Soils: the substrate upon whichmany plants depend for water and nutrients

--primary medium to support plants

--also acts as shelter for billions of micro and macro organisms

--bacteria, insects, worms, rodents: all help mix the soil and keep it healthy

--nutrients are cycled from living to nonliving components, soil always in a state of flux

• Parent material

• Climate

• Vegetation

• Topography

• Age

Soil Processes

Parent material

ClimateVegetation

Weathering

Parent Material

Granite (igneous rock): mineral content includesAl+, Fe+, Ca+, K+, Mo+ (molybdenum)

--provides diverse nutrients to vegetation

Serpentine (also igneous): mineral content isCr (chromium), Mg+, Ni+ in high concentrations

--limited nutrients = limited plant growth

Weathering

--can be physical or chemical

--physical includes freeze/thaw, scouring by wind and sand to break down rock

Weathering--chemical occurs when water dissolves substances in rock from chemical reactions

--can release salts, calcium sulfides

--rocks react and erode differently based on mineral content, e.g., limestone vs. granite

Water and pHPure water spontaneously associates and disassociates constantly

H2O H+ + OH-

--1 per 550 million molecules does this every instant

Water is rarely pure. Other substances added to itwill break bonds and form new ones with either H+ or OH-

Acids: substances that leave excess H+

Bases: substances that leave excess OH-

e.g., NH3 (ammonia) is a base: H2O + NH3 NH4 + OH-

CO2 dissolves in water: CO2 + H2O H2CO3 (carbonic acid)

H2CO3 disassociates in soil to H+ + HCO3- making

the soil more acidic, influences weathering rate ofparent material

pH scale is from 1-14

1-6 is acidic, higher amounts of H+

7 is neutral, or pure water

8-14 is basic, higher amounts of OH-

Most rain is at pH 5, or slightly acidic

--will weather limestone quickly, especially if acidity increases

--higher acidity will also affect soil processes, bacterial decay will slow, etc.

Vegetation--plants absorb and store nutrients until released with decay

--storage time can be short or long, depending on species of plant

--some vegetational debris can make soils more acidic with decay, e.g., pine needles, and influence weathering rates of parent material

Topography--important for drainage, sedimentation, e.g., valley bottoms vs. slopes where soils accumulate

--also rainshadow effect

Age--time it takes to weather bedrock, release nutrients and develop soils

--as soil develops, it forms horizons based on water and nutrient flow

mineral soil

surface litter

zone of accumulation (illuviation)

zone of leaching (eluviation)

weathered parent material

unweathered parent material

Figure 4.1 (EFB)

Soil Patterns

Grasslands: grass species short-lived, rapid decay every year, warm areas but low rainfall

Forests: trees are longer-lived species, slower decay, more acidity in soil from pine needles, more leaching of nutrients may occur

Soil TextureBased on particle sizes of soil and can influence soil horizon development

Three size classes: sand: 0.05 – 2.0 mm diameter silt: 0.002 – 0.05 mm clay: < 0.002 mm

The relative proportion of each in a soil affects drainage

Chart shows how soils can be classified by texture

Figure 4.2 (EFB)

Why is clay loam best?Water moves through soil as either:

1. Gravitational: drains out2. Capillary: held between grains3. Hygroscopic: strong adhesion to surface

of smallest soil particles

Field capacity: maximum amount of capillary and hygroscopic water a soil can hold

--determined 1 to 3 days after a soaking rain, after gravitational water drains through

--most plants can absorb capillary water down to -15 bar water potential (permanent wilting point)

MicellesCharged clay particles

--formed by crystallization of minerals that weather from bedrock, pH dependent to have charge

--negative charges capture and hold cations (+ ions) in soil, don’t leach away e.g., Ca+, Na+, K+, Mg+, Fe+, Si+, NH4

+ (ammonium)

--plants break bonds and can absorb these nutrients

--problem is that each cation varies in strength of bond it forms, with H+ as strongest e.g., H+ Ca+ Mg+ K+ Na+

strong weak

--thus, H+ ions can displace all others off micelles and cause more leaching. If acidity increases, this can be a problem

For these reasons, then, clay loam is best—it maximizes field capacity and has lots ofmicelles to hold nutrients

Negative ions also can be held by micelles:may attach to a cation held on micelle, which actsas a ‘bridge’

e.g., NO3- (nitrate), PO4

- (phosphate), SO4- (sulfate)

Figure 4.7 (EFB)

Figure 4.8 (EFB)

Soils classifications by processesthat form them

Podzolization: highly acidic soils, e.g., pine forests, resulting in highly leached A and E horizons, dark B

--typical of cold, northern coniferous forests, where rainfall exceeds evaporation

Laterization: deeply weathered soils where decay is rapid and high Fe content, e.g., tropical forests

--rainfall exceeds evaporation, no humic buildup and little leaching to A horizon

--high Fe content gives reddish color, hardens like bricks when exposed to sun, drying, when forests are cleared

Soils classifications by processesthat form them

Salinization: salt buildup in top soils from reverse leaching, e.g., dry arid regions and deserts

--evaporation exceeds rainfall, causes reverse leaching from ground water

--can be salts, but also carbonates (alkali) that form hard crust on surface, limiting plant growth