Environmental Science, Soil Conservation, and Land Use Management

Educator Background

Sustainability

Sustainability is existence maintained over time. A sustainable system is one where current and developing practices can continue without exhausting the natural resource on which that existence depends. In other words, resources are renewed or replenished rather than depleted. Sustainability promotes balance with the natural world, avoids damaging that balance, and encourages development in harmony with the natural world.

Best Management Practices

Best management practices (BMPs) are activities that landowners and managers (whether urban, suburban, or rural, in forestry or agriculture) can use to help conserve soil and water resources. These practices help reduce human effects and remain sustainable in the long run. BMPs are proven to reduce soil erosion and pollution and improve water and environmental quality. Management practices can address the source of the problem, the outcome, or both. BMPs for human activities that impact soil quality and amplify natural soil degradation processes are described in detail in Chapter 6 in the Know Soil Know Life book. BMPs for agricultural soils are summarized in Table 6-1 of the book.

Natural Processes Affecting Soil Degradation

Natural processes that affect the soil in both natural and agricultural ecosystems include erosion, acidification, desertification, and salinization.

Erosion

Erosion is a threat to sustained agricultural production – and therefore to sustaining human populations and cultures. Erosion occurs when soil particles are detached, transported, and deposited. Although erosion occurs in natural ecosystems, the rate of soil formation (see Chapter 2) in humid and semiarid regions is approximately equal to the rate of erosion, so a somewhat constant amount of soil remains in place. Because of the way they are managed, agricultural ecosystems may experience accelerated wind and water erosion, so soil erodes faster than it forms. If more soil erodes than forms, the amount of available soil decreases over time, threatening long-term sustainability.

You can see the long-term effects of water erosion in the shape of landscapes in many regions of the United States and around the world. Rivers meander and change course as stream banks erode. Weathering and water erosion create beaches, which continue to change due to tidal erosion as well as erosion from hurricanes and tsunamis. Rivers, lakes, estuaries, bays, and water bodies receive runoff that carries sediment (displaced soil) from construction sites, agricultural land, and urbanized areas. Such sediments fertilize floodplains and deltas, but are also a prominent source of water pollution. Water erosion is caused by two detaching forces: raindrop impact and flowing water. Three types of water erosion can occur: sheet, rill, and gully. Water control methods attempt to decrease the kinetic energy of the water by limiting soil detachment, decreasing transportation, and encouraging deposition.

Wind erosion damages millions of hectares of land in the United States annually, especially in the Great Plains. Wind erosion is worse in the western Great Plains than in the eastern parts for several reasons. The west receives less precipitation, so there is less vegetation on the soil surface. The landscapes are generally flatter, with few hills and trees to slow the wind at the soil surface. The soils also tend to have less organic matter, and therefore less "glue" to hold aggregates together, making the soils more susceptible to wind erosion. Although all soils can be moved by wind, sands are the particles that begin moving first. The moving sand particles detach silt and clay particles, where they can be transported great distances. Sand particles are heavier and are moved a few centimeters to a few kilometers, but silt particles can be carried for a few hundred kilometers and clays for thousands. There are three types of wind erosion: surface creep, saltation, and suspension. The main principles for controlling wind erosion are similar to those for controlling water erosion: protect the surface and reduce energy.

Desertification

Desertification is the extreme degradation of productive land in arid and semiarid regions. It is a natural process associated with global climate changes. With time, as the climate changes, even forests may become deserts. The Petrified Forest National Park, for example, now sits in the middle of the Arizona desert. Desertification can also be brought on by improper management practices (INSERT see image – Image 6-24 from KSKL). Grasslands adjacent to deserts are usually characterized by low precipitation amounts that can vary greatly from year to year. Although they have low productivity and carrying capacity, such grasslands are typically used as rangeland for grazing cattle, sheep, goats, or other livestock. Putting more animals on an area than it can support results in overgrazing: too much of the grass is harvested, and the perennial grasses decrease in vigor and death with continued overgrazing. The lack of soil cover increases the potential for erosion of soil and grass seeds. Without sufficient precipitation, the perennial grasses do not recover or reseed themselves, and shrubs and annual grasses more characteristic of desert vegetation encroach. Range management BMPs include controlling the livestock stocking rate and grazing intensity, keeping livestock out of sensitive areas, and providing livestock with alternative locations for water and shade. Adequate vegetative cover should be maintained to prevent accelerated erosion.

Acidification

In sub-humid and humid regions, at times during the year, the amount of precipitation exceeds what the soil can hold and plants can use. Acidification occurs when base cations leach from the soil in the excess water, leaving acidic cations behind. The pH decreases as the soil becomes more acid. This process can be accelerated by the application of certain fertilizers. Producers managing acid farmlands regularly apply lime (calcium carbonate or similar minerals) to mitigate the acid in the soil.

Salinization

Arid and semiarid lands are subject to another devastating challenge to their soil's productivity: salinization, or the buildup of salts. The natural growth potential of plants in these regions is limited by lack of water, even though the soils can be very fertile. Consequently, farmers and governments invest in irrigation technologies. Without proper management, irrigation can turn soil into a salty, degraded mess. Like ocean water, freshwater also contains salts, though in lesser amounts. Salinization occurs when the water evaporates from soil. The previously dissolved salts get left behind and begin to accumulate in the soil. Since these areas are dry, very little rainfall is available to wash out, or leach, these salts. Although it can occur when groundwater is used, salinization is most commonly associated with irrigation from rivers. River water in dry regions already contains elevated concentrations of salts, and high evaporative demand leads to some of these salts accumulating in the surface soil. In addition, poorly managed excessive irrigation also leads to rising groundwater tables in lowlands. As brackish (saline) groundwater rises, capillary action (see Chapter 2) pulls this water up to the surface where the water evaporates or is used by plants, leave the salts behind to accumulate on the soil surface. High salt concentrations are harmful to growing plants because salts "compete" with plants for water.

Human Activities Affecting Soil Degradation

Human activities such as deforestation, intensive tillage, excessive livestock grazing, poorly managed irrigation, and chemical fertilizer inputs, and low returns of crop residues accelerate erosion, acidification, desertification, and salinization beyond natural rates on a global scale. Population growth requires more living space and services, so every year natural and agricultural ecosystems are destroyed to build housing subdivisions, water treatment facilities, roads, schools, shopping malls, factories, and recreational facilities. All of these activities contribute to the worldwide trend of soil degradation.

Deforestation

Wood is a renewable, carbon-neutral energy source when managed correctly, but when done improperly timber harvesting often leads to soil degradation and other environmental issues. The effects of tree loss on soil are significant. Trees and shrubs provide shade that reduces soil temperature, which in turn reduces evaporation. Trees and shrubs also shield the ground from the force of raindrops, so the removal of this vegetation can expose soil to rain splash. This loosens soil and dislodges soil particles, eroding soil and create a more impermeable bare surface, which increases runoff. On steeper slopes, the topsoil is removed or degraded, and the biological health and productivity of the soil also decreases.

Urban and Rural Issues

Every year, urbanization removes more land from food production, putting more pressure on the remaining farmland to produce more crops and livestock. In developing countries, urbanization is an increasing threat to food production and security. People live near water, and the best soils for producing food are often the same soils where cities are built. As cities expand, more highly productive soil is converted for urban purposes. Food production is driven farther away from the water, often into soils with more marginal productive capacity, and which may be more susceptible to erosion and other forms of degradation. Urbanization also contributes to runoff and erosion potential because of the preponderance of houses, roads, parking lots, and other impermeable surfaces. When native landscapes are converted to urban uses, runoff increases several-fold. Soil loss due to construction activity is a small fraction of the whole but is still a common occurrence. Construction sites are often stripped of all vegetation, leaving bare, compacted soils with little infiltration, increasing runoff and erosion potential. Regulations and construction codes now require mitigation and erosion control practices to be used during construction.

Land Application of Manures and Treated Wastes

One of the greatest challenges associated with human, livestock, and food production wastes and byproducts is the quantity of nutrients they contain.

Manures and byproducts of waste treatment systems are organic, non-commercial nutrient sources that can be used on farms, lawns, and golf courses. These wastes all contain nutrients that when applied to land may enter surface waters through runoff and erosion or through leaching. Nutrients found in surface and groundwater include nitrogen, phosphorus, potassium, sulfur, iron, carbon, manganese, boron, and cobalt. When nutrients get into surface waters, nutrient overload (eutrophication) results

Most meat animals today are fed in confined animal feeding operations (CAFOs). Feed (along with the nutrients in the feed) are imported from elsewhere. Concentrating so many animals in one place greatly increases the efficiency of production. However, the amount of manure and effluent (liquid waste from runoff or washing production floors) produced per area increases several-fold with CAFOs. These operations are considered point sources of pollution and are regulated under federal law. The wastes contain essential plant nutrients and are typically disposed of through land application. However, when the wastes are applied improperly or over-applied, the result can be increased nutrient loads in leachates to groundwater and runoff that reaches the surface waters. Further, when wastes are applied to the same land over many years, nutrients may increase to levels that limit plant growth. Waste management is a BMP that includes the land application and treatment by soil of animal, human (residential or municipal), and food-processing wastes in an environmentally sound manner.

Mineland Reclamation

There is little doubt that surface and underground mining have dramatic environmental effects on the soil and the surrounding landscape. Mine wastes affect soil's physical, chemical, and biological properties and overall soil quality. Wide areas are exposed to erosion due to the nature of mining and construction activities. A variety of technologies aim to minimize erosion effects, including silt fencing, straw bale barriers, sedimentation ponds, and revegetation.

Agriculture and Soil Degradation

Historical methods of agricultural production include inverting the soil, which buries crop residues and breaks up soil aggregates, leaving a bare, uniform surface in which to plant seeds. Bare soil aggregates are exposed to energy in raindrops and wind. Such conditions promote accelerated water and wind erosion. Erosion moves the finer particles from the surface. These particles take nutrients and organic matter with them, decreasing the fertility and productive potential of the soil. The effect on agriculture is the loss of fertile ground, which necessitates increased use of fertilizer to sustain crop yield, which in turn increases the cost of food production.

To control erosion, farmers employ a variety of BMPs: reduced tillage, conservation tillage, residue management, contour plowing, strip cropping, cover crops, crop rotations, diversions, terraces, grassed waterways water control structures, and buffer strips.