Soil Physical Properties and Profiles

Created by:

Mark W. Bowen

Department of Geography and Urban Planning

University of Wisconsin Oshkosh

Oshkosh, WI 54901
Phone: 920-424-7114
E-mail:

Purpose:

The purpose of this laboratory experiment is to familiarize you with basic soil physical properties (color, structure, and texture) of soil samples and how these properties vary with depth in a soil profile.

Essential Learning Outcomes:

Describe soil color using a Munsell Soil Color Chart

Classify soil structure

Determine soil texture by hand texturing

Identify soil Master and subordinate horizons

Understand how soils vary with depth

Equipment supplied:

Munsell Soil Color ChartColored pencilsMud bucket

Water bottleSoil samplesSoil cores

Soil core racksTape measureWeak acid

Field Book for Describing and Sampling Soils (Version 3.0)

Instructor’s notes:

This laboratory requires you to provide the students with 5 hand samples of soil samples and two soil cores or monoliths. The lab can easily be split into two separate exercises, or the core section can be omitted if you do not have access to soil cores or monoliths. The Field Book for Describing and Sampling Soils (Version 3.0) is freely available as a PDF file at:

There is no answer key available for this exercise as all answers depend upon the soil samples provided to the students.

Background:

For this exercise we will be describing basic soil properties (color, texture, and structure) in hand samples. Soil color has little bearing on the function or use of soils, but provides information about soil properties and conditions. Color is most influenced by organic matter content, water content, and oxidation state. Also, soil color is the most obvious feature of a soil and one of the easiest properties to measure. We will determine soil color using a Munsell Color Chart. When using the chart to determine a soil color, soil is compared to a small color chip that is described by hue (red/yellowness), value (light/dark),and chroma (intensity of brightness) in that order.

Example:10 YR 5/6 chroma

Hue value

(10 yellow/red, value 5, chroma 6)

Soil structure refers to the arrangement of primary soil particles into groupings called aggregates or peds. Structure greatly influences water movement, heat transfer, aeration, and porosity in soils. There are five basic structural shapes, some of which are subdivided: spheroidal (granular or crumb), plate-like, blocky (angular or subangular), prism-like (columnar or prismatic), and structureless (single-grain or massive).

Soil texture describes the size of soil particles within the fine earth fraction, that portion excluding gravels, cobbles and boulders. Thus, soil texture is a combination of the sand, silt, and clay fractions. There are three basic soil textural classes: sandy soils, clayey soils, and loamy soils; loams are combinations of sands, silts, and clays in nearly equal portions. The basic textural classes can be subdivided and from coarsest to finest texture are: sand, loamy sand, sandy loam, loam, silt loam, sandy clay loam, clay loam, silty clay loam, sandy clay, silty clay and clay. Texture can be further subdivided into fine and coarse factions (i.e. fine sandy loam etc.). For this laboratory exercise we will classify soil texture using the “Feel” method.

Soil Taxonomy is a hierarchical system developed in the United States to group/classify soils. This system is based on a set of properties that can be objectively identified in the field and measured in the lab. Examples of properties used in Soil Taxonomy include moisture and temperature patterns throughout the year, color, texture, horizons, pH, organic matter content, etc. The combination of various properties is used to define diagnostic soil horizons and the presence or absence of diagnostic horizons is used to classify the soil. There are eight diagnostic surface horizons, termed epipedons, and nineteen diagnostic subsurface horizons. There are six classes within Soil Taxonomy: order, suborder, great group, subgroup, family, and series (from broadest to most specific).

A soil profile is a vertical section of soil exposing several layers or horizons. Profiles can be measured and sampled as exposed faces (i.e. road cuts, streams banks, etc.) or from soil cores. Soil horizons are layers or zones of soils paralleling the surface, with properties and characteristics distinct from soils above or below the horizon. The uppermost soil horizons are generally the most weathered or altered soils, while the lowest horizons are typically the least weathered and are most similar to the soil’s parent material. Soils can exhibit a range of master horizons including: O, A, E, B, C, and R. O horizons are surface organic horizons and are not always present, A horizons are surface mineral horizons and are common in all profiles, E horizons are subsurface horizons dominated by eluviation or leaching and are not very common, B horizons are subsurface horizons dominated by illuviation or accumulation and require some degree of weathering of the profile to be present, C horizons are subsurface horizons with minimal weathering and resemble the parent material, and the R horizon is bedrock or unweathered parent material found at the base of the profile. Special characteristics distinct to a master horizon may also be indicated with the designation of a lowercase letter immediately following the capital letter that indicates the master horizon (i.e., Ap indicates the “A” master horizon has been modified by plowing). These subordinate distinctions include special physical properties and/or the accumulation of material such as clays or salts. There are currently 30 possible subordinate classifications.

Exercises:

Part 1 – Soil Physical Properties (Hand samples)

Use the Munsell Color Chart to classify soil color for the following samples:

Tip: Most (not all) soils have a hue of 7.5YR or 10YR. See the example on this first page of this laboratory exercise for proper notation of Munsell color.

Use pages 2-53 & 2-54 in the Field Book for Describing and Sampling Soils Version 3.0 to determine structure type for the following samples:

Use the “feel method” to classify texture for the following samples (See Attached Handout):

Tip: You want to start with enough soil to make a ball slightly smaller than a golf ball. Do your test over one of the mud buckets to reduce the mess.

Which sample do you think has the highest organic matter content? How can you tell?

Which sample do you think has the greatest water holding capacity? Why?

Which sample do you think has the greatest aeration rate? Why?

Which sample do you think would be best for agriculture? Why?

Feel Method Flowchart

Modified from S.J. Thien. 1979. A flow diagram for teaching texture by feel analysis.Journal of Agronomic Education. 8:54-55.

Part 2 – Soil Profile Descriptions

Core:

For this core, you must identify and label the master and subordinate horizons anddetermine Munsell and soil structure for each horizon. Provide a sketch of the soil profile using colored pencils – in addition to coloring, include horizon boundaries and structure in your drawing.

Soil Profile HorizonMunsell Color and Soil Structure

Core:

For this core, you must identify and label the master and subordinate horizons and determine Munsell and soil structure for each horizon. Provide a sketch of the soil profile using colored pencils – in addition to coloring, include horizon boundaries and structure in your drawing.

Soil Profile HorizonMunsell Color and Soil Structure

Tips for identifying soil horizons:

Remember your options for master soil horizons are O,A, E, B, C, and R or a combination of two if it is a transitional horizon (e.g., AB, BC, CB). See lab introduction, lecture notes, and your textbook for descriptions of master soil horizons. Below is a list of common subhorizon designations. Not all master horizons will include a subhorizon designation.

Subhorizons you may encounter:

Ap – tillage or other disturbance of the surface layer; typically only a few inches of the top of the core, lighter color compared to underlying material, and little or no structure (appears more like loose sediment than soil)
Bt – accumulation of clays; hard zone; often slightly redder; small peds are difficult to crush between your fingers
Bc or Cc – concretions or nodules; may be small white carbonate nodules or small black to purplish manganese nodules; carbonate nodules fizz if a weak acid is applied, while manganese will leave a dark streak on a piece of paper
Bg or Cg – strong gley; horizon will have a dull bluish-greenish-gray color (chroma is typically ≤ 2)
Bk or Ck – carbonate accumulation; whitish or has white lines in former root spaces; can determine the presence of carbonate by applying a weak acid and seeing if it fizzes
Bw – incipient/weak horizon formation; if you think you have a B horizon but it is not well-developed

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