Soil and Its UsesOutline:Soil FormationSoil PropertiesErosionSoil ConservationLand - Portion of the world not covered by water.Soil - Mixture of minerals, organic material, living organisms, air, and water.Soil FormationSoil formation begins with fragmentation of parent material.母岩母岩- Ancient rock layers with recent deposits from lava or glacial flows. 熔岩或冰河流动熔岩或冰河流动Weathering 风化风化- Factors bringing about fragmentation 破碎破碎 or chemical change of parent material.Mechanical Weathering 物理风化物理风化, 原生矿物原生矿物 Results from physical forces reducing size of rock particles without changing the chemical nature of the rock.Freezing and Thawing CyclesErosion from Wind and Moving WaterChemical Weathering 化学风化，化学风化，次生矿物次生矿物 - Involves the chemical alteration of rock in such a manner that it is more likely to fragment or be dissolved.Rock fragments exposed to atmosphere may oxidize, or otherwise chemically change.Humus - Decaying organic material.mixed with the top layers of rock particles, and supplies some of the needed nutrients to plants.Increase the water-holding capacity and inorganic nutrient solubility (acid cond.)creates crumbly soil which allows adequate water absorption and drainage.Other factors influencing soil formation:Burrowing animals bring nutrients up from deeper soil layers, improving soil fertility.Plant rootsBacteria and Fungi (decomposers)1 cm soil formation: 15 years (warm, humid climate) to hundreds of yearsSoil PropertiesTexture 质地，矿物质机械组成质地，矿物质机械组成 ： Various particles are mixed in many different combination, resulting in soil classification. Determined by the size of soil particles. Gravel 石砾石砾, d2mm Sand 砂粒砂粒, 0.05-2.0mmSilt 粉砂粒粉砂粒, 0.002-0.05mmClay 粘粒粘粒, 0.002mmToo many large particles leads to extreme leaching.Too many small particles leads to poor drainage.Soil classification: Clay 粘土粘土Silt 淤泥淤泥Sand砂土砂土 Loam壤土壤土Ideal for agricultureStructure - Refers to the way various soil particles clump together.土壤结构：土壤结构：土壤颗粒的空间排列方式及稳定土壤颗粒的空间排列方式及稳定程度，孔隙的分布及结合状况。程度，孔隙的分布及结合状况。How to change?Measured by? Friability - Soils ability to crumble. vs 粘结粘结性，结持性性，结持性In good soils 2/3 of the intra-soil spaces contain air after the excess water has drained.Oxygen for root cells and soil organism.Protozoa, nematodes, earthworms, insects, algae, bacteria, and fungi are typical inhabitants of soil.Soil ProfileSoil Profile 土壤质地剖面形态土壤质地剖面形态 - A series of horizontal layers of different chemical composition, particle size, and amount of organic matter.Horizon 发生层发生层 - Each recognizable layer of the profile.O Horizon - Litter - Un-decomposed or partially decomposed organic material.A Horizon - (Topsoil) Uppermost layer - contains most of the soil nutrients and organic matter.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 - BedrockMost 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.Forest Soils - Topsoil layer is relatively thin, but topsoil leachate forms a subsoil that supports substantial root growth. (High rainfall areas)Soil Erosion 土壤侵蚀土壤侵蚀Erosion Wearing away and transportation of soil/topsoil by wind and water.Floodplain, delta, muddy water after stormEffect: leave a degraded area and bury vegetation where it settles.Natural process accelerated by agricultural practice. Worldwide 25.4, China 5 Billion tons/yrMade worse by deforestation and desertification.High-tech agricultural practices on poor land increase erosion and lead to the transport of associated fertilizers and pesticides.Most current agricultural areas lose topsoil faster than it can be replenished.Wind erosion Most common in dry, treeless areas. After desertification.Soil ConservationWhen 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.Water erosion control runoffConservation PracticesContour 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.Wind erosionWindbreaks - Planting of trees or other plants that protect bare soil from the full force of the wind.TillagePlowing has multiple desirable effects:Weeds and weed seeds are buried / destroyed.Crop residue is turned under - decays faster and helps build soil structure.Leached nutrients brought closer to surface.Cooler, darker soil is brought to top and warmed.ProblemEach 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.