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 is can be physically disruptive to soilfauna 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 soiltype, 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

Accelerated erosion of topsoil due to human activities and poor agricultural land management is a potentially serious issue. The areas most vulnerable to soil erosion include locations with thin organic (A and O) horizons and hilly terrains (see Figure Water Erosion Vulnerability).

Some amount of soil erosion is a natural process along sloping areas and/or in areas with soft or noncohesive materials susceptible to movement by water, wind, or gravity. For instance, soil material can be mobilized in strong windstorms, along the banks of rivers, in landslides, or by wave action along coastlines. Yet most topsoil erosion results from water influenced processes such as in rivers, creeks, ravines, small gullies, and overland flow or sheetwash from stormwater runoff. Although some soil erosion is natural, anthropogenic (human-induced) processes have greatly accelerated the erosion rate in many areas. Construction and agriculture are two of the more significant activities in our modern society that have increased erosion rates. In both cases, the erosion of topsoil can be significant if poor land management practices are used or if the area is geologically sensitive. 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 soiland displacement of the original prairie grasses (that once held the soil together) also contributed to the crisis. 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.

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” (see Figure Aquatic 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) (see Figure Aquatic Dead Zones). This issue is of great importance because the dead zones are near inhabited coastlines with commercially and ecologically vital aquatic life.

One of the most notorious dead zones (second to the Baltic Sea) is an 8,500 square mile region in the Gulf of Mexico (see Figure Aquatic Dead Zones). 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 (see Figure Aquatic Dead Zones), to improve the health and sustainability of our soil, land, and aquatic ecosystems.