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