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Soil Soil
SoilSoil Formed by 1- weathering of rocks, Formed by 1- weathering of rocks,
2- deposition of sediment, and 3-2- deposition of sediment, and 3-decomposition of organic materialdecomposition of organic material
Soil CompositionSoil Composition Minerals (45%) - Weathered rockMinerals (45%) - Weathered rock Organic Material (5%) - Leaves, animal Organic Material (5%) - Leaves, animal
dung, dead stuffdung, dead stuff Water (25%)Water (25%) Air (25%)Air (25%)
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
Nutrient CyclingNutrient Cycling
Nutrients are Nutrients are cycled between cycled between plants, animals plants, animals and soiland soil
Biogeochemical Biogeochemical CyclesCycles CarbonCarbon NitrogenNitrogen HydrologicHydrologic PhosphorusPhosphorus
Soil PropertiesSoil Properties Soil AciditySoil Acidity
4-8 pH = most 4-8 pH = most soils soils
pH affects pH affects solubility of plant solubility of plant nutrientsnutrients
6-7 pH = Optimum 6-7 pH = Optimum soil (nutrients are soil (nutrients are maximally maximally available)available)
Soil PropertiesSoil Properties Soil TextureSoil Texture
Sand: 2mm–0.05 mmSand: 2mm–0.05 mm Silt: 0.05mm–0.002 mmSilt: 0.05mm–0.002 mm Clay: <0.002 mmClay: <0.002 mm Loam: a roughly equal concentration of sand, Loam: a roughly equal concentration of sand,
silt and claysilt and clay
Water Water
High permeability Low 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
Soil PropertiesSoil Properties
Types of SoilTypes of Soil Clay – Very fine particlesClay – Very fine particles
Low permeability to water, prone to Low permeability to water, prone to waterloggingwaterlogging
Compacts easilyCompacts easily Nutrient-rich due to negatively charged surface Nutrient-rich due to negatively charged surface
(able to hold onto important plant nutrients: K(able to hold onto important plant nutrients: K++, , CaCa2+2+, NO, NO2-2-))
Types of SoilTypes of Soil SiltSilt
Particles intermediate in size between Particles intermediate in size between sand and claysand and clay
Erodes easilyErodes easily Often found on riverbanksOften found on riverbanks Has a silky feel (like flour)Has a silky feel (like flour)
Types of SoilTypes of Soil
SandSand coarser than siltcoarser than silt Water flows through too fast for most cropsWater flows through too fast for most crops
Types of SoilTypes of Soil LoamsLoams
A mixture of clay, sand, silt and humusA mixture of clay, sand, silt and humus Best soil for cropsBest soil for crops
From left to rightSandyLoamClay
Soil TriangleSoil Triangle
Soil Soil Rate of Soil Formation - Factors:Rate of Soil Formation - Factors:
Parent MaterialParent Material TimeTime ClimateClimate OrganismsOrganisms TopographyTopography
Tropical SoilsTropical Soils Infertile soilsInfertile soils Topsoil often thin (nutrients in overlying Topsoil often thin (nutrients in overlying
plant life)plant life) Prone to chemical weathering (humus + Prone to chemical weathering (humus +
water = acidic solution)water = acidic solution)
Desert/Arctic SoilsDesert/Arctic Soils
ThinThin Made up of mostly rock fragments Made up of mostly rock fragments
(evidence of mechanical weathering)(evidence of mechanical weathering)
Soil Problems: ErosionSoil Problems: Erosion Soil ErosionSoil Erosion
Causes: water runoff, Causes: water runoff, wind, steep slopewind, steep slope
Why a problem?Why a problem? Loss of soil fertility as Loss of soil fertility as
organic material & organic material & nutrients are erodednutrients are eroded
More fertilizers must be More fertilizers must be used to replace nutrientsused to replace nutrients
Case Study:Case Study:The American Dust Bowl The American Dust Bowl
1930s1930s Great Plains region subject to Great Plains region subject to droughtdrought Natural grassland vegetation Natural grassland vegetation
had been removedhad been removed Replaced by shallow-Replaced by shallow-
rooted annual crops rooted annual crops Winds blew soil as far east Winds blew soil as far east
as NYCas NYC Inspired Soil Conservation Inspired Soil Conservation
ActAct
Soil Problems:Soil Problems:Nutrient Mineral DepletionNutrient Mineral Depletion
Often in arid /semi-Often in arid /semi-arid areasarid areas
Elevated salt Elevated salt concentrations concentrations toxic to plantstoxic to plants
Soil Problems: Soil Problems: SalinizationSalinization Soil SalinizationSoil Salinization
Gradual accumulation of salt in the soil, Gradual accumulation of salt in the soil, usually due to improper irrigation techniquesusually due to improper irrigation techniques
Salinization SolutionsSalinization Solutions
Prevention Cleanup
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 drainagesystems (expensive)
Soil Problems: Soil Problems: DesertificationDesertificationThe degradation of once-fertile rangeland, The degradation of once-fertile rangeland,
agricultural land, or tropical forest into agricultural land, or tropical forest into nonproductive desertnonproductive desert
DesertificationDesertification
Consequences
Causes
Worsening drought
Famine
Economic losses
Lower living standards
Environmentalrefugees
Overgrazing
Deforestation
Surface mining
Erosion
Salinization
Soil compaction
Crop RotationCrop Rotation Plant a series of different crops in the same Plant a series of different crops in the same
field over a period of years field over a period of years Corn <-> legumes (ex: soybean)Corn <-> legumes (ex: soybean)
Soil ConservationSoil Conservation Conservation Conservation
Tillage / “no-till” - Tillage / “no-till” - Minimize Minimize disturbance of soildisturbance of soil
Strip Strip CroppingCropping
TerracinTerracingg
Soil ConservationSoil Conservation Contour PlowingContour Plowing
Plowing around hill Plowing around hill instead of up-downinstead of up-down
Strip CroppingStrip Cropping Alternate strips of Alternate strips of
different crops along different crops along natural contoursnatural contours
TerracingTerracing Creating terraces on Creating terraces on
steep slopes to prevent steep slopes to prevent erosionerosion
WindbreaksWindbreaks
Preserving Soil FertilityPreserving Soil Fertility Organic fertilizersOrganic fertilizers
Animal manure, crop residue, bone meal, Animal manure, crop residue, bone meal, compostcompost
Slow release of nutrients available as material Slow release of nutrients available as material decomposesdecomposes
Inorganic fertilizersInorganic fertilizers Manufactured from chemical Manufactured from chemical
compounds (high fossil fuel use)compounds (high fossil fuel use) SolubleSoluble
Fast acting, short lastingFast acting, short lasting Mobile: easily leach into Mobile: easily leach into
groundwater supplies groundwater supplies
Soil Conservation Policies in Soil Conservation Policies in USUS Soil Conservation Act 1935Soil Conservation Act 1935
Authorized formation of Soil Conservation Authorized formation of Soil Conservation Service, now called Natural Resource Service, now called Natural Resource Conservation Service (NRCS)Conservation Service (NRCS)
Assess soil damage and develop policies to Assess soil damage and develop policies to improve soilimprove soil
Food Security Act (Farm Bill) 1985Food Security Act (Farm Bill) 1985 Requires farmers with highly erodible soil to Requires farmers with highly erodible soil to
change farming practiceschange farming practices Instituted Conservation Reserve ProgramInstituted Conservation Reserve Program
Pays farmers to stop farming highly erodible Pays farmers to stop farming highly erodible landland