physical properties (part 2)

how does soil high in sand content affect wind erosion? high clay content?

sand - moderate (high if fine sand)

clay - low if aggregated

how does soil high in sand content affect shrinking/swelling? high clay content?

sand - very low

clay - moderate to very high

how does soil high in sand content affect compaction risk ? high clay content?

sand - low

clay - high

how does soil high in sand content affect nutrient adsorption capacity ? high clay content?

sand - low

clay - high

compacted clay has ______ porosity and a _____ structure

very low, platy

what do soil textural classes convert

relative % sand, silt, clay (by weight) into a name (texture class)

what do texture classes represent

various combinations of sand, silt, clay

how are soil textural classes found

Bouyoucos hydrometer

what textural classes are in very fine textural group

HC - heavy clay

what textural classes are in fine textural group

SiC - silty clay

C - clay

SC - sandy clay

what textural classes are in medium textural group

SiCL - Silt clay loam

CL - clay loam

SCL - sandy clay loam

L - loam

SiL - silty loam

Si - silt

what textural classes are in coarse textural group

SL - sandy loam

LS - loamy sand

S - sand

what are some positives as clay content increases (5)

fertility potential increases

water holding capacity increases

leaching decreases

aggregation increases

risk of wind erosion decreases

what are some negatives as clay content increases (7)

number of large pores decreases

aeration decreases

drainage decreases

risk of runoff and water erosion increases

swelling/shrinking increases

risk of compaction increases

risk of surface crushing increases

what are some positives as sand content increases (4)

aeration increases

drainage increases

risk of compaction decreases

does not crust when wet soils dry after tillage/seeding

what are some negatives as sand content increases (4)

drought risk increases

risk of wind erosion increases

leaching increases

fertility potential decreases

when is the risk of wind erosion higher for sandy soils

in medium to fine sand sizes (smaller sizes)

why do soils high in sand have an increase in drought risks

they have low water holding capacity

what are medium textured soils

soils found in between the extremes of coarse and fine textured soils

what is beneficial about medium textured soils

they have all the benefits of sand and clay without the severe limitations associated with extremes of either coarse or fine soils

what are characteristics of medium textured soils

1. high water retention

2. good drainage

3. good aeration

4. low risk of compaction

5. reduced risk of wind erosion

what is soil structure

the arrangement of soil solids (sand, silt, clay) into aggregates

larger aggregates are often composed of _____

clusters of smaller aggregates

peds vs clods

peds - naturally formed large aggregates and separated by planes of weakness and have distinct shapes

clods - management (artificial) induced large aggregates

what separates peds in soil

planes of weakness

how are aggregates formed biologically? chemically? physically?

bio - via microbial activity (influenced by OM content) and vegetation (roots)

chem - influenced by amount of clay where calcium help bind clay and OM components

phys - freeze/thaw, wetting/drying, root pressure

does an anion or cation help bind clay and OM compounds together

cation - calcium

what does aggregation create between and within aggregates

pore spaces

soil structure influences (5)

porosity

aeration

water movement/holding capacity

heat transfer

erosion risk

soil texture vs soil structure

texture

• little human control

• difficult to change

• related to size and relative amounts of clay, silt, sand

structure

• greatly influenced by management (artificial activities)

• changes with management

• related to arrangement of sand, silt, clay and SOM

does soil structure involve OM content? soil texture?

structure - yes

texture - no

what are five factors affecting soil structure

texture, freeze/thaw and wetting/drying, tillage, dominant cation in soil, SOM content

what % of clay is required for aggregates to form

minimum %15

how does freeze thaw and wet/dry affect soil mass

by creating lines of weakness in the soil

what happens if freeze thaw and wet/dry cycles occur too often

may destroy aggregates (especially if soil is low in OM and high in clay)

why should you never till when soil is too wet

it causes soil puddling and crusting

what is soil puddling

the complete loss of soil granular structure

what do puddled soils form when dried

hard crust

what is a surface compaction example

soil crusting

what type of soils is soil crusting more likely to occur

high in clay (>30%) and low in OM (4-5%)

what are the dominant cations in soil that bridge clay particles together into aggregates

calcium, magnesium, aluminum

what element disperses pr separates clay into individual particles

sodium

what soil structure forms as a result of excess sodium

columnar

_____ promotes aggregation and provides water aggregate stability

SOM

is OM or clay a better glue for aggregate formation

OM

_____ promotes aggregation but cannot give water aggregate stability

clay

only factor that effectively gives water stability to soil aggregates

SOM

6 types of soil structure

granular

platy

blocky

prismatic

columnar

structureless

_____ structure helps water infiltration and seed germination

granular

what soils are granular structures found

surface soils (ex. Ah)

____ peds are spheroidal

granular

peds are thin, flat, horizontal plates

platy

how are platy structures formed

naturally in eluviated horizons or by clayey soil compaction

example of horizon with platy structure

Ae

what are the two types of blocky soil structures

angular and subangular

example of a horizon where angular blocky structure can be found

Bt (high content of illuviated clay(

what do both types of blocky structures promote

drainage, aeration and root penetration

prismatic vs columnar

prismatic - spikelike top

columnar - flat or rounded top

example of a horizon where prismatic structures can be found

Bm (arid and semi arid regions)

example of a horizon where columnar structures can be found

Bn or Bnt (high in sodium)

what is structureless soil structure

no observable aggregation or structural units

two types of structureless soils

single grain (sand)

massive (solid mass without aggregates)

what is soil consistence

the soil’s resistance to deformation (force needed to crush soil aggregate between thumb and forefinger)

what are the three soil moisture conditions for describing consistence

dry, moist, wet

degree of soil _____ when dry? ______ when moist? ______ when wet?

dry = hardness

moist = toughness

wet = plasticity

what three things does soil consistence depend on

1. clay content

2. dominant clay mineral

3. OM content

soil consistence correlates with soil ____

tilth