SOIL 460 - Exam 1

define soil particle-size distibution

differentiate sand, silt, and clay

course fragment > 2 mm (so soil is less than 2 mm)

sand: 0.05 - 2.0 mm

silt: 0.002 - 0.05 mm

clay: < 0.002 mm

explain how particle-size distribution is measured

can be measured with:

• sieve analysis

  • coarse fragments and sand fractions

• hydrometer

  • suspension density through time

• pipette

  • timed sampling at a known depth

• laser diffraction

  • light-scattering estimate of size distribution

• texture by feel

  • field estimate, not a laboratory

    

prep for a particle size analysis (PSA) with…

• removal of organic matter (combustion at high temp)

  • treat with hydrogen peroxide (oxidize)

• removal of carbonates (acid treatment)

  • interferes with dispersion, issue in some glacial soils in the midwest

• removal of iron oxide cements (chemical treatment)

  • bids clays, not really a concern in the midwest

• dispersion (breaking apart) of soil aggregates

  • required for all soils

  • using sodium hexametophosphate

describe sieve, hydrometer, pipette, and laser methods

sieve: sample is passed through screens of various sizes

sedimentation: particle settling in a fluid based on size (calculations of Stoke’s Law)

explain stoke’s law

predict how particle-size distribution affects infiltration and water retention

soil composition

45% mineral matter

5% organic matter

50% water and air

formation of sand & silts

sand and silt are produced from the physical breakdown of rocks

• feldspar is the most common mineral in the crust of the earth, but the most common mineral in soil is quartz

  • quartz is physically hard and very resistant to breakdown (chemically inert)

  • offers no plant nutrients

  • particle density of soil matches the density of quartz

formation of clay

• clay forms from chemical alteration of mineral (like feldspar)

  • hydrolysis reaction: feldspar + water → clay mineral + ions in solution

• has high surface area

• charges surfaces (both positive and negative)

• attracts and retains nutrients and water

USDA sand subclasses

very coarse sand: 2.00 - 1.00 mm

coarse sand: 1.00 - 0.50 mm

medium sand: 0.50 - 0.25 mm

fine sand: 0.25 - 0.10 mm

very fine sand: 0.10 - 0.05 mm

very coarse sand

2.00 - 1.00 mm

coarse sand

1.00 - 0.50 mm

medium sand

0.50 - 0.25 mm

fine sand

0.25 - 0.10 mm

very fine sand

0.10 - 0.05 mm

coarse fragment class

textural modifiers for rock fragments

sieve

sample is passed through screens of various sizes

• works best for particles > 0.05 mm (sand fraction)

  • labor and time intensive

  • size and shape influence results (rounder shape is better results)

  • loss of sample (especially with wet sieving)

sedimentation

particle settling in a fluid based on size

sedimentation principles: an object moving at a constant speed (terminal velocity) is acted upon by forces equal in magnitude

• forces accelerating it (downwards)

  • downward force (F_g): particles accelerate due to gravity

• forces slowing it (upwards)

  • upward force (F_d): resistance/drag due to viscosity of fluid

sedimentation concept:

• particles uniformly mixed (at time₀)

• at time_n all particles with a diameter (d) > some critical diameter are below that line

• larger diameter particles (more massive) settle faster

sedimentation assumptions:

• particles are not affected by thermal (or other) motions

• particles are smooth and spherical

• particles have the same density

• particles settle independently (no collisions)

• smooth flow of fluid around particles (no turbulence)

particle density

ρ_s = 2.65 g cm^-3

density of fluid (water)

ρ_w = 1.00 g cm^-3

stoke’s law, time to settle

pipette method

procedure:

• suspend soil sample in cylinder

• as specified times, lower a pipette into the solution

• extract a sample an oven-dry it to determine a mass

• calculate corresponding particle diameter using (Stoke’s Law)

notes: generally straightforward and accurate

hydrometer

procedure:

• hydrometer measures the specific gravity (relative density of a solution)

approaches to account for changes hydrometer effect depth:

• determine diameter for a given depth

  • adds additional computations

  • diameter will likely not represent the boundary between USDA size class - will need to extrapolate

• use standardized sampling times

  • assumes some error (generally small)

  • easy

  • no universal agreement on times

    • quick method = 40 sec for sand 2 hr for sand and silt

    • recommended: 40 sec for sand, 8 hr for sand and silt

    • convenience: 40 sec, 1.5 hr, and 24 hr

notes:

• more particles = more mass = higher specific gravity/density

• use of equipment is quick, easy, and straightfoward

• determination of sampling times/depths is more complicated

geometric mean diameter

geometric mean standard deviation

a measurement of variability/dispersion around a mean

soil

an open porous system consisting of soils, liquids, and gases in which matter and energy are exchanged within the soil system and externally

energy in soil

chemical: water, redox reactions, mineral transformations

biological: transpiration, photosynthesis, respiration

thermal: heat

soil health

continued ability of soil to function as vital ecosystem

• maximize continuous living roots

• minimize disturbances

• maximize soil cover

• maximize biodiversity

accuracy

like how close to the bullseye

precision

multiple values grouped together (how close they are together)