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NAME: Soil quality
BRIEF DESCRIPTION: Soils vary greatly in time and space. Over time-scales relevant to geoindicators, they have both stable characteristics (e.g. mineralogical composition and relative proportions of sand, silt and clay) and those that respond rapidly to changing environmental conditions (e.g. ground freezing). The latter characteristics include soil moisture and soil microbiota (e.g. nematodes, microbes), which are essential to fluxes of plant nutrients and greenhouse gases. The soils of boreal regions are estimated to hold the equivalent of some 60% of the current atmospheric carbon: long-term warming is expected to increase decomposition and drying, thus potentially releasing huge volumes of methane and CO2. Recent research shows that the Alaskan tundra no longer serves as a carbon sink, but has begun to release significant quantities of carbon.
Most soils resist short-term climate change, but some may undergo irreversible change such as lateritic hardening and densification, podsolization, or large-scale erosion. Soil properties and climatic variables such as mean annual rainfall and temperature can be related by mathematical functions known as climofunctions.
Chemical degradation takes place because of depletion of soluble elements through rainwater leaching, overcropping and overgrazing, or because of the accumulation of salts precipitated from rising groundwater or irrigation schemes. It may also be caused by sewage containing toxic metals, precipitation of acidic and other airborne contaminants, as well as by persistent use of fertilizers and pesticides. A widespread problem is the retention in the soil organic matter and clay minerals of potentially toxic metals and radionuclides (e.g. Cu, Hg, Pb, Zn, 226Ra, 238U). These and other chemical components may be catastrophically released as what are commonly referred to as `chemical time bombs' where the pH of the soil is decreased by acidification or where other environmental disturbances (e.g. erosion, drought, land use change) intervene. Soils also act as a primary barrier against the migration of organic contaminants into groundwater. Key indicators are pH, organic matter content, sodium absorption ratio, cation exchange capacity, and cation saturation.
Physical degradation results from land clearing, erosion and compaction by machinery. Soil structure may be altered so that infiltration capacity and porosity are decreased, and bulk density and resistance to root penetration are increased. Such soils have impeded drainage and are quickly saturated: the resultant runoff can cause accelerated erosion and transport of pollutants such as pesticides [see soil and sediment erosion]. The key soil indicators are texture (especially clay content), bulk density, aggregate stability and size distribution, and water-holding capacity.
SIGNIFICANCE: As one of Earth's most vital ecosystems, soil is essential for the continued existence of life on the planet. As sources, stores, and transformers of plant nutrients, soils have a major influence on terrestrial ecosystems. Soils continuously recycle plant and animal remains, and they are major support systems for human life, determining the agricultural production capacity of the land. Soils buffer and filter pollutants, they store moisture and nutrients, and they are important sources and sinks for CO2, methane and nitrous oxides. Soils are a key system for the hydrological cycle [see groundwater chemistry in the unsaturated zone]. Soils also provide an archive of past climatic conditions and human influences.
HUMAN OR NATURAL CAUSE: Soils may be degraded or enhanced by both natural processes and human activities. Human activities influence soil properties by causing increases in bulk density from agricultural tillage and road operations and in acidification from inorganic fertilizers and acid rain. Soil degradation is one of the largest threats to environmental sustainability: over the past half century the productivity of more than 1.2 billion ha of land (an area larger than China and India together) has been significantly lowered. It is estimated that about 15% of the soils between latitudes 72° N and 57° S have been degraded by human activities - 7% through loss of soil nutrients and/or organic matter (mainly in Africa and South America), 4% because of soil salinization (mainly in Asia), about 3% by physical deterioration (compaction, sealing and crusting), and 1% by soil pollution, the latter two mainly in Europe.
ENVIRONMENT WHERE APPLICABLE: Any land surface, especially agricultural and afforested areas.
TYPES OF MONITORING SITES: Undisturbed lands, such as uncultivated grasslands and forests, can provide reference sites for comparison with changes in soils subject to human activities related to forestry, agriculture and urbanization.
SPATIAL SCALE: patch to landscape / regional
METHOD OF MEASUREMENT: Routine physical, chemical and morphological descriptions. Chemical degradation can also be monitored by analysis of groundwater [see groundwater quality].
FREQUENCY OF MEASUREMENT: Every 1-10 years
LIMITATIONS OF DATA AND MONITORING: Soils can vary considerably in chemical, physical and biological properties, both vertically through the soil profile, and horizontally, so that it may be difficult to select representative sites for monitoring.
APPLICATIONS TO PAST AND FUTURE: The morphology and chemistry of soils as determined in soil profiles may record past changes in the environment as, for example, iron oxides that accumulated due to flooding of low-lying areas, charcoal fragments produced by forest fires, or pottery shards produced by early humans. The properties of older soils (paleosols), whether buried or not, are indicators of past climates and can be used to predict the impacts of future climatic changes. High rainfall periods causing weathering and leaching are manifested in the clay content and mineralogy, clay coatings and the silica/sesquioxide ratio. Granular soil structures can result from biological activities associated with grasslands ecosystems or from frost action.
POSSIBLE THRESHOLDS: Threshold values for chemical and physical degradation vary according to the usage of soils for agricultural, forestry, waste disposal, and other purposes.
Acton, D.F. & L.J.Gregorich (eds.) 1995. The health of our soils - toward sustainable agriculture in Canada . Centre for Land and Biological Resources Research, Ottawa: Agriculture and Agri-Food Canada.
Batjes, N.H. & E.M.Bridges 1992. A review of soil factors and processes that control fluxes of heat, moisture and greenhouse gases. Technical paper 23, Wageningen: International Soil Reference and Information Center.
Klute, A. (ed) 1986. Physical and mineralogical methods. Methods of soil analysis: Part 1 . American Soil Science Society Agronomy Monograph 9.
Page, A.L., R.H.Miller & D.R.Keeney 1986. Chemical and microbiological properties. Methods of soil analysis: Part 2 . American Soil Science Society Agronomy Monograph 9.
Peirce, F.J. & W.E.Larson 1996. Quantifying indicators for soil quality. In Berger, A.R. & W.J.Iams (eds). Geoindicators: Assessing rapid environmental changes in earth systems :309-321. Rotterdam: A.A. Balkema.
Ringrose-Voase, A.J. & G.S.Humphrey (eds) 1994. Soil micromorphology: studies in management and genesis . Amsterdam: Elsevier.
OTHER SOURCES OF INFORMATION: ISRIC maintains a database of world soils. Departments of agriculture, geological surveys, AIPEA (R. Schoonheydt, Secretary-General Association Internationale pour l'...tude des Argiles, COK Katholieke Universiteit Leuven, Kardinaal Mercierlaan 92, B-3001 Heverlee, Belgium), FAO, ISSS.
RELATED ENVIRONMENTAL AND GEOLOGICAL ISSUES: Accelerated contamination of groundwater can occur if the sorption capacity of soils for potentially toxic chemicals is exceeded by elevated levels from human activities. There is an extensive body of knowledge dealing with physical and chemical changes in soils under cultivation.
OVERALL ASSESSMENT: Soil quality is a sensitive indicator of natural and human-induced perturbations of the environment: changes may affect the quality of surface and groundwater. Monitoring changes in soil properties can assist in predicting the future value of soils for agricultural, forestry and other purposes.
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