Soil texture

Soil structure

Density and porosity

Tilth

The relative proportion of sand, silt, and clay sized particles that lends a distinctive “feel” to the soil

Fundamentally it is the most important property of the soil Structure Water holding capacity Tillage Nutrient supply Drainage

Soils are a mixture of different sized mineral particles

Sand 0.05 – 2 mm

Mainly quartz

Gritty

Silt 0.002 – 0.05 mm

Mostly quartz and primary silicate minerals

Powdery

Clay < 0.002 mm

Secondary minerals

Sticky

Not a component of texture, but affect soil management

Gravel 2 – 75 mm (3 in.)

Cobbles 75 – 254 mm (10 in.)

Stones > 254 mm

Texture Lay Categories

Sand Coarse textured Light

Loamy sand

Sandy loam

Loam Medium textured

Sandy clay loam

Sandy clay

Silt Heavy

Silt loam

Silty clay loam Fine textured

Silty clay

Clay loam

Clay

Fig. 2-1 Page 11

Feel test (see Table 2-2 in textbook) Qualitative Good for quick in-field determination Organic matter influences feel Easiest on moist soil

Physical analysis Soil is dispersed with Na and suspended Density measured over time with hydrometer Uses Stoke’s Law to predict settling based on particle

size Available at most soil testing labs ~ $25

2 x 2 in. blocks

8 together (large particle) 4” x 4” x 6 sides = 96 sq. in.

8 separate (small particle) 2” x 2” x 6 sides x 8 blocks = 192 sq. in.

The smaller the particles, the greater the surface area

Clay has 1000 x more surface area than sand

Clays also have internal surface area (deck of cards)

Soil reactions occur on particle surfaces

An acre plowlayer of clay has 25,000,000 acres of surface (How many acres in Wisconsin?)

Bulk density = mass per soil volume High soil bulk density can limit plant growth

In soils dry mass is used

Often expressed in units of grams/cubic centimeter (g/cc)

Particle density = mass per particle volume Averages 2.65 g/cc

Maximum soil bulk density if zero porosity = particle density

Silt loam soil has 50 % porosity, what is the bulk density?

Soil porosity = pore space In a field soil some are water-filled, some are air-filled Important for gas exchange in soils Need to get O2 to roots for respiration needed for nutrient

uptake Need to “ventilate” CO2 from soil microbial activity

Less than 10% air-filled pore space has been show to limit plant growth K uptake is reduced Available N turns to a gas

Plants may die after 48 hours in saturated conditions

Bulk density (Db) = Ws

Vpore + Vparticle

Particle density (DP) = Ws

Vparticle

Re-arrange = Vpore = Db or solid fraction

Vpore + Vparticle Dp

Porosity = 1 - Db or pore fraction

Dp

Weight of 50 cc of soil _____

Bulk density _____

Volume of water: 50 cc

Suspension volume _____

Particle volume _____

Particle density _____

Db / Dp _____

Porosity (1 – Db/Dp) _____

Soil Texture Bulk Density (g/cc) Porosity (%)

Coarse textured (sandy) 1.6 40

Medium textured (loamy) 1.3 51

Fine textured (clayey) 1.2 55

Well structured soil

Rocky soil

The combination of individual soil particles to form aggregates having a defined shape, size, and distinctness Peds

Millions of particles “glued” together Biological secretions

Oxides

Static charge

Wetting/drying

Freezing/thawing

Internal pressure

Presence of certain ions (Ca vs. Na)

Tillage management

Earthworm activity

Crop rotation

0

0.5

1

1.5

2

2.5

3

CC CSb SbC SbSb

SIZ

E (

mm

)

FALL MB FALL CH NO-TILL

Structure-less Structured

Single-grained

Puddled Granular Platy Blocky Prismatic Columnar

Type Profile

Location Size Permeability

Granular Surface Topsoil

1 – 10 mm High

Platy Between

Topsoil and Subsoil

Thin Irregular

Poor

Blocky Upper Subsoil 5 – 50 mm Moderate

Prismatic/ Columnar

Lower Subsoil 10 – 100 mm Moderate to

Slow

Avoid operations on wet soils

Control traffic

Limit vehicle load

Till at friable

consistence

Add organic matter Crop residue

Manure

Compost

Rotate to forage

Physical condition of the soil with respect to plant growth

A combination of …..

Texture

Structure

Management

Maintain tilth

Work when friable

Avoid unnecessary operations

Add organic matter

Maintain residue

Rotate crops

Optimize fertility and pH