Soil Structure, Density, and Porosity

Laboratory #4

Objectives

Understand the concept of soil structure and how it differs from soil texture.

Understand how soil structure influences other soil properties.

Learn how soil structure affects other soil properties and why it is important to maintain soil structure.

Learn how land use and management affects soil structure.

Definition

Soil structure is the arrangement of the primary soil particles (sand, silt, and clay) and other soil materials into discrete aggregates.

Peds

Structural units are called peds, and have distinct boundaries and well-defined planes of weakness between the aggregates.

Peds consist of primary particles bound together by cementing agents like organic matter, clay, and hydrous oxides of iron and aluminum.

Peds can take several shapes.

Ped Shapes

Granular Blocky Prismatic Columnar Platy Single-grained Massive

Granular Structure

Resembles cookie crumbs and is usually less than 0.5 cm in diameter.

Commonly found in surface horizons where roots have been growing.

http://soil.gsfc.nasa.gov/pvg/granular.gif

http://soils.usda.gov/technical/manual/images/fig3-30_large.jpg

Blocky Structure

Irregular blocks that are usually 1.5 - 5.0 cm in diameter.

Can be subangular or angular blocky.

http://soil.gsfc.nasa.gov/pvg/blocky.gif

http://www.dpi.vic.gov.au/dpi/vro/glenimages.nsf/Images/gl167_profile/$File/gl167_profile.jpg

Prismatic Structure

Vertical columns of soil that might be a number of cm long.

Usually found in lower horizons.

http://soil.gsfc.nasa.gov/pvg/prismatic.gif

http://soils.usda.gov/technical/manual/images/fig3-27_large.jpg

Columnar Structure

Vertical columns of soil that have a salt "cap" at the top.

Found in soils of arid climates.

http://soil.gsfc.nasa.gov/pvg/columnar.gif http://soils.usda.gov/technical/

manual/images/fig3-28_large.jpg

Platy Structure

Thin, flat plates of soil that lie horizontally.

Usually found in compacted soil.

http://soil.gsfc.nasa.gov/pvg/platy.gif http://soils.ag.uidaho.edu/soilorders/i/Arid_03.jpg

Single-grained Structure Soil is broken into individual particles that do

not stick together. Always accompanies a loose consistence. Commonly found in sandy soils.

http://soil.gsfc.nasa.gov/pvg/singlegrained.gif

Massive Structure

Soil has no visible structure, is hard to break apart and appears in very large clods.

http://soil.gsfc.nasa.gov/pvg/massive.gif http://soils.usda.gov/technical/manual/images/fig3-31_large.jpg

Grade of Soil Structure

The terms weak, moderate, or strong are used to describe the grade or how stable the peds are and how hard they are to break apart.

What do you think the grade would be for this picture?

http://soils.usda.gov/technical/manual/images/fig3-27_large.jpg

Class of Soil Structure

The size or class of the peds is described as fine, medium, or coarse.

How does structure affect water movement in soils? In soils with good structure, the pore

space that occurs between peds is relatively large and facilitates water and air movement.

Well-developed structure is very important in clayey soils.

Clayey soils with poor structure restrict water and air movement.

Degree of Water Movement

http://ohioline.osu.edu/b905/images/006.jpg

StructureStructure

Water Water MovementMovement

Altering Soil Structure

Unlike texture, structure can be altered by tillage or traffic.

Tilling soils that are too wet, or compacting soils with heavy equipment can break down the natural structural units. http://www.ny.nrcs.usda.gov/

programs/images/tractor-tillin.jpg

Bulk Density

Soil bulk density is the mass per unit bulk volume of soil that has been dried to a constant weight at 105 °C.

Bulk Density Example

If we have a soil that weighs 50 grams after being oven-dried and has a volume of 30 cm3, what will be the bulk density?

It would be 50 g/30 cm3 or 1.67 g/ cm3.

Bulk Density in the Field

Bulk density of organic soils can be very low, such as 0.5 g/cm3.

Clayey soils have higher bulk densities. Compacted clay soils have very high bulk

densities, between 1.6 and 1.8 g/cm3.

Particle Density and Porosity

Particle density is the mass per unit volume of soil particles.

Particle density is a relatively constant parameter and is sometimes assumed to be 2.65 g/cm3.

Bulk Density vs. Particle Density

http://soil.gsfc.nasa.gov/pvg/pd3.htm

Porosity

Soil porosity is the percentage of a soil that is pore space or voids.

The average soil has a porosity of about 50%, and the pores are filled with air or water depending on the moisture content.

Sands have larger pores, but less total pore space than clays.

If both bulk density and particle density are known, the total porosity can be calculated using these values.

Soil Permeability

Soil permeability is the ease with which air, water, or plant roots penetrate or pass through soil.

Soils with large pores that are connected are more permeable.

Rainwater soaks in readily and moves down through the soil profile.

Clayey soils can have greater total porosity than sand and still be less permeable than sand since the pores are small.

Soil Strength

Soil strength is defined as the amount of force required to move or rearrange soil particles.

Soil strength is an important physical property affecting plant growth and seedling emergence.

It is affected by three main factors; moisture content, soil texture, and bulk density.

Moisture Content & Soil Strength

Water content is the most important factor determining soil strength.

The lower the soil water content, or the drier the soil, the greater the soil strength.

Soils that are saturated, or waterlogged have weak soil strengths.

Moisture Content

Which soil would have the greater soil strength?

http://www.wy.blm.gov/botany/pics/cob-duckswamp-500wfo.jpg

http://www.research.noaa.gov/spotlite/archive/images/drysoil.jpg

Soil Texture & Soil Strength

Soil texture also affects soil strength. Soil strength of aggregated soils increases as clay

content increases. Poorly aggregated or single grain soils (sands, loamy

sands, sandy loams) usually have the weakest soil strengths unless they are cemented or compacted.

Individual particles of single grain (sandy) soils are easy to rearrange, but these soils are susceptible to compaction, sometimes resulting in the formation of hard pans.

Bulk Density & Soil Strength

Bulk density is the third factor affecting soil strength.

As bulk density of a given soil increases soil strength also increases.

Remember that soil is composed of solids and pores, and the greater the bulk density the greater the amount of solids, and the smaller the amount of pore space.

Soil Management Problems

Two examples of management problems caused by increasing soil strength are soil crusts and tillage pans.

A soil crust is a thin soil layer that forms at the soil surface following heavy rains.

Tillage or hard pans are high bulk density (>1.7 g/cm3) layers that occur within the Ap and E horizons.

Compaction caused by traffic of equipment, vehicles, or even foot traffic often increases soil strength to levels that restrict root penetration and plant growth.

Penetrometers

The force required to push a rod into the soil is a measure of soil strength.

Penetrometers are devices used to measure the resistance of a soil to penetration to estimate the effect of compaction on growth, and to detect layers of different soil strength.

http://agnews.tamu.edu/dailynews/stories/SOIL/photos/May0505a-lr.jpg

Questions?