soils exam 2

What is frost heaving?

Frost heaving is the upward movement of soil, fence posts or roots caused by freezing temperatures. as soil freezes, water moves towards the freezing front and forms ice lenses, expanding upwards.

1st step in frost heaving

soil begins to freeze from top down, surface cools

2nd step in frost heaving

capillary action and matric potential pull unfrozen water up towards the freezing layer

3rd step in frost heaving

water accumulates and forms ice lenses

4th step in frost heaving

ice expands and pushes things up

ice lenses

layers of ice in the soil

what are two conditions needed for frost heaving?

continuous water supply, and freezing temperatures

water moves…

towards more negative potential

What are the three gradients responsible for attracting water to the freezing front of ice lens?

osmotic, thermodynamic, and moisture

what soil is most susceptible to frost heaving?

silty soils

How would you manage frost heaving?

improve drainage, compact soil, use coarse backfill, or design frost-protected foundations

Bulk density formula

BD = Md/Vt (mass of oven dry soil divided by total soil volume)

bulk density units

g/cm³

organic soil BD range

0.3 - 0.9g/cm³

mineral soil BD range

1.1 - 1.6g/cm³

Particle density for mineral soils (PD)

2.65

Particle density (PD) for organic soils

~1.3

Total porosity formula

f = 1 - BD/PD

Gravimetric water content (0g)

water mass per unit dry soil mass

Gravimetric water content (0g) formula

0g = Mwet-Mdry/Mdry

Volumetric water content (0v)

water volume per total soil volume

Volumetric water content (0v) formula

0v = 0g x BD

Which soil has the lowest available water?

sand

Which soil has the highest available water?

silt loam (best balance of macro and micropores)

Which soil holds the most water at saturation?

clay

Which soil has the least water at field capacity?

sand (no water retention)

Which soil has the most tightly held water (plants can’t get it)

clay

Soil with the fastest drainage?

sand

soil with the slowest drainage?

clay

soil structure most restrictive to roots

platy

Soil structure best for root growth?

granular

A soil with high BD has…

low porosity and few micropores

A soil with low BD is likely…

organic, well-aggregated, high porosity

As BD increases, porosity…

decreases

A well aerated soil has at least…

50% pore space

Water moves from ______ potential to ______ potential.

higher to lower

Which matric component dominates in unsaturated soil?

matric potential

Which component becomes important when salts accumulate?

Osmotic potential

Which component dominates above the water table?

gravitational potential

Capillary fringe is…

zone above water table where water rises due to matric potential

Air-filled porosity =

total porosity - volumetric water content

which soil has the highest capillary rise?

clay (micropores will water upwards)

Frost heaving is strongest in which soil

silty soils (best capillarity)

Which roots are most vulnerable during frost heaving?

Root crops (large-non fibrous roots)

What do water bars prevent?

runoff concentration and erosion

Slash left on skid bars helps by…

Reducing compaction by protecting soil surface

A culvert should be installed …

at natural drainage paths/intermittent streams

Which soil has the highest infiltration rate?

sand

Which soil has the lowest infiltration rate?

clay

Which soil provides the most plant available water?

silt loam

If a soil has a high BD and low porosity its likely…

compacted or mineral dominated

If a soil has low BD and high porosity, it’s likely…

organic or highly aggregated

Hysteresis

the relationship between soil water content and soil water potential. differs depending on if the soil is wetting or drying

Hydric soils

soils that form under conditions of saturation

biological zero

the temperature which many biological processes slow at

The conductivity of a dry soil increases with…

increasing bulk density

What value is used for a hectare furrow slice when bulk density is unknown?

2.2 million kg

soil structure

arrangement of soil particles into larger aggregates/peds

soil texture 

how many particles of sand silt or clay in soil

soil moisture potential

a measure of the energy state of water in the soil, determining its availability to plants and the direction of its movement

soil water characteristic

graph that shows relationship between the amount of water a soil holds and the energy it takes for a plant to extract it

wetland

area, where soil is saturated for a prolonged period where the temperature is>5. they have hydric soils, wetland hydrology, and hydrophytic vegetation

Adiabatic lapse rate

rate that a parcel of air changes temp as it rises or sinks (without exchanging heat with it’s surroundings)

destructive soil sampling

soil can’t be used (coring, coning)

nondestructive soil sampling

preserves soil so it can be studied (gamma ray, Lidar)

three things used to determine soil compaction

water quantity, texture, pressure

hygroscopic coefficient

maximum amount of water that 100 grams of dry soil can absorb from the atmosphere under specific conditions

How does soil structure happen?

sand, silt, and clay particles clump together into aggregates through physical, chemical, and biological processes. Biological activity from microorganisms and roots, along with organic matter, acts as a natural "glue". Chemical bonds from minerals and cations also bind the soils together.