UN-FAO A. Healthy soils are the basis for healthy food production. B. A tablespoon of normal topsoil has more microorganisms than the entire human population on Earth. C. It can take up to 1,000 years to form 1 cm of top soil. D. Current rate of soil degradation threatens capacity to meet the needs of future generations. E. Soils help to combat and adapt to climate change by playing a key role in the carbon cycle.
Overview Rock weathering Functions of soil Soil forming factors Soil properties
Objectives You should be able to: Understand rock weathering; Explain the importance of soils in ecosystems and for human societies; Discuss the five soil forming factors; Describe soil properties.
Keywords and Concepts Rock weathering Soil forming factors and weathering Soil horizons (layers) Soil types (orders) Soil properties Physical (texture, structure, bulk density) Chemical (ph, CEC, redox potential)
Rock weathering Weathering is a general term encompassing many processes by which parent rocks are broken down. Mechanical (or physical) weathering is the fragmentation of rocks without chemical changes. Wind abrasion Rock splitting by the freezing of water Growth of roots in rock crevices Chemical weathering occurs when parent rock materials react with acidic and oxidizing substances. Usually chemical weathering involves water, and mineral constituents are released as dissolved ions.
Physical weathering - wind
Physical weathering water
Physical weathering - ice
Chemical weathering of the mineral apatite Ca 5 (PO 4 )3OH + 4H 2 CO 3 5Ca 2+ + 3HPO 4 2- + 4HCO 3 - + H 2 O
Biological weathering - plant roots
Rock weathering can remove atmospheric CO 2! The net effect of silicate mineral weathering is to convert CO 2 into dissolved HCO 3-. A representative reaction is: 2CO 2 + 3H 2 O + CaAl 2 Si 2 O 8 (Anorthite)---> Ca 2+ + 2HCO 3 - + Al 2 Si 2 O 5 (OH) 4 (Kaolinite) The dissolved HCO 3 - is carried to the ocean by rivers and, if accompanied by dissolved Ca 2+ or Mg 2+, the carbon is removed from the oceans as Ca-Mg carbonate minerals. In this way, the overall process results in the removal of CO 2 from the atmosphere. From Robert A. Berner (SCIENCE, V 276, April 1997, p 544): The spread of deeper rooted vascular plants during the Devonian Period (400 to 360 million years ago) likely accelerated rock weathering. This enhanced chemical weathering would have resulted in enhanced removal of atmospheric CO 2.
Important factors involved in rock weathering 1. Parent rock types (e.i., igneous: basal, granite; or sedimentary: sandstone, limestone) As a general rule of thumb, the higher is aluminum-silicates content of the rock type, the less susceptible to weathering. 2. Climate (e.i., precipitation, temperature, wind, etc.) 3. Organisms (Plant roots, microorganisms, etc.) Because the primary chemical weathering mechanism is carbonic acid: H 2 O + CO 2 <---> H + + HCO 3 - <---> 2H + + CO 3 2-4. Topography More water at the bottom of a hill than on the slopes, and the slopes that directly face the sun will be drier than slopes that do not. 5. Time All of the above factors assert themselves over time, often hundreds or thousands of years (e.i., Hawaii islands).
Soil functions
Anchorage for plants
Storage and recycling of nutrients Mineral pool Organic pool
Water filtering
Habitat for many organisms Mole Nematode Mite Protozoa Springtail
Douglas fir roots with mycorrhizae Left: Wheat roots (with root hairs) growing in sandy soil Rhizosphere Image Gallery: http://ic.ucsc.edu/~wxcheng/wewu/
Soil formation processes What is soil formation? Most soils are mineral soils formed by the weathering of solid rock masses into unconsolidated materials, except for organic soils that mostly develop from plant residues. Soil formation consists of two inter-connected parts: (1) the production and accumulation of unconsolidated materials by weathering and subsequent movements; and (2) horizon development involving changes within the loose material over time.
Soil forming factors Parent material Time Topography Climate Biota
Soil forming factors: Parent materials (rocks, loess, glacial till, alluvium, etc) Climate (precipitation, temperature, wind, etc ) Biota (organic and living) Topography (or relief, modify water and temperature) Time (without time, nothing changes) Interactions of all
Soil properties Texture Structure ph Other chemical properties nutrients Biological properties
Soil properties: Texture 40% 40% 20%
Soil properties: Structure Soil aggregates: Clumps of soil particles bound together by roots, polysaccharides, fungal hyphae, or minerals
Soil properties: water holding capacity Soil texture and structure: important for: Water holding capacity Drainage Aeration
Soil properties: ph ph = -log (H + ) (H + ) = 0.01 M ph = 2 Soil ph influences many organisms and biological processes (e.g. decomposition)
Soil properties: ph
Soil properties: nutrients Macro nutrients: nitrogen (N), phosphorous (P), potassium (K), sulfur (S), calcium (Ca), magnesium (Mg) Micro nutrients: boron (B), chlorine (Cl), copper (Cu), iron (Fe), manganese (Mn), molybdenum (Mo), cobalt (Co), zinc (Zn)
Soil properties: biological properties Soil organisms affect other soil properties (e.g. structure) and play an important role in soil processes (e.g. decomposition).
Soil classification Soil horizons
Germany Spotosol Minnesota Spotosol Australia Oxisol Minnesota Mollisol
USDA Classification: Soil Orders Order: Entisol Inceptisol Aridisol Vertisol Mollisol Alfisol Spodosol Ultisol Oxisol Histosol Andisol Diagnostic Characteristics: Simple soils, no subsoil diagnostic horizons Soils with minimum development, little or no subsoil clay accumulation Soils of hot and dry regions No subsoil diagnostic horizons, much clay Thick, soft, dark mineral soil of grasslands Subsoil accumulation of clay, not strongly leached Subsoil accumulation of iron Subsoil accumulation of iron, strongly leached Extemely leached, Fe, Al oxides, and quartz left Dark organic soil, little mineral matter Mineral soil formed on volcanic ash parent material
Ultisol
Mollisol
Aridisol
Human activity: soil degradation Activities causing soil erosion and desertification: Improper cultivation/overgrazing Deforestation Industrial and municipal pollution
Soil degradation is a global environmental problem. From D. Pimentel et al. 1995, SCIENCE, Vol.267:1117-1123: Of the 75 billion tons of soil eroded worldwide each year, about two-thirds come from agricultural land. If we assume a cost of $3 per ton of soil for nutrients, $2 per ton for water loss, and $3 per ton for off-site impacts, this massive soil loss costs the world about $400 billion per year, or about $67 per person per year.
How fast is soil formation rate? Tennessee Valley, Marin county, CA From: Helmsath et al. 1997. Nature 388:358-361
Summary Soil functions: Anchorage for plants Storage and recycling of nutrients Water filtering Habitat for many organisms Soil forming factors: Parent material Climate Topography Time Biota (including humans)
Summary Soil classification soil horizons Soil properties: Texture Structure ph Nutrients Biological properties
More interest in soils? Take ENVS 161A/L: Soils and Plant Nutrition/Lab