Chapter 12: SoilsI. Soil and RegolithA. intro1. soil: relatively thin layer of mineral matter that normally contains a considerable amount of organic material that is capable of supporting living plantsa. topmost layer of the lithosphereb. average worldwide depth of 6”2. soil developmenta. weathering: physical and chemical disintegration of rock exposed tothe atmosphere and to the action of water percolating down from the surface  regolith formationb. regolith: layer of broken and partly decomposed rock particles that covers bedrock; either residual or transportedc. soil is the upper portion of regolithB. Soil as a Component of the LandscapeII. Soil-forming FactorsA. The Geologic Factor1. parent material: source of the rock fragments that make up soila. from residual bedrock or transported materialb. chemical composition and physical characteristics influence resulting soil2. in early stages of soil formation this factor dominates, eventually influence of parent material may be completely obliteratedB. The Climatic Factor1. temperature and moisture: climatic variables of greatest significance2. generally chemical and biological processes in soils accelerated by high temperatures and abundant moisture a. soils tend to be deepest in warm, humid regions b. soils are shallowest in cold, dry regions3. water carries dissolved chemicals and tiny particles of matter in suspension through soil, vertically and laterallya. rearranges the chemical and physical components of soilb. contributes to variety and availability of plant nutrients4. generally climate the most influential factor in soil development in the long runC. The Topographic Factor1. slope and drainage: 2 main topographic features that influence soil development2. slopea. deepest soils usually on flat land b. on steep slopes with rapid erosion, soils tend to be thin and immaturely developed Chapter 12: Soils – p. 1 of 123. if there is insufficient natural drainage, most often in valley bottoms or flat terrain, soil tends to be waterlogged with a high proportion of organic materialD. The Biological Factor1. organic matter in soila. organic matter: living and dead plants and animalsb. soil by volume: ~50% mineral matter; ~50% air and water; small fraction organic matterc. organic matter vitally important: gives life to soil and “makes it morethan just dirt”2. role of organic matter in soila. plant roots: provide passageways for drainage and aeration and vital link between soil nutrients and growing plantsb. animals: mixing and plowing of soil; soil enrichment with waste productsc. microorganisms: help release nutrients from dead organisms by decomposing organic matter into humus and converting nutrients toforms usable by plantsE. The Chronological Factor1. soil-forming processes are slowa. from sediment more quicklyb. from bedrock relatively slowly2. in grand scale of geologic time soil can be formed and reformed, but in the dimension of human time, soil is a nonrenewable resource III. Soil ComponentsA. Inorganic Materials 1. ~50% of soil volume is mineral matter2. sand and silt a. fragments of the wasting rockb. common minerals1) quartz, composed of silica (SiO2)2) feldspars and micas3. clay a. smallest particles – colloidal in size: larger than molecules but too small to be seen with naked eyeb. flat platelets with relatively large surface area c. important influence on soil chemical activity1) chemical reactions occur at the surfaces of soil particles2) sheets of platelets are negatively charged3) attract positively charged ions (cations)4) many essential plant nutrients occur in soil solution as cations 5)  clay is reservoir for plant nutrient and soil water B. Organic Matter1. usually <5% of total soil volume, but enormous influence2. litter: collection of dead plant parts that accumulate on the soil surfaceChapter 12: Soils – p. 2 of 123. humus: dark colored, gelatinous, chemically stable organic matter on or in the soil that results from decomposed plants and animalsa. “black gold”b. utmost importance to agriculture: it loosens the structure and lessens the density of the soil  facilitates root development c. catalyst for chemical reactions d. reservoir for plant nutrients and soil water C. Soil Air1. ~ 50% volume of average soil is made up of pore spaces 2. interstices: interconnecting passageways that allow air and water to penetrate soil3. ideal soil: filled half with air, half with waterD. Soil Water1. sources of soil watera. downward percolation of rain and snowmeltb. upward capillary movement of ground water2. soil water loss:a. percolation down into ground waterb. upward capillary movement to the surface by evaporation or plant transpiration3. functions of water:a. effective solvent: dissolves nutrients and makes them available to plant rootsb. leaching: process in which gravitational water dissolves soluble materials and carries them downward in solution to be redeposited at lower levels1) depletes top soil of soluble nutrientsc. moves particles around1) eluviation: process by which gravitational water picks up fine particles of soil from the upper layers and carries them downward2) illuviation: process by which fine particles of soil from the upperlayers are deposited in the lower layersd. facilitates many of the chemical reactions with clay and actions of microorganisms that produce humus 4. soil-water balance: the relationship between gain, loss, and storage of soil watera. water gained through percolation and capillary action is diminished largely through evapotranspirationb. primary determinant: temperature and humidityc. how much water available to plants is far more important than amount of precipitation 1) warm weather increases evapotranspiration  diminished soil-water supply2) cool weather slows evapotranspiration  more soil moisture retentionChapter 12: Soils – p. 3 of 12d. amount of soil moisture available for plant use is the difference between field capacity and wilting point 1) field capacity: the maximum amount of water that can be retained in the soil after the gravitational water has drained awaya) most soil pore spaces are filled with water2) wilting point: the point at which plants are no longer able to extract moisture from the soil because the capillary water is all used up or evaporatede. soil-water budget: an accounting that demonstrates the variation of the soil-water balance over timef. hypothetical soil-water budget for a northern hemisphere midlatitude