Modified: 10/4/98
The soil is a moist environment, teaming with living organisms that produce weak acids. The moisture and acids bathing rock minerals causes them to dissolve, a process called chemical weathering 1. The soluble salts released when a rock mineral weathers are either absorbed by plant roots to become the ash or plant minerals 1, serving as vital nutrients for growth, or the salts are leached into rivers and groundwater, ultimately reaching the sea. The insoluble salts released by weathering become clays 1, fine-grained minerals with a high surface area that retain nutrients and toxicants on their surfaces.
Moisture is essential for life and the amount of biomass in an ecosystem is correlated with rainfall. Increased rainfall means greater biological activity, in general, and greater biological activity means more acidity 1 for mineral weathering 2. Rock minerals dissolve to produce soluble salts (ash or plant minerals) and clays. Everything else being equal (time, parent material, land form), the greater the rainfall the higher the clay 2 content in the soil.
Burning vegetation in forests and grasslands produces ash 2, which is nothing more than the plant minerals absorbed through plant roots. This ash is rich in essential nutrients and is very soluble, making it readily available for vegetation sprouting after the fires. Furthermore, many ecosystems are adapted to periodic fires that release the nutrients stored in accumulated plant residue.
Wetlands 1 typically form where the drainage of water, surface or subsurface, is poor. The problem is not whether a wetland site will hold water, but rather how to get rid of excess water when the wetland is drained. Poor surface drainage means that water falling on the land surface does not run off or runoff 1 from other places flows into the wetland site. Poor surface drainage means the water table 1 is high, perhaps because it is near a marsh or stream or lake.
Peat bogs occur in depressions where the water table 2 meets the land surface. Sometimes the peat bog begins as a small pond or lake that gradually fills with humus. Wherever the water table comes close to the land surface, water draining from the surface of the landscape quickly enters the groundwater 1. This process replenishes the groundwater, but it can also lower groundwater quality if contaminants are dissolved in the runoff 2.
Flooding is a natural, recurring phenomena. It becomes a problem when it causes the loss of life and property. Land use that increases runoff 3, by removing vegetation or loss of wetlands 2, will increase river flow and the liklihood of flooding. Land use that attempts to deny rivers full access to their flood plains results in the loss of life and property when levee fail. Solutions to the loss of life and property to flooding seems always come back to land use decisions, none of which are simply or easy.
The soils belonging to the same soil order 1 share common characteristics. ENTISOLS and INCEPTISOLS, for instance, have poorly defined or weakly expressed horizons simply because the duration of soil development is very short. Some soils occur in special settings. VERTISOLS have large amounts of clays 3 that shrink when they dry and swell when they are wet while ANDISOLS form in volcanic deposits or HISTOSOLS form in the deep organic deposits found in peat bogs.
Some soil orders occur in specific climate zones: ARIDISOLS in deserts and MOLLISOLS in grasslands. Forest ecosystems span humid climates ranging from cool to warm tropical. The sequence of soil orders found in humic forest ecosystems ranges from SPODOSOLS (cool), through ALFISOLS and ULTISOLS (temperate and warm subtropical) to OXISOLS (warm tropical). Simply knowing the soil order says volumes about the location and natural history of the soil.
Though MOLLISOLS form under grassland ecosystems and ALFISOLS under temperate forest ecosystems, the soils belonging to these two soil orders 2 have at least one characteristic in common. The most fertile soils typically belong to these orders. These soils are fertile because they retain significant amounts of basic (alkaline) cations such as potassium, calcium and magnesium. The mere presence of basic cations tells us these soils have been spared intense or prolonged mineral weathering or leaching 1.
Some of the water falling on the surface of the soil flows into the pores, root channels, insect burrows and cracks within the soil while the remainder runs off the surface. Gravity pulls the water downward through a network of pores between mineral grains and soil aggregates 1. As it percolates downward, the water carries with it soluble salts and clays released by mineral weathering , depositing them lower in the soil profile 1. Leaching 2 is the transport of salts and colloids downward by percolating water, removing them from the upper horizons in the profile and depositing them in the lower horizons.
Let's assume the mine tailings contain sulfide 1 minerals that react with oxygen and water to produce sulfuric acid 1. Acidic mine drainage also dissolves metals 1 from the residual ore minerals remaining in the tailings. Treatment of mine drainage may simply neutralize acidity 2 by adding limestone (calcium carbonate). Neutralizing acidity will also remove many of the metals dissolved in the drainage water, but further chemical treatment that specifically removes toxic metals may be necessary. The end product of this treatment is a higher quality water that is no longer acidic and contains less dissolved metals and a sludge containing the metals removed by the water treatment process. If the levels of toxic metals are sufficiently high, the sludge may be considered a hazardous waste.
The sulfuric acid produced when the sulfide minerals in the mine tailings oxidized before the tailings were flooded would make the water very acidic because sulfuric acid is a very strong acid. If the tailings remained flooded for a long time and if acid- and metal-tolerant microbes began to grow, then the tailings would become anaerobic 1 and the pH would rise as the sulfuric acid was converted back into sulfides, reversing the process when the sulfides oxidized into sulfuric acid. The dissolved metals would combine with the sulfides to form the same insoluble minerals originally in the ore. If the flooding was brief and no acid- and metal-tolerant microbes had time to grow, then the flooding would only increase the risk that acidic, metal- containing water would contaminate the groundwater 2.
Soil erosion is the loss of soil by wind or water. The removal of humus from the A-horizon or the exposure of a clay-rich B-horizon is a type of soil degradation. Soils can be degraded by compaction or the destruction of soil aggregates 2, resulting in less aeration and slower water movement. Soils can be degraded by the accumulation of salts, acidity 3, excessive nutrients, or pollutants. A change in the properties of a soil caused by human activity that reduces the vitality of the soil ecosystem is degradation or loss of soil quality 1.
The weathering of rock minerals produces clays and the decomposition of plant and animal residue produces humus. Adhering to the negatively-charged surface of clays 4 and humus are cations, positively-charged atoms, that can exchange 1 with cations dissolved in water. The weathering of minerals releases alkaline cations such as sodium, potassium, calcium, and magnesium that raise the pH and make the soil alkaline 1. Biological activity releases acidic hydrogen cations that lowers pH and makes the soil acidic. Increases in soil acidity 4 is buffered--or dampened the way a shock absorber dampens rapid vibrations--because new acids are quickly neutralized by basic cations released from the surfaces of clays and humus.
Biological activity pumps acidity 5 into soils while mineral weathering 3 pumps alkalinity 2. Increased biological activity follows increases in temperature. Everything else being equal (rainfall, the amount of water percolating downward through the soil, aeration), increased acidity follows increased temperatures.
Biological activity produces weak acids as plant roots absorb ions through their roots, as soil organisms respire, and as microbes decompose residue and humus. The acid produced by the oxidation of sulfide 2 minerals, sulfuric acid 2, is much stronger than the acids produced by biological activity and will acidify soil more rapidly and to a greater degree. The net effect is substantial loss of soil quality 2 because the soil pH becomes unfavorable to all but a few acid-tolerant microbes.
Gleying 1 is a technical term that describes the gray, blue, purple or green soil colors that occur in soils that have been waterlogged 1 for prolonged periods of time. Anaerobic 2 microbes flourish in the absence of air, reducing iron and manganese minerals. The chemical reduction of iron and manganese produces the characteristic gley colors.
Both gleying and mottling 2 develop if a soil is waterlogged for extended periods of time. Gleying implies uniform coloration while mottling means splotchy coloration. Prolonged waterlogging 2 without aeration produces gleying. Mottling occurs when the soil becomes partially aerated between episodes of waterlogging.
Continental glaciers deposit thick blankets of till, an unsorted jumble of material ranging from clay to boulders, as the ice front retreats at the end of an ice age dropping in place the rubble frozen in the ice. Thick till fills in the low places making the landscape relatively flat. Buried beneath the till lie the former stream and river valleys that collected and carried away the runoff 4 from the pre-glacial landscape. The slow process of geologic erosion make take tens or hundreds of thousands of years to cut a new network of streams into the fabric of the till plane, gradually restoring the ancient watersheds 1 obliterated by the ice sheet.
Miller & Gardiner (pp. 211-214) discuss composting and make it clear that the conditions in a well-managed compost pile rarely occur in nature. Composting accelerates the humification processes, for one thing. The pathogenic organisms in compost must be destroyed, otherwise pathogens will cause disease in the plants growing in the compost or in the humans or animals consuming the plants.
The compost heap is an excellent insulator, trapping the heat produced by microbes digesting the residue. As heat builds in the compost pile, a susccession of increasingly heat tolerant microbes digest the reside, killing pathogenic microbes in the processes. Heat build up occurs during the early stages of decomposition. After the pathogens are destroyed, the manager of the compost pile will aerate the pile and perhaps add fertilizer to foster the efficient decay of the remaining residue. Incidentally, microbial decomposition is fundamentally the same as "burning" the residue, a process that releases plant minerals (ash 3) to build microbial biomass. That release of nutrients makes them more readily available to plants, hence the fertilizer value of compost.
A pedon 1 is a body of soil in the landscape that can be classified as belonging to a particular soil series 1, meaning it has recognizable horizons that distinguish it as a soil different from other soils. A soil mapping unit is defined by the scale of the soil map. The larger the scale of the map, the more general the unit represented on the map. The two maps inside the covers of our textbook (Miller & Gardiner), "Global Soil Regions" and "Dominant Soil Orders of the U.S. and Puerto Rico" use soil orders 3 as map units. The soil map "Soil regions of Wisconsin" uses nontechnical map units (forested or prairie soils). County soil survey 1 maps may use a single soil series or a soil association, which is a map unit containing two or more soil series that occur together in the landscape in a predictable pattern.
It depends on what one means by "the same place". It is not uncommon to find soils belonging to two different soil series 2 (meaning their profiles 2 are sufficiently different they are recognizably different from each other) within the distance of a few meters. The reason there are two different soils so close together is because the factors that cause soil development (parent material, climate, vegetation, topography, and time) some how differ significantly. Of course, the climate cannot change significantly over a distance of a few meters, but subtle changes in the other factors can be significantly different over distances as small as a few meters.
The majority of soil organisms are aerobic, meaning they require oxygen to live, and all require moisture. As the water content increases, water replaces air in soil pores. Above a certain water content organisms consume the oxygen in soil pores faster than air can diffuse from the soil surface. The organisms that depend on air either suffocate or go dormant and microbes adapted to live in the absence of oxygen (anerobic microbes 3) become active. The anaerobes remain the dominant active community until the soil moisture level decreases sufficiently for air to diffuse into the soil depths, bringing back to life the dormant aerobic microbes.
Acidity 6 and alkalinity 3 define the two limits of pH, between them is neutrality. Biological activity or sulfide acidification produces acidity that lowers soil pH. Mineral weathering releases soluble salts that produce alkalinity, raising soil pH.
Wisconsin and Florida lie in different climate zones, supporting very different vegetation communities and soil orders 4. Disregarding such things as parent material and topography, we can draw a few conclusions about the soils from these two locations. First, there is a difference in the duration of soil development. Wisconsin was covered by a glacier until about 10,000 years ago. The retreating glacier exposed unweathered parent material as soils in this pristine landscape began to develop. The landscapes of Florida have not been renewed by fresh deposits of unweathered minerals for hundreds of thousands or millions of years, exposing the soils there to a much longer duration of soil formation.
The climate in Wisconsin is temperate subhumid supporting a mix of grassland and forest vegetation while Florida experiences a humid, subtropical climate. Thus, the intensity of mineral weathering and the overall level of biological activity are significantly greater in Florida soil ecosystems relative to those in Wisconsin. The net result is fewer unweathered rock minerals, more clays 5, and more acidity in Florida soils. Ultisols are relatively common in Florida, but not to be found in Wisconsin. Mollisols, common in southern Wisconsin, are not found in Florida. Alfisols occur in both states, but Florida Alfisols will be more acidic and have greater clay accumulation in the B-horizon than in Wisconsin.This is exactly the case in New England. The parent material for the soils in that region is granite, a rock containing few alkaline 4 elements to neutralize soil acidity. Runoff from frozen soil during Spring snowmelt provides little opportunity for the meager capacity of the soil to neutralize Winter's accumulated acidity. Acidic runoff washes into lakes and streams where it kills aquatic flora and fauna. The most severe ecological damage from acid rain 1 occurs in aquatic ecosystems and damage to foliage in ecosystems exposed to acidic fog.
Soil health 1 relates to the vitality of the soil ecosystem and is independent of any particular use of the soil. Suitability rates the limitations on the use of the soil for a specific purpose (crop production, pasture for grazing, woodland for timber production, woodland for wildlife habitat, etc).
Map scale determines the appropriate mapping unit 1. For instance, there are 12 soil orders 5 in the US Soil Taxomony and 15,000 recognized soil series 3. One can imagine a map of the US showing the distribution of 12 types of soils but how could you display 15,000 different soils? The soils of a single county may belong to only one or two soil orders but may include 20 soil series. A county soil map with only two soil order map units would convey much useful information but one displaying 10-20 soil series or soil associations would be far more informative.
All soils belonging to a given Land Capability Class 1 are regarded as having similar severity of limitations without regard to what causes the limitation. Soils belonging to the same Capability Subclass have limitations arising from the same factor (slope steepness, wetness, shallow or stony, etc.)
Each time a unique soil is identified, a soil whose profile 3 is significantly different from other recognized soils, it is named after the place. Soils with similar profiles may be identified at different locations at a later time, but they will bear the name given to that soil when it was first discovered.
Soil surveys 2 provide the technical justification for restricting certain land use practices. Land use planners, engineers, developers, farmers, conservationists and others use either the basic soil geography information or the assessments of land use suitability or limitations to select sites for specific purposes or management practices.
The some of the original soil surveys 3 were made more then 50 years ago. Since that time our understanding of soils, how soils should be classified and the uses made of soil survey information have all changed. The new soil surveys updates information that is needed for modern uses.