Texture

Structure

Bulk Density and Particle Density

Pore space

Color

 Temp.

Strength

Sand, silt and clay - The size of these soil particles in the soil make up soil texture

-Critical to soil behavior and management

Sand

-Size

-Texture

-made of

-Pore size

-Surface area

-Structure

- 2.0 - 0.05 mm dia

-gritty, visible to naked eye

-made of primarilay quartz

-Macropores= fast drainage and free air exchange

-LESS surface area= very low water holding capacity

-Non-cohesive, single grained (won't stick together)

Silt

-size

-texture

-made of

-Surface area

-Pore size

-Minerals for plant nutrients

-.05 - .002 mm dia

-not visible to naked eye, smooth

-made of primarily quartz and primary silicate minerals (feldspars, hornblende, micas)

-Moderate surface area= moderate water holding capacity

-Smaller pores, but more pores than sand.

-Silicate minerals for plant nutrients: K, Ca, Mg, Fe, Mn

Clay

-size

-texture

-made of

-Surface area/water hold capactiy

-Structure

-pore size

- <.002 mm dia

-colloidal: <1 mm very small

-Primarily secondary silicate minerals

-High surface area= high water holding capactiy

-Stacking, plate shaped particles

-Small, convoluted micropores

-Faster rate of weathering= more horizontal development

- Higher capactiy to hold water

-Higher area for colonizations by microbes

- Higher attraction between particles. Results in forming soil aggragates

- Higher capacity of particles to hold plant nutrients

Soil Structure

-Define (2 parts)

-Arrangement of soil particles into groupings called aggragates (peds) that are seperated by planes of weakness (macropores).

-Pattern of pores and peds influences movement of water and air.

How soil aggragates form and stabilize?

Fine Textured Soil

-What type of processes?

-Particle attractions - pos. and neg.

-What keeps clay dispersed

-Physical, chemical processes of aggragation dominate

-Polyvalent cations: Ca²⁺ and Al³⁺ b/w clay particles attract other particles and act as bridges= pull and hold particles together.

-Positively charged edges of clay platelets attract negatively charged planar surfaces

-If Na+ is not present, clay particles remain dispersed= Repulsion (columnar)

How soil aggragates form and stabilize?

Coarse Textured soils

-Dominant agg. process

-Types of aggragates

-Water stability

-Granular structure in surface horizon

-Biological processes of aggragation dominant - microbial activity (air, OM) in surface

-Macroaggragates and microaggragates

-High water stability

-organic glues (polysaccharides) produced by bacteria and fungi, decomposing OM and earthworm casts.

~microbes in guts break down polysaccharides, bacterial cells

Tillage/plowing

-positive and negative effects

-Positive: break up large clods, incorporate OM, increase total porosity

-Negative: wet soil, can increase the rate of OM decomposition, crush stable aggragates, decrease macroporosity

-Adding OM: lower BD, higher porosity

Soil Pore Space

-between aggragates

-critical for

-optimum percentage

- not made of solid material

- air and water movement, root growth

-25% by volume (compacted suboil) to 60% (well aggragated, high OM surface horizon

- Optimum percentage: 50%

-Fast air movement

-Fast water drainage

-easy root growth

-biopores formed in roots, EW and insects (create continuous pores)

-important in clay soils

Micropores

-movment

-paw

-in what soils

-can roots respire?

-Very slow air movement

-Very slow water movement

-anoxic: roots can't respire (No CO₂)

-most water in micropores not available to plants

-Dominant pore size in fine textured, unaggragated soils

Bulk Density

-increased and decreased

-pore space

-root growth

-aeration

-water infiltration

D_b

-mass per unit volume of dry soil (solids and pores)

-Greater the pore space, the lower the BD

-Increased BD means less pore space, which means poor root growth. Reduced aeration and water infiltration

Partical Density

-Determined by...

-Average number

D_p

-mass per unit volume of soil solids only (g/cm³)

-Determined by chemical composition and crystal structure of mineral

-Varies b/w 2.6 - 2.75

Calc. percent soil pore space

-From what

-equation

-From BD and PD measurement:

% pore space = [1 - (BD/PD)] * 100

Effect of soil texture and structure on BD and porosity:

Fine

- Fine textured soils have lower BD and higher total pore space than sandy soils

-Protect soil structure

Well-structured soils have:

-pore size, BD

-pore space

-Large pores b/w soil aggragates, lower BD

-Higher total pore space due to macropores

BD of soil influences:

Soil strength and root growth

-BD

-types of soils and root penetration/growth

-pores

-High BD = high strength

-compacted, clayey soils

-Dry soils have greater resistance to root growth/penetration than wet soils

-Pores in compacted, fine textured soil are too small for roots to penetrate; roots must push particles aside

Measure of how difficult it is for roots to push particles aside to grow and capacity of soil mass to withstand stresses without rupturing or being deformed (engineering).

-No traffic

-roots subsoiled 3-4 ft *plowpan

-allows water drainage

-roots to grow

Ag. Management Factors affect BD:

-tillage and BD

-Field equipment and BD

-Long-term, intense tillage increases BD by increasing rate of OM decomposition and destroying soil structure.

-Field equipment used in wet soils increases BD by causing soil compaction layer (plow pan) which stops roots

-Minimize tillage to reduce loss of OM

-Lower traffic in fields and use of tillage equipment causing pan development

-Till at optimum soil moisture to avoid compaction

-Mulch surface with crop residues and include grasses and cover crops in crop rotation

-Add OM

-Protect aggragates from rain impact

Soil Color

-provides

-OM

-Oxidation: well and poor

-Provides information about OM content and drainage

-OM gives dark brown-black topsoil

-Oxidation state of Fe and Mn oxides

-well-drained: oxidized Fe bright red brown colors (oxygen!)

-poor-drained: reduced Fe dull, gray blue

-Hue: redness or yellow

-Value: lightness/darkness, 0=black

-Chroma: intensity/brightness - color saturation, 0=neutral gray

Soil Temperature:

-Affects...

-More important than air temp...

-Optimum temp

-Affects chemical, physical, and biological processes occuring in soil

-frozen, water can't be in phys or chem weathering

-More important than air temp in controlling:

-seed germination (BIG DEAL)

-water or nutrient uptake

-Optimum temp for plant growth depends on plant type

-Opt. temp. for microbial processes = 30 deg C (86 deg F)

-Nitrogen transformations, decomposition of OM

Microbial processes and soil temp:

-influence...

-Biological zero

-metabolic activity rate

-Influences nitrogen cycling and OM decomposition

-Biological Zero: 5 deg C= metabolic activity stops

-Rate of metabolic activity doubles w/ every 10 deg C increase in temp. over biological zero to optimum.

-Keep surface mulched!

-Alternate freezing and thawing of soil water: forces objects upwards in soil, especially when soil is WET

Classify soil by strength: Cohesive

-soil seperate

-pores

Clay content >15%

-strength down when pores filled with water, results in sudden failure

-Strength up when dry and compact

Classify soil by strength: non-cohesive

-soil seperate (wet, dry)

-angle of repose

-texture

-Sands, quicksand when saturated

-dry: strength depends on frictional forces b/w grains

-Angle of Repose: steepest angle to which material can be piled without slumping

- Smoothed, round sand grains can't be piled as high as rough, interlocking grains

Noncohesive, loosely packed sand grains held together at their contact points by gypsum, clay, or water under tension.

-Arid and semi-arid regions (cementation is stable)

-water breaks apart the cementing agents, in wet soils, and disperses the clays

-liquifaction

Sand grains

-Angle of repose: how much you can pile up before it slumps

-Coarse grains can pile better than smooth grains

-Dry: strength depends on frictional forces of grains