6 Geologic Processes Plate Tectonics—Large plates of crust and outer mantle (lithosphere) slowly moving over liquid surface of mantle. – Heat from earth core causes movement. Plates are pulling apart in some areas, and colliding in others. Building processes counteracted by processes tending to tear down land.
8 Weathering Weathering—Factors bringing about fragmentation or chemical change of parent material. – Mechanical —Results from physical forces reducing size of rock particles. Temperature changes and abrasions are primary agents. – Chemical—Rock fragments exposed to atmosphere may oxidize, or otherwise chemically change.
10 Soil and Land Land—Portion of world not covered by water. Soil—Mixture of minerals, organic material, living organisms, air, and water. Role of organisms in soil development is very important.
11 Soil Formation Parent material—Ancient layers of rock, or more recent deposits from lava flows or glacial activity. Humus—Decaying organic material. – Mixes with top layers of rock particles, and supplies needed nutrients to plants. – Creates crumbly soil which allows adequate water absorption and drainage. Soils can take 100’s to 1000’s of years to fully develop. “non-renewable” resource?
17 Soil Properties Structure—Refers to the way various soil particles clump together. – In good soils 2/3 of the intra-soil spaces contain air after the excess water has drained. Friable—Crumbles easily. – Protozoa, nematodes, earthworms, insects, algae, bacteria, and fungi are typical inhabitants of soil.
19 Soil Profile Soil Profile—A series of horizontal layers of different chemical composition, physical properties, particle size, and amount of organic matter. Horizon—Each recognizable layer of the profile.
20 Soil Horizons - “Topsoil” O Horizon— (Litter)—Un-decomposed or partially decomposed organic material. A Horizon—(Topsoil) Uppermost layer— contains most of the soil nutrients and organic matter. E Horizon—Formed from leaching darker materials. – Not formed in all soils. – Usually very nutrient poor.
21 Soil Horizons - “Subsoil” B Horizon—(Subsoil) Underneath topsoil. Contains less organic matter and fewer organisms, but accumulates nutrients leached from topsoil.—Poorly developed in dry areas. C Horizon—Weathered parent material, very little organic material. R Horizon—Bedrock.
23 Soil Profiles Mixtures – Over 15,000 separate soil types have been classified in North America. However, most cultivated land can be classified as either grassland or forest soil. Grassland Soils—Usually have a deep A Horizon—Low rainfall limits topsoil leaching. A Horizon supports most root growth.
24 Soil Profiles Forest Soils—Topsoil layer is relatively thin, but topsoil leachate forms a subsoil that supports substantial root growth. (High rainfall areas) – Tropical Rainforests Two features of great influence: High temperatures Rapid decomposition, little litter. High rainfall Excessive leaching of nutrients.
26 Soil Erosion Erosion—Wearing away and transportation of soil by wind, water, or ice. – Worldwide removes 25.4 billion tons/yr. – Made worse by deforestation and desertification. – Poor agricultural practices increase erosion and lead to the transport of associated fertilizers and pesticides.
28 Soil Erosion Most current agricultural areas lose topsoil faster than it can be replenished. Wind erosion may not be as evident as water erosion, but is still common. – Most common in dry, treeless areas. – Great Plains have had four serious bouts of wind erosion since late 1800s.
30 Soil Conservation Practices When topsoil is lost, fertility is reduced or destroyed, thus fertilizers must be used to restore fertility. – Raises food costs and increases sediment loads in waterways. – Over 20% of U.S. land is suitable for agriculture, but virtually all of it requires some form of soil conservation practice.
31 Soil Conservation Practices Contour Farming—Tilling at right angles to the slope of the land. Each ridge acts as a small dam. Strip Farming—Alternating strips of closely sown crops to slow water flow. Terracing—Level areas constructed at right angles to the slope to retain water. – Good for very steep land.
33 Soil Conservation Practices Waterways—Depressions in sloping land where water collects and flows off the land. Windbreaks—Planting of trees or other plants that protect bare soil from full force of the wind.
34 Conventional vs. Conservation Tillage Plowing has multiple desirable effects: – Weeds and weed seeds are buried or destroyed. – Crop residue is turned under. Decays faster and builds soil structure. – Leached nutrients brought to surface. – Cooler, darker soil brought to top and warmed.
35 Problem: Each trip over the field is an added expense to the farmer, and at the same time increases the amount of time the soil is open to erosion via wind or water. – Reduced Tillage— Uses less cultivation to control weeds and prepare soil, but generally leaves 15-30% of soil surface covered with crop residue after planting.
36 Problem: – Conservation Tillage—Further reduce amount of disturbance and leaves 30% or more of soil surface covered with crop residue. Mulch Tillage—Tilling entire surface just prior to planting. Strip & Ridge Tillage—Tilling narrow strips. No Till—Place seeds in slits.
37 Pros of Reduced Tillage Wildlife gain food and cover. Less runoff—reduced siltation of waterways. Row-crops can be planted in sloped areas. Fewer trips means lower fuel consumption. Double-cropping Fewer trips means less soil compaction.
38 Cons of Reduced Tillage Plant residue may delay soil warming. Crop residue reduces evaporation and upward movement of water through the soil. Accumulation of plant residue can harbor plant pests and diseases requiring more insecticides and fungicides.
39 Protecting Soil on Nonfarm Land By using appropriate soil conservation practices, much of the land not usable for crops can be used for grazing, wood production, wildlife production, or scenic and recreational purposes.
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