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  1. 1. The Geography of Soils Properties and Processes
  2. 2. Overview Soil: Definition, Description, Characteristics Soil Properties Soil Chemistry Soil Biology Soil Formation Soil Taxonomy Soil Issues
  3. 3. Soil Soil is a mixture of both mineral and organic matter Maintained by microscopic lifeforms Soil Profiles Erosion: eluviation Pedon, Polydedon Transport: leaching Deposition: illuviation Horizons O: Organic matter: litter and humus: soil moisture retention, mobilization of nutrients, provision of nutrients A: Humus and clays (water retention of humus limits eluviation) E: Sand and silt (clays and other oxides eluviated down to B) B: Clays and oxides illuviated from E deposited here C: Weathered bedrock, exclusive of the bedrock, not affected by biota R: Bedrock
  4. 4. Soil Properties Color Soils rich in organic matter are dark or black Iron and Aluminum oxides give soil a reddish color Texture Gravel Sand 2mm Silt 50 μm Clay 2 μm Structure Consistency (wet, moist and dry) Porosity: affects inflitration improved by biotic action harmed by compaction (increases runoff) Soil Moisture
  5. 5. Texture
  6. 6. Structure
  7. 7. Soil Moisture Soil moisture related to Organic matter content Texture (esp. to fine particles) Loss of organic matter can lead to Loss of soil moisture retention Loss of A horizon to E horizon Eluviation of fine particles from E Illuviation of fine particles to B Further loss of soil moisture retention
  8. 8. % Soil Moisture % Fine Particles % Organic matter
  9. 9. Soil Chemistry Soil Atmosphere (N2, CO2) Nutrients take the form of ions Cations: Na+, K+, Mg2+, Ca2+, NH4+ Anions: NO3- Colloids acts as nutrient storage soil particles (especially clay) organic matter Soil moisture acts as a transport agent by solution Root hairs have a slight negative character and attract positive ions Cation Exchange Capacity (CEC) is a measure of how easily nutrients pass between colloids
  10. 10. Acidity and pH Acidity refers to the concentration of Hydrogen ions (H+) H+ ions occupy colloidal spaces, crowding out other nutrients and lowering fertility pH = -log[H+] pH < 7, acidic pH = 7, neutral, water pH > 7, basic (alkaline)
  11. 11. Soil Biology Critical for cycling nutrients Performed largely by bacteria Fungae, nematodes, springtails, earthworms, etc. (form food chains) Nitrogen cycle, Carbon cycle Sources of Nitrogen Atmosphere (N2) Litter, Through decomposition, also biological, provides NH4+ from organic compounds Inorganic fertilizer (NPK) Utilizability of Nitrogen Nitrogen gas, inert, unusable by plants Nitrite, toxic to plants Nitrate and Ammonium utilizable by plants
  12. 12. Nitrogen Cycle Plants must rely on soil microbiota to process nitrogen into a form it can use Nitrogen fixation: bacteria change atmospheric N into various nitrogen compounds legumes Nitrification: Ammonium changed into nitrite by “Type I” organisms (nitrosomona); nitrite turned into nitrate by “Type II” organisms (nitrobacter) Denitrification: nitrogen compounds turned back into nitrogen gas by bacteria (20 – 40% of fertilizer is wasted this way) Plants return nitrogen to the cycle through leaf litter and decomposition A diverse soil biota is necessary for proper functioning
  13. 13. Carbon Cycle Plants take in carbon dioxide, add it to their tissues Other nutrients, including N, are taken in Tissues becomes deposited in soil through leaf litter Builds up soil organic matter Acts as a form of carbon storage Contributes N to the soil
  14. 14. Soil Formation Dynamic factors Climatic – weathering of parent material; texture; pedogenic regimes Biologic – organic matter additions Passive factors parent material: affects nutrient availability topography and relief: accumulation and depth time Lots of it (hundreds to thousands of years), under stable conditions Paleosols Human factor Rapid loss of soil
  15. 15. Classification of Soil Types Pedogenic regimes An earlier system of classification based on climatic factors Replaced earlier texture classification system with one that was more focused on development USDA Soil Taxonomy Replaced pedogenic regimes as a system, but includes them as a diagnostic feature Uses diagnostic horizons to classify soils epipedon: Upper most surface layers, exclusive of C and R diagnostic subsurface horizons: A and B horizons Twelve orders and multiple suborders
  16. 16. Pedogenic Regimes Relates soil formation to climate Soil types are closely related to these regimes climate is not the sole factor in determining soil type, however
  17. 17. Soil Taxonomy: Orders Oxisols Aridisols Mollisols Alfisols Ultisols Spodosols Entisols Inceptisols Vertisols Histosols Andisols Gelisols
  18. 18. Oxisols Tropical soils Laterization Iron and aluminum oxides (oxidation; reddish color) Heavily leached A horizon (leaching occurs throughout the year) Loss of nutrients and colloidal material poor fertility and low CEC Exposure to air and water creates hardpan (plinthite, laterite) Rainforest soil Can be farmed sustainably slash and burn, with long fallow (low demand) Vulnerable to erosion if activities not done right (1000s of tons per sq. km per year)
  19. 19. Aridisols Desert soils (arid climates) shallow horizons poor colloidal structure (fewer fine particles) lacking in organic matter poor water retention contributes to flash flooding leaching highly seasonal High evapotranspiration Salinization Evaporation deposits salts near the soil surface
  20. 20. Mollisols Grassland soils world’s richest soil type deep and rich organic layer (humus) due to deep grass roots Calcification More CO2 in the soil atmosphere causes leaching of calcium and magnesium carbonates illuviation into B and C horizons caliche, kunkur
  21. 21. Alfisols Name taken from Aluminum and Iron content (Al, Fe) Moderately weathered forest soil Considered to be a moist version of mollisols Note similarities to Oxisols
  22. 22. Ultisols Highly weathered forest soils Heavily leached A horizon with residual Fe and Al oxides Intermediate between Oxisols and Alfisols
  23. 23. Spodosols Northern coniferous forest soils Humid continental mild summer cool, moist climates Humus and fine particles eluviated from A horizon Uppermost horizon lacks clay and humus, tends to have a sandy texture and has a light color Podzolization Low pH (acidic) leaching of fine particles and nutrients
  24. 24. Entisols recENT soils Not climate dependent Insufficient time passage to develop horizons Insufficient weathering of parent material lacking fine particles and nutrients Occurence: areas of recent deposition slopes, floodplains, tidal mudflats, dune and desert sands
  25. 25. Inceptisols Soils in earliest stages of development Eluviation and illuviation are evident, but distinct horizons have not yet formed Low fertility
  26. 26. Gelisols Cold and frozen soils the presence of permafrost is a defining characteristic Form very slowly due to low temperatures slowing chemical and physical weathering disturbance to these soils has long lasting effects cryoturbation freeze-thaw cycle of frost tends to churn the soil
  27. 27. Andisols Named after the Andes Volcanic parent material pyroclastic material Rapidly weather into colloidal material high fertility, high CEC Distributed around subduction zones Pacific Ring of Fire especially
  28. 28. Vertisols Expandable clay soils At least 30% expandable clay (shrink and swell) Climatic variables Subhumid to semi-arid moisture, moderate to high temperature Regions with highly variable seasonal moisture balances Distinct dry season following wet season Form under tropical and subtropical grasslands and savanna Disastrous for construction (don’t buy a house built on vertisols)
  29. 29. Histosols Organic soils Formed from the accumulation of organic matter lakebeds > bogs, marsh and peat > histosols Poor drainage
  30. 30. Human-Environment Interaction Issues with Soils Irrigation and Salinization More common in arid environments The accumulation and evaporation of irrigated water causes salts to build up in the soils, and ultimately ruins its fertility Drainage tiles Groundwater issues, Colorado River Basin issues
  31. 31. Soil degradation results from various practices that destroy agriculturally viable land agricultural practices that leave soil vulnerable to erosion overgrazing, vegetation removal, over exploitation 20% drop in yield over next 20 years at moderate erosion rates Urbanization 3 – 5 million acres of US farmland are lost each year 38% decrease in world’s farmable land since 1950 1380 million acres 12 – 15 million acres per year Agriculturally viable soil takes hundreds to thousands of years to form Soil as a non-renewable resource? Agriculture as soil mining?