Chapter 15 Soil Resources
Overview of Soil Unit The Rock Cycle Igneous, sedimentary and metamorphic rock types What is soil? Soil Properties Major Soil Orders Soil Problems Soil Conservation Soil Reclamation
The Rock Cycle The rock cycle is the natural process in which rocks transform from one rock type into another rock type over time, a type of natural recycling. Igneous rocks, which are formed from magma cooling underground, or lava above ground are formed from previous igneous, metamorphic, or sedimentary rocks which have become melted Sedimentary rock is formed by deposition and consolidation of mineral and organic material and from precipitation of minerals from solution. The processes that form sedimentary rock occur at the surface of the Earth and within bodies of water Metamorphic rock is the result of the transformation of an existing rock type, the protolith, in a process called metamorphism, which means "change in form".
Rock Cycle: Igneous
Sedimentary Rocks
Metamorphic Rock
The Rock Cycle
Soil Uppermost layer of earth’s crust that supports plants, animals and microbes Soil Forming Factors Parent Material Time Climate Organisms Topography Types: clay, sand, silt, and gravel (including a mixture of one or more or all
What is Soil? Produced very slowly – almost nonrenewable Production by… Weathering of rock (parent material) – chemical & mechanical this adds inorganic components Deposit of sediments by erosion Introduction of living organisms – succession the biotic component Decomposition of organic materials and dead organisms Development is slow 200-1000 years to produce 1 inch of topsoil Soil is different in different areas
Soil Composition Mineral Particles (45%) Organic Material (5%) Weathered rock Provides essential nutrients for plants Organic Material (5%) Litter, animal dung, dead remains of plants and animals, humus (picture) Water (25%) Air (25%)
Soil Composition Pore space 50% of soil Soil air- good for aeration Soil water- provides water to roots
Soil Horizons O-horizon A-horizon B-horizon C-horizon Rich in organic material A-horizon Topsoil B-horizon Lighter colored subsoil C-horizon Weathered parent material
Soil Organisms There are millions of microorganisms in 1 tsp of fertile agricultural soil
Soil Organisms Soil organisms provide ecosystem services Examples Def: Important environmental benefits that ecosystems provide Examples Decaying and cycling organic material Breaking down toxic materials Cleansing water Soil aeration (especially done by earthworms)
Nutrient Cycling Nutrients are cycled between plants, organisms and soil Example Bacteria and fungi decompose plant and animal wastes They are transformed into CO2, soil nutrients and water
Soil Properties Soil Texture Relative proportion of sand, silt and clay Sand: 2mm-0.05mm Silt: 0.05mm-0.002mm Clay: >0.002mm
Soil Properties Soil texture affects soil properties Coarse textured soil (sandy) Will not hold water well- flows through easily Fine textured soil (high in clay) Poor drainage Low oxygen levels in soil Due to negatively charged surface, able to hold onto important plant nutrients (K+, Ca2+, NO2-)
Soil Properties
Soil Properties Soil Acidity Measured using pH scale 0-7 = acidic 7 = neutral 7-14 = basic pH of most soils range from 4-8 Affects solubility of certain plant nutrients Optimum soil pH is 6-7 This is the pH where plant nutrients are most available to plants Soil amendments (ex: lime) can be used to achieve this pH
Major Soil Groups Variations in soil forming factors cause variation in soils around globe Soil Taxonomy Separates soils into 12 orders Subdivided into more than 19,000 soil series that vary by locality Five common soil orders Spodosols, alfisols, mollisols, aridosols, oxisols
Major Soil Groups Spodosols Form under coniferous forests O-horizon composed of needles E-horizon is ash-gray under A-horizon Not good farmland- too acidic
Major Soil Groups Alfisols Brown to gray-brown A-horizon Precipitation high enough to leach most organics and nutrients out of O-, A- and B-horizons Soil fertility maintained by leaf litter
Major Soil Groups Mollisols Found in temperate, semi-arid grassland Very fertile soil Thick, dark brown/ black A-horizon Soluble nutrients stay in A-horizon due to low leaching
Major Soil Groups Aridosols Found in arid regions of all continents Low precipitation preclude leaching and growth of lush vegetation Development of salic horizon possible
Major Soil Groups Oxisols Found in tropical and subtropical areas with high precipitation Very little organic material accumulation due to fast decay rate B-horizon is highly leached and nutrient poor
Soil Problems Soil Erosion Why a problem? Def: wearing away or removal of soil from the land Caused primarily by water and wind Why a problem? Causes a loss in soil fertility as organic material and nutrients are eroded More fertilizers must be used to replace nutrients lost to erosion Accelerated by poor soil management practices
Case in Point: American Dust Bowl Great Plains have low precipitation and subject to drought 1930-1937 severe drought No natural vegetation roots to hold soil in place Replaced by annual crops Winds blew soil as far east as NYC and DC. Farmers went bankrupt
Soil Problems Nutrient Mineral Depletion
Soil Problems Soil Salinization Often in arid and semi-arid areas Def: gradual accumulation of salt in the soil, usually due to improper irrigation techniques Often in arid and semi-arid areas The little precipitation that falls is quickly evaporated Leaves behind salts Salt concentrations get to levels toxic to plants
Soil Problems Desertification Typically a human-induced condition Def: degradation of once-fertile rangeland, agricultural land, or tropical dry forest into nonproductive desert Typically a human-induced condition Change in vegetation changes climate, further decreasing precipitation levels
Soil Conservation Conservation Tillage Crop Rotation Residues from previous year’s crops are left in place to prevent soil erosion Includes no tillage Crop Rotation Planting a series of different crops in the same field over a period of years Lessens pest and insect disease
Soil Conservation Contour Plowing Strip Cropping Terracing Plowing around hill instead of up-down Decreases soil erosion Strip Cropping Alternating strips of different crops along natural contours Terracing Creating terraces on steep slopes to prevent erosion Terracing
Preserving Soil Fertility Organic fertilizers Animal manure, crop residue, bone meal and compost Nutrient available to plants only as material decomposes Slow acting and long lasting Inorganic fertilizers Manufactured from chemical compounds Soluble Fast acting, short lasting Environmentally sound to limit use Mobile- easily leach and pollute groundwater
Soil Reclamation Two steps Best way to do this is shelterbelts Stabilize land to prevent further erosion Restoring soil to former fertility Best way to do this is shelterbelts Row of trees planted to reduce wind erosion of soil
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 Required farmers with highly erodible soil had to change their farming practices Instituted Conservation Reserve Program Pays farmers to stop farming highly erodible land