SOILS SOILS

What is Soil??What is Soil??Soils are the transition between the biotic and abiotic Soils are the transition between the biotic and abiotic

worlds. worlds.

Soil is a mixture of:Soil is a mixture of:1.1. Minerals Minerals

2.2. WaterWater

3.3. GasesGases

4.4. HUMUS-Dead “things” that have broken down and become HUMUS-Dead “things” that have broken down and become organic material—decayed plant and animal remains organic material—decayed plant and animal remains

(thanks to decomposing fungi (thanks to decomposing fungi

and bacteria) and bacteria)

Soil Composition (idealized)Soil Composition (idealized)

What Makes Up Soil

Air25%

Water25%Humus

5%

Minerals45%

Why are soils important?Why are soils important?Soils are important:Soils are important:

As a habitat for growing cropsAs a habitat for growing crops FoodFood Fabrics/DyesFabrics/Dyes Rubber/Building MaterialsRubber/Building Materials

Medium for photosynthetic organisms Medium for photosynthetic organisms Medium for decomposersMedium for decomposers As foundations of buildingsAs foundations of buildings As beds for roads and highwaysAs beds for roads and highways As an absorbent of domestic wastes in rural areas as well as As an absorbent of domestic wastes in rural areas as well as

a depository for other wastesa depository for other wastes As a filter for pollution that comes from rain and water As a filter for pollution that comes from rain and water

runoffrunoff

What determines how soil forms?What determines how soil forms?

There are THREE main contributing There are THREE main contributing factorsfactors

1.1. Length of time soils have been developing—the Length of time soils have been developing—the AGE AGE of the soils of the soils

2.2. The materials (rocks) from which they form- The materials (rocks) from which they form- these are called these are called PARENT MATERIALPARENT MATERIAL

3.3. The The ENVIRONMENTENVIRONMENT in which they developed in which they developed (climate, vegetation, soil life, topography)(climate, vegetation, soil life, topography)

Good rule of thumb: 500-1000 yrs per inch of topsoilGood rule of thumb: 500-1000 yrs per inch of topsoil

How do parent materials get into How do parent materials get into soils?? soils??

WEATHERINGWEATHERING: Any process where rock : Any process where rock breaks down (changes chemically and breaks down (changes chemically and physically)physically)

There are two types of weathering:There are two types of weathering:Physical or Mechanical WeatheringPhysical or Mechanical Weathering: Large rock : Large rock

mass is broken into smaller fragments of the mass is broken into smaller fragments of the same type-NO CHEMICAL CHANGEsame type-NO CHEMICAL CHANGE

Chemical WeatheringChemical Weathering: mass of rock is decomposed : mass of rock is decomposed by chemical reactionsby chemical reactions

What is happening in these photos?What is happening in these photos?

Let’s take a closer look at Let’s take a closer look at frost wedging…frost wedging…

1.Water collects in cracks in rock

2.Water expands when it freezes

3.Water melts; Pieces of rock break off

Rock

Chemical WeatheringChemical Weathering Most chemical weathering involves gases in Most chemical weathering involves gases in

the atmosphere (oxygen, carbon dioxide, the atmosphere (oxygen, carbon dioxide, acid gases) and/or wateracid gases) and/or water

ExampleExample: Think about metal rusting: Think about metal rusting You go from metal to rust—a chemical You go from metal to rust—a chemical

reaction has taken placereaction has taken place

Contrast with EROSION (Contrast with EROSION (movement!movement!) by ) by wind, water, icewind, water, ice

Topsoil FormationTopsoil Formation

Detritus-based Soil EcosystemDetritus-based Soil Ecosystem

All these happy little critters break down dead plant and animal

material and aerate the soil—they make topsoil!

What is a soil profile?

Soil ProfilesSoil Profiles Soil Profiles - Soil Profiles - the sequence of layers (horizons) from the sequence of layers (horizons) from

the surface downward to rock or other underlying the surface downward to rock or other underlying materialmaterial

Soil LayersSoil Layers- “O”- organic horizons, litter derived from dead plants and - “O”- organic horizons, litter derived from dead plants and

animalsanimals- - A- TOPSOIL horizons which lie at or near the surface [A- TOPSOIL horizons which lie at or near the surface [Eluvial- Eluvial-

Characterized as zones of maximum leaching, Characterized as zones of maximum leaching, E = ExitE = Exit]]

- - B- SUBSOIL [sometimes illuvial I = Into]B- SUBSOIL [sometimes illuvial I = Into]- C- unconsolidated parent material under A&B layers- C- unconsolidated parent material under A&B layers-- Bedrock Bedrock

Soil profile example

Maryland Soil

Profile

What are some soil properties? What are some soil properties?

TextureTexture pHpH Porosity Porosity PermeabilityPermeability Nutrient Levels—Nitrogen/PhosphorusNutrient Levels—Nitrogen/Phosphorus ColorColor

SOIL TEXTURESOIL TEXTURE

Soil textureSoil texture: The way a soil "feels": The way a soil "feels"Depends on the fraction of each size of particle in Depends on the fraction of each size of particle in

the soilthe soil SandSand, , siltsilt, and , and clayclay are names that describe are names that describe

the size of individual particles in the soil.the size of individual particles in the soil. SANDSAND are the largest particles and they feel are the largest particles and they feel

"gritty.“"gritty.“ SILTSILT are medium sized, and they feel soft, silky or are medium sized, and they feel soft, silky or

"floury""floury" CLAYCLAY are the smallest sized particles, and they feel are the smallest sized particles, and they feel

"sticky" and they are hard to squeeze."sticky" and they are hard to squeeze.

Why care about soil texture?Why care about soil texture?

Predicts soil properties-fertility, drainage, etc.Predicts soil properties-fertility, drainage, etc. Sandy soilsSandy soils

low organic matterlow organic matter Poor retention water and nutrients (it flows through)Poor retention water and nutrients (it flows through)

As silt and clay fractions increaseAs silt and clay fractions increase, , More organic matterMore organic matter Better bufferedBetter buffered Better retention of water and nutrientsBetter retention of water and nutrients

But you can have TOO much clayBut you can have TOO much clay Hard to cultivate-too sticky when wet; too hard when dryHard to cultivate-too sticky when wet; too hard when dry Shrinks and swellsShrinks and swells

The best soils are a mix of all three (20% clay, 40% sand, 40% The best soils are a mix of all three (20% clay, 40% sand, 40% silt) and are called LOAMsilt) and are called LOAM

Soil pHSoil pHWhat does pH measure?What does pH measure? It’s a measure of how acidic or basic of a solution is on a 0 to It’s a measure of how acidic or basic of a solution is on a 0 to

14 scale (REM: inverse log of [H+]?)14 scale (REM: inverse log of [H+]?) We actually measure pH of soil SOLUTIONWe actually measure pH of soil SOLUTION The pH of the soil solution affects how much of soil nutrients The pH of the soil solution affects how much of soil nutrients

are available to plants. are available to plants. TOO ACID or TOO BASIC, can cause important nutrients TOO ACID or TOO BASIC, can cause important nutrients

(like nitrogen and phosphorus) to be (like nitrogen and phosphorus) to be unavailableunavailable..

At these extremes (too acid and too basic), METALS also At these extremes (too acid and too basic), METALS also released into soil, causing TOXIC effects in plantsreleased into soil, causing TOXIC effects in plants

PESTICIDE TIE-IN: If soil is too acidic, applied pesticides, PESTICIDE TIE-IN: If soil is too acidic, applied pesticides, herbicides, and fungicides herbicides, and fungicides will not be absorbedwill not be absorbed (held in the (held in the soil) and they will end up in runoff soil) and they will end up in runoff

Plant pH PreferencesPlant pH Preferences

Plants the like strongly acid soil (pH 4.0 to 5.0)

Plants that prefer slightly acid soil (pH 5.0 to 6.5)

Plants that prefer neutral soil

(pH 6.0-8.0)

• Sundew• Pitcher Plants• Venus flytrap• Azalea• Rhododendron• Camellia

• American holly• Orchids• Many evergreen

trees and shrubs• Strawberries• Potatoes• Carrots• Fescue grasses

• Alfalfa• Bluegrass• Most vegetables (lettuce, tomato)• Grains (corn,

wheat)

Soil Porosity and PermeabilitySoil Porosity and Permeability

PorosityPorosity- volume of pores (spaces) per volume - volume of pores (spaces) per volume of soil—sometimes referred to as “aeration”of soil—sometimes referred to as “aeration”

PermeabilityPermeability- rate of flow of materials through - rate of flow of materials through soilsoil

Infiltration—water getting into soilInfiltration—water getting into soil

Soil NutrientsSoil Nutrients

NitrogenNitrogen

PhosphorusPhosphorus

•Nutrients: essential chemical elements needed for the growth of healthy plants

•Plants get nutrients from soil after the nutrients have dissolved in the soil solution (the water around soil particles)

•Each nutrient has specific roles in producing healthy plants

Needed for chlorophyll, growthNeeded for chlorophyll, growthLow nitrogen causes yellowy leavesLow nitrogen causes yellowy leaves

Needed for roots, energy Needed for roots, energy Low phosphorus causes purply leaves,Low phosphorus causes purply leaves,

poor rootspoor roots

Sources of ImagesSources of Images

GLOBE Program- Agricultural Soils GLOBE Program- Agricultural Soils http://http://ltpwww.gsfc.nasa.gov/globe/agrisoil.htmltpwww.gsfc.nasa.gov/globe/agrisoil.htm

http://ltpwww.gsfc.nasa.gov/globe/basics.htmhttp://ltpwww.gsfc.nasa.gov/globe/basics.htm http://mather.ar.utexas.edu/AV/CRP369K/Lecthttp://mather.ar.utexas.edu/AV/CRP369K/Lect

ures/Soils.pdfures/Soils.pdf http://gushwalogy.org/APES/EnvironPowerPt/http://gushwalogy.org/APES/EnvironPowerPt/

EnvironPowerPts/Agriculture%5B1%5D.pptEnvironPowerPts/Agriculture%5B1%5D.ppt