Earth Systems and Resources D. Soil and Soil Dynamics: • Rock cycle • Formation • Composition • Physical and chemical properties • Main soil types • Erosion and other soil problems • Soil conservation Rock Cycle Three Types of Rocks • Igneous – from heat, pressure (magma) • Sedimentary – formed when sediments (sand, mud, gravel, shells) are compressed • Metamorphic – form when sedimentary/igneous rocks are subjected to high pressure and T http://www.youtube.com/watch?v=sLKXIVHoh54& edufilter=vH8ntV7keCE-ZLl5wRMzNg Rock Cycle Weathering – breaking down rock into soil Climate(T + water) + biotics Creates erosion Physical Chemical Biological Chemical Weathering Biotic Weathering • Roots • Lichens • Animals produce CO2 which diffuses into soil, reacts with H2O & forms carbonic acid (H2CO3) Soil Composition • • • • Sand – largest type of soil; visible with the eye Silt - flour; barely visible Clay – very tiny, fine; electron microscope Larger than 2mm in diameter = gravel/stones (no value to plants) Soil Horizons • O horizon - uppermost layer; organic material (plant decay) • May be thick or thin (desert, tropical rain forests) • A horizon – mineral mixed with humus • O + A = topsoil Soil Horizons • B – subsoil – clay; rich in Fe, Al compounds (acid deposition) • Lighter colored • C – rock fragments • Saturated with groundwater • Usually no roots • Borders parent bedrock Soil Texture • The percentages (by weight) of different sized particles of sand, silt and clay • How soil particles are organized and clumped together. (sand, silt, clay) Soil Horizons • • • • Mature soils are layered Infiltration – percolation Leaching – dissolving minerals Type of soil determines percolation and leaching. Soil Composition • Porosity – volume and space between particles • Porosity = permeability • Sand, silt, clay http://techalive.mtu.edu/meec/module06/Percolation.html Mosaic of closely packed pebbles, boulders Weak humusmineral mixture Desert Soil (hot, dry climate) Dry, brown to reddish-brown with variable accumulations of clay, calcium and carbonate, and soluble salts Alkaline, dark, and rich in humus Clay, calcium compounds Grassland Soil semiarid climate) Forest litter leaf mold Acidic, light-colored humus – little topsoil Humus-mineral mixture Iron and aluminum mixed with clay Light, grayishbrown, silt loam Dark brown firm clay Tropical Rain Forest Soil (humid, tropical climate) Deciduous Forest Soil (humid, mild climate) Acid litter and humus Light-colored and acidic Humus and iron and aluminum compounds Coniferous Forest Soil (humid, cold climate) Soil pH • Determines solubility of minerals • Most soils - 4.0 to 8.0 • Pygmy Forest in California (2.8-3.9) • Death Valley, California, (10.5) Factors Effecting Soil • Depth - depends on erosion, plant life, etc. • Color – • Dark soil, rich, lots of organic matter (leaf decay) • Light soil (sand) very little organic matter • Slope – runoff, erosion Erosion • • • • • • After plowing/harvesting 6.4 billion tons of soils per year (U.S.) Renewable but recycles slowly Unsustainable Lowers fertility Overloads lakes, rivers with sediments Case Study: Soil Erosion in the U.S. • 1985 Farm Act - subsidy for taking highly erodible land out of production and replanting it with soil-saving plants for 1015 years Reducing Erosion • No-till • Minimum Tillage (reduced tillage) Reducing Erosion • Terracing • Contour Plowing - plowing across the slope • Windbreaks - also help retain soil moisture, supply some wood for fuel, and provide habitats for birds • Strip cropping – a row crop (corn) is alternated in strips with another crop that completely covers the soil: • Helps prevent the spread of pests and plant diseases Cover Cropping (alley cropping) – several crops are planted together in strips or alleys between trees and shrubs that can provide shade (which reduces water loss by evaporation) and helps to retain and slowly release soil moisture. Surface Creep – mountains/sand dunes Desertification • About one-third of the world’s land has lost productivity due to desertification • Drought • Human activities that reduce or degrade topsoil*** Salinization • Repeated irrigation causes salt buildup Solutions Soil Salinization Prevention Cleanup Reduce irrigation Flush soil (expensive and wastes water) Stop growing crops for 2–5 years Switch to salttolerant crops (such as barley, cotton, sugarbeet) Install underground drainage systems (expensive) Fig. 13-15, p. 281 Transpiration Evaporation Evaporation Evaporation Waterlogging Less permeable clay layer Waterlogging: 1. Precipitation and irrigation water percolate downward. 2. Water table rises. Soil Conservation • Fertilizers can help restore soil nutrients, but runoff of inorganic fertilizers can cause eutrophication pollution. • Organic fertilizers: from plant and animal (fresh, manure, or compost) materials. • Commercial inorganic fertilizers: Active ingredients contain nitrogen, phosphorous, and potassium and other trace nutrients. Fertilizers • Organic Fertilizers – animal manure, crop residues, bone meal, and compost • Inorganic Fertilizers – man-made from chemical compounds • Benefits – exact compositions are known; they are soluble & thus immediately available to the plant • Costs – quickly leach away (pollutes water); doesn’t increase water holding capacity of the soil as organic fertilizers do.