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Published bySherman Walsh Modified over 9 years ago
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Soil
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Soil Formed by 1- weathering of rocks, 2- deposition of sediment, and 3- decomposition of organic material Soil Composition Minerals (45%) - Weathered rock Organic Material (5%) - Leaves, animal dung, dead stuff Water (25%) Air (25%)
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SOIL LAYERS Leaf litter – leaves, animal waste, insects & decomposers Topsoil – humus (decomposed organic matter); inorganic soil; root systems; insects & decomposers Subsoil – inorganic matter; mixture of sand, silt, clay, and gravel Weathered parent rock
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Nutrient Cycling Nutrients are cycled between plants, animals and soil Biogeochemical Cycles Carbon Nitrogen Hydrologic Phosphorus
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Soil Properties Soil Acidity 4-8 pH = most soils pH affects solubility of plant nutrients 6-7 pH = Optimum soil (nutrients are maximally available)
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Soil Properties Soil Texture Sand: 2mm–0.05 mm Silt: 0.05mm–0.002 mm Clay: <0.002 mm Loam: a roughly equal concentration of sand, silt and clay
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Water High permeabilityLow permeability Infiltration/Percolation – downward movement of water through soil Leaching – as water seeps down, it dissolves soil nutrients in upper layers & carries it to lower layers Soil texture determines porosity & permeability
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Soil Properties
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Types of Soil Clay – Very fine particles Low permeability to water, prone to waterlogging Compacts easily Nutrient-rich due to negatively charged surface (able to hold onto important plant nutrients: K +, Ca 2+, NO 2- )
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Types of Soil Silt Particles intermediate in size between sand and clay Erodes easily Often found on riverbanks Has a silky feel (like flour)
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Types of Soil Sand coarser than silt Water flows through too fast for most crops
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Types of Soil Loams A mixture of clay, sand, silt and humus Best soil for crops From left to right Sandy Loam Clay
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Soil Triangle
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Soil Rate of Soil Formation - Factors: Parent Material Time Climate Organisms Topography
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Tropical Soils Infertile soils Topsoil often thin (nutrients in overlying plant life) Prone to chemical weathering (humus + water = acidic solution)
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Desert/Arctic Soils Thin Made up of mostly rock fragments (evidence of mechanical weathering)
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Soil Problems: Erosion Soil Erosion Causes: water runoff, wind, steep slope Why a problem? Loss of soil fertility as organic material & nutrients are eroded More fertilizers must be used to replace nutrients
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Case Study: The American Dust Bowl 1930s Great Plains region subject to drought Natural grassland vegetation had been removed Replaced by shallow- rooted annual crops Winds blew soil as far east as NYC Inspired Soil Conservation Act
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Soil Problems: Nutrient Mineral Depletion
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Often in arid /semi- arid areas Elevated salt concentrations toxic to plants Soil Problems: Salinization Soil Salinization Gradual accumulation of salt in the soil, usually due to improper irrigation techniques
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Salinization Solutions PreventionCleanup Reduce irrigation Switch to salt- tolerant crops (such as barley, cotton, sugar beet) Flushing soil (expensive, water intensive) Not growing crops for 2-5 years Install under- ground drainage systems (expensive)
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Soil Problems: Desertification The degradation of once-fertile rangeland, agricultural land, or tropical forest into nonproductive desert
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Desertification Consequences Causes Worsening drought Famine Economic losses Lower living standards Environmental refugees Overgrazing Deforestation Surface mining Erosion Salinization Soil compaction
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Crop Rotation Plant a series of different crops in the same field over a period of years Corn legumes (ex: soybean) Soil Conservation Conservation Tillage / “no-till” - Minimize disturbance of soil
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Strip Cropping Terracing Soil Conservation Contour Plowing Plowing around hill instead of up-down Strip Cropping Alternate strips of different crops along natural contours Terracing Creating terraces on steep slopes to prevent erosion Windbreaks
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Preserving Soil Fertility Organic fertilizers Animal manure, crop residue, bone meal, compost Slow release of nutrients available as material decomposes Inorganic fertilizers Manufactured from chemical compounds (high fossil fuel use) Soluble Fast acting, short lasting Mobile: easily leach into groundwater supplies
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Soil Conservation Policies in US Soil Conservation Act 1935 Authorized formation of Soil Conservation Service, now called Natural Resource Conservation Service (NRCS) Assess soil damage and develop policies to improve soil Food Security Act (Farm Bill) 1985 Requires farmers with highly erodible soil to change farming practices Instituted Conservation Reserve Program Pays farmers to stop farming highly erodible land
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