, such as in sandy or gravelly soils, can lead to less available water to plants, especially during dry seasons when the water table is low. Too much water, in poorly drained regions, can lead to anoxic conditions in the soil, which may be toxic to some plants.

Several elements obtained from soil are considered essential for plant growth. Macronutrients, including C, H, O, N, P, K, Ca, Mg, and S, are needed by plants in significant quantities. C, H, and O are mainly obtained from the atmosphere or from rainwater. These three elements are the main components of most organic compounds, such as proteins, lipids, carbohydrates, and nucleic acids. Oxygen generally serves as an electron acceptor and is required by roots of many plants. The other six elements (N, P, K, Ca, Mg, and S) are obtained by plant roots from the soil and are variously used for protein synthesis, chlorophyll synthesis, energy transfer, cell division, enzyme reactions, and osmotic or ionic regulation.

Micronutrients are essential elements that are needed only in small quantities, but can still be limiting to plant growth since these nutrients are not so abundant in nature. Micronutrients include iron (Fe), manganese (Mn), boron (B), molybdenum (Mo), chlorine (Cl), zinc (Zn), and copper (Cu). There are some other elements that tend to aid plant growth but are not absolutely essential.

Micronutrients and macronutrients are desirable in particular concentrations and can be detrimental to plant growth when concentrations in soil solution are either too low (limiting) or too high (toxicity). Elemental nutrients are useful to plants only if they are in an extractable form in soil solutions, such as an exchangeable cation, rather than in a solid mineral grain. As nutrients are used up in the microenvironment surrounding a plant's roots, the replenishment of nutrients in soil solution is dependent on three aspects: (a) the rate of dissolution/alteration of soil minerals into elemental constituents, (b) the release rate of organically bound nutrients, and (c) the rate of diffusion of nutrients through the soil solution to the area of root uptake.

Many nutrients move through the soil and into the root system as a result of concentration gradients, moving by diffusion from high to low concentrations. However, some nutrients are selectively adsorbed by the root membranes, such that elemental concentrations of solutions within plants may differ from that in soil solutions. Most nutrients exist as exchangeable cations that are acquired by roots from the soil solution—rather than from mineral or particle surfaces. Inorganic chemical processes and organic processes, such as the action of soil microorganisms, can help to release elemental nutrients from mineral grains into the soil environment.

Soil Health and Sustainability

Overall soil health can generally be defined as the capacity of the soil to function in a way that infiltrates water and cycles nutrients to support plant growth. Long term health of native soil is in many cases improved by disturbing the soil less, growing a greater diversity of crops, maintaining living roots in the soil, and keeping the soil covered with residue. Stable soil aggregates are important for soil health as they promote proper infiltration and thus limit the amount of water runoff —this has the added benefit of reducing soil erosion and downstream flooding and sedimentation.

Management of soil on farms may include use of tillage, fertilizer, pesticides, and other tools that may improve soil health if used correctly; however, significant damage to soil may result otherwise. Tillage with a plow or disk can be physically disruptive to soil fauna and microbes. The complex relations between soil and plant life, which have evolved into a sustainable relationship in the natural world, can be disturbed chemically by misuse or overuse of fertilizers or pesticides. Thus, to maintain soil health, one needs to understand the chemical, biological, and physical processes that operate in the natural soil profile. To the extent possible, we must work with the complexity of processes that function in a healthy soil and limit our disturbances to only those that are clear, practical necessity. Biodiversity is another important aspect to consider, because increasing the biodiversity of plants that are grown in soil can limit disease and pest problems and allow for a better functioning food web. More diversity in plants above ground leads to more diversity in the subsurface food web. Consequently, increasing the diversity of appropriate crop rotation in agricultural lands can ultimately lead to better soil health and limit problems in the long run.

Agriculture and Food Capacity

Soils on arable lands globally are a resource to society with potential use for food production. Production is ultimately limited by soil type, climate, hydrology, and land management. The native soil type is what has been provided by the land, from centuries or millennia of soil development, typically under mostly natural conditions under native plant vegetation. The effect of human populations may have been to drain land for cultivation (affecting hydrology), to modify the landscape, build structures, and to remove native vegetation. Some modifications have aided with food production. Others have had unintended consequences of causing land degradation, such as salinization, topsoil erosion, compaction, pollution, desertification, or depletion of soil nutrients.

Some of these issues are of serious concern in developing countries where oversight and regulations protecting the land may not be in place. For instance, overgrazing and rapid deforestation of the land, and generally poor land management, can lower the organic matter content of surface soils, thus lowering fertility and increasing the likelihood of topsoil erosion due to removal of the protective vegetative covering. As the world's population continues to increase, we will need to find ways to continually increase (or more effectively utilize) food production capacity from an essentially fixed amount of arable land worldwide. As population density has increased, crop yields and the numbers of acres in production have been continually increasing, with technological advances and more land in agriculture. This is not a sustainable trend, though, since the land area on earth is finite. In fact, some prime farmland is even being removed from production in developed countries as urbanization and land development occur on the ever-expanding edges of population centers. Efforts will need to be made to preserve enough high yield farmland to be sustainable for future generations.

Soil Compaction, Tillage, and Sustainable Practices

In modern agricultural practices, heavy machinery is used to prepare the seedbed, for planting, to control weeds, and to harvest the crop. The use of heavy equipment has many advantages in saving time and labor, but can cause compaction of soil and disruption of the natural soil biota. Much compaction is reversible and some is unavoidable with modern practices; however, serious compaction issues can occur with excessive passage of equipment during times when the soil has a high water content. The problem with soil compaction is that increased soil density limits root penetration depth and may inhibit proper plant growth.

Current practices generally encourage minimal tillage or no tillage in order to reduce the number of trips across the field. With proper planning, this can simultaneously limit compaction, protect soil biota, reduce costs (if performed correctly), promote water infiltration, and help to prevent topsoil erosion (see below). Tillage of fields does help to break up clods that were previously compacted, so best practices may vary at sites with different soil textures and composition. Crop rotation can also help to reduce bulk density with planting of crops with different root depth penetration. Another aspect of soil tillage is that it may lead to more rapid decomposition of organic matter due to greater soil aeration. Over large areas of farmland, this has the unintended consequence of releasing more carbon and nitrous oxides (greenhouse gases) into the atmosphere, thereby contributing to global warming effects. In no-till farming, carbon can actually become sequestered into the soil. Thus, no-till farming may be advantageous to sustainability issues on the local scale and the global scale.

Soil Erosion

Soil can take hundreds or thousands of years to mature. Therefore, once fertile topsoil is lost, it is not easily replaced. Erosion is the biggest cause of soil degradation. Soil productivity is reduced as a result of losses of nutrients, water storage capacity and organic matter. The two agents of erosion are wind and water, which act to remove the finer particles from the soil. This leads to soil compaction and poor soil tilth. Human activities such as construction, logging, and off-road vehicle use promote erosion by removing the natural vegetation cover protecting the soil.

Agricultural practices such as overgrazing and leaving plowed fields bare for extended periods contribute to farmland erosion. Each year, an estimated two billion metric tons of soil are eroded from farmlands in the United States alone. The soil transported by the erosion processes can also create problems elsewhere (e.g. by clogging waterways and filling ditches and low-lying land areas).

Wind erosion occurs mostly in flat, dry areas and moist, sandy areas along bodies of water. Wind not only removes soil, but also dries and degrades the soil structure. For instance, in the 1930's, drought conditions and poor land management methods (lack of cover crops and rotation) combined to result in severe wind erosion and dust storms in the Great Plains of the United States, which came to be known as the Dust Bowl. Deep plowing of soil and displacement of the original prairie grasses (that once held the soil together) also contributed to the crisis. Wind stripped large areas of farmlands of topsoil, and formed clouds of dust that traveled as far as the eastern United States.

Water erosion is the most prevalent type of erosion. It occurs in several forms: rain splash erosion, sheet erosion, rill erosion and gully erosion. Rain splash erosion occurs when the force of individual raindrops hitting uncovered ground splashes soil particles into the air. These detached particles are more easily transported and can be further splashed down slope, causing deterioration of the soil structure. Sheet erosion occurs when water moves down slope as a thin film and removes a uniform layer of soil. Rill erosion is the most common form of water erosion and often develops from sheet erosion. Soil is removed as water flows through little streamlets across the land.

Severe sheet and rill erosion on highly erodible soils in Cass County, Iowa after heavy rains. The field had no protection against soil erosion.

Gully erosion occurs when rills enlarge and flow together, forming a deep gully. |

Once the natural topsoil is eroded by wind or water, it is only slowly renewable to its former pre-eroded condition. It may take anywhere from several decades to hundreds of years to millennia, under replanted native vegetation, to restore the soil to a relatively natural (pre-disturbed) state with its original physical, chemical, and biological characteristics. Furthermore, when soil is eroded, the particles become sedimented downstream in streams, rivers, lakes, and reservoirs. If rapid, this sedimentation can deteriorate the water quality with sediment and agricultural chemicals. Better land management practices, such as more limited tillage or no-till practices, can help to greatly limit soil erosion to a rate that is sustainable over the long term. Practices today are somewhat improved overall, but more improvement in agricultural practices are needed over large areas of farmland in the United States and other countries to bring us on a path to long-term sustainability of agricultural lands.

Deforestation due to logging, construction, or increased fire occurrences can also cause significant increases in soil erosion in many areas globally and may be a particular problem in developing countries. Removal of the natural cover of vegetation enhances erosion since plant foliage tends to buffer the intensity of rainfall and roots hold soil together and prevent breakup and erosion. Furthermore, decomposing plant material provides a protective cover of organic material on the soil surface. Watersheds with large areas of construction or deforestation can experience several times the natural erosion rate. In such watersheds, streams can become clogged with unwanted sediment that disturbs the natural ecosystem and infills valuable wetland areas, in addition to the problem of valuable topsoil loss from upland areas.

Salinization

When considerable quantities of salt accumulate in the soil in a process known as salinization, many plants are unable to grow properly or even survive. This is especially a problem in irrigated farmland. Groundwater used for irrigation contains small amounts of dissolved salts. Irrigation water that is not absorbed into the soil evaporates, leaving the salts behind. This process repeats itself and eventually severe salinization of the soil occurs. A related problem is water logging of the soil. When cropland is irrigated with excessive amounts of water in order to leach salts that have accumulated in the soil, the excess water is sometimes unable to drain away properly. In this case it accumulates underground and causes a rise in the subsurface water table. If the saline water rises to the level of the plant roots, plant growth is inhibited.

Fertilizer Runoff, Ecological Effects, and Dead Zones

Nutrients in soil and water are generally beneficial when they exist at naturally occurring levels. Nitrogen fertilizers have been applied to farm fields for decades in order to maximize production of agricultural lands. However, an unintended consequence is that the same nutrients can be detrimental to aquatic ecosystems when introduced excessively for agricultural or other purposes. Nitrogen (N) and Phosphorus (P) are introduced by fertilizers that are used intensively in agriculture, as well as golf courses and some lawns and gardens. Farm animal waste and sewage also provide large amounts of reactive N and P. Phosphorus was formerly used heavily as an additive in laundry and dishwater detergents, but since the 1970's it has been phased out in both through a combination of state and federal regulations. Overall, our modern society has altered the global N and P cycles such that there is an overabundance in many settings.

Although atmospheric nitrogen gas is abundant, the gas is neither reactive nor utilized by most plants. Reactive nitrogen, in nitrate and ammonia fertilizers, is utilized by plants at some rate. However, excessive nutrients (not utilized) are often washed into drainage ways, streams, and rivers during rainfall and storm events. High N and P levels in surface water runoff have the effect of dramatically increasing algae growth downstream due to eutrophic conditions. The algal blooms have the unwanted effect of strong decreases in dissolved oxygen, which is needed for survival of fish and other aquatic life. Enhanced algae growth can thus disrupt normal functioning of the ecosystem and cause what are known as "dead zones." The waters may locally become cloudy and colored a shade of green, brown, or red. Eutrophication can occur naturally, but it has been greatly enhanced due to the use of fertilizers. As a result of eutrophication, many coastal waters contain increasingly problematic dead zones where major rivers discharge nutrient-rich agricultural runoff or poorly treated sewage and wastewater (e.g. Gulf of Mexico, Chesapeake Bay, Baltic Sea, East China Sea). This issue is of great importance because the dead zones are near inhabited coastlines with commercially and ecologically vital aquatic life.

The red circles show the size of many of our planet's dead (hypoxia) zones, whereas the plain black dots are dead zones of unknown size. Darker blue colors show high concentrations of particulate organic matter, an indication of overly fertile waters (high in N and P). Most dead zones occur in downriver of agricultural areas (with overused fertilizer) or areas of high population density with poorly treated wastewater.

One of the most notorious dead zones (second to the Baltic Sea) is an 8,500 square mile region in the Gulf of Mexico. The Mississippi River dumps high-nutrient runoff from its drainage basin that includes vast agricultural lands in the American Midwest. Increased algal growth produced by these nutrients has affected important shrimp fishing grounds in the Gulf. The primary source of the nutrients is the heavily tile-drained areas of farmland in the Midwest corn and soybean belt (SW Minnesota, N Iowa, NE Illinois, N Indiana and NW Ohio). Improved soil drainage systems over the past century or more have allowed for effective transport of nitrate compounds as stormwater runoff into drainage basins (Ohio River, Wabash River, Illinois River, Missouri River, etc.) that feed into the Mississippi River. In other words, the same drainage tiles that allow for the agricultural benefit of having rich bottomland/wetland soils in production, have the disadvantage of increased and more rapid movements of nitrate solutes to the Gulf of Mexico. Such large-scale problems, across state governmental boundaries, can only be fully addressed in the future with a national system of incentives, regulations, or laws.

In addition to fertilizers, Nitrogen inputs to watersheds can also include atmospheric deposition, livestock waste, and sewage, but nitrogen fertilizers comprise a significant majority of the input to monitored streams, particularly in springtime when much fertilizer is applied. Possible solutions to this problem include encouraging farmers to apply a more limited quantity of fertilizer in the spring (only as much as necessary), rather than in the fall, to allow for considerably less time for stormwater or meltwater runoff. Other solutions include maintaining cover crops, or restoring wetlands in key locations to contain nitrate losses. An overall strategy that limits the excess capacity of nutrients can simultaneously benefit farmers (by limiting cost), the ecology of stream watersheds and coastal ecosystems (also locally stressed by oil spills and other pollution). Over the long term, more efforts will need to be made in the Mississippi River Basin, and globally in similarly stressed agricultural or urban watersheds, to improve the health and sustainability of our soil, land, and aquatic ecosystems.

Soil Conservation

Because soil degradation is often caused by human activity, soil conservation usually requires changes in those activities. Soil conservation is very important to agriculture, so various conservation methods have been devised to halt or minimize soil degradation during farming. These methods include: construction of windbreaks, no-till farming, contour farming, terracing, strip cropping and agroforestry.

Creating windbreaks by planting tall trees along the perimeter of farm fields can help control the effects of wind erosion. Windbreaks reduce wind speed at ground level, an important factor in wind erosion. They also help trap snow in the winter months, leaving soil less exposed. As a side benefit, windbreaks also provide a habitat for birds and animals. One drawback is that windbreaks can be costly to farmers because they reduce the amount of available cropland.

One of the easiest ways to prevent wind and water erosion of croplands is to minimize the amount of tillage, or turning over of the soil. In no-till agriculture (also called conservation tillage), the land is disturbed as little as possible by leaving crop residue in the fields. Special seed drills inject new seeds and fertilizer into the unplowed soil. A drawback of this method is that the crop residue can serve as a good habitat for insect pests and plant diseases.

Contour farming involves plowing and planting crop rows along the natural contours of gently sloping land. The lines of crop rows perpendicular to the slope help to slow water runoff and thus inhibit the formation of rills and gullies. Terracing is a common technique used to control water erosion on more steeply sloped hills and mountains. Broad, level terraces are constructed along the contours of the slopes, and these act as dams trapping water for crops and reducing runoff.

Strip cropping involves the planting of different crops on alternating strips of land. One crop is usually a row crop such as corn, while the other is a ground-covering crop such as alfalfa. The cover crop helps reduce water runoff and traps soil eroded from the row crop. If the cover crop is a nitrogen-fixing plant (e.g. alfalfa, soybeans), then alternating the strips from one planting to the next can also help maintain topsoil fertility.

Agroforestry is the process of planting rows of trees interspersed with a cash crop. Besides helping to prevent wind and water erosion of the soil, the trees provide shade which helps promote soil moisture retention. Decaying tree litter also provides some nutrients for the interplanted crops. The trees themselves may provide a cash crop. For example, fruit or nut trees may be planted with a grain crop.