Soils Lab 2 on soil texture includes a review of the idea soil composition, a detailed review of how to calculate soil texture mechanically, and finally how to calculate soil density and porosity.   Assignment activities include taking provided data and determining soil textural class and as well as density and porosity.   Calculations are reviewed in detail in the lab materials with review questions include materials from lecture materials from ‘Soil Architecture and Physical Properties’ (Chapter 4).   

Instructions

You should name your file with your last name, underscore exercise number.  For instance, a word file with my lab exercise 2 would be named ‘Pope_SoilsLab2 ‘.  

Intro Soils – Lab 2
Soil Texture, Density, and Porosity

o Lecture Materials: Soil Architecture and Physical Properties (Ch 4)

o Labs submitted without advised instructions will result in a 3 point deduction:

 Proper document name (LastName_SoilsLab2)

 Name included in document

 Legible numbering and spacing including questions with answers

 Use of spell and grammar check

o Submission Closes Sunday evening, February 5, 2016 with to Module 2.

o Labs submitted on or prior Monday, February 1, 2016 will receive feedback with the opportunity
to resubmit the lab. Do not miss out on a great opportunity to be ensure understanding of the
materials and increase your lab grade.

Lab 2 – Soil Texture, Density, and Porosity

Introduction
Soil physical properties greatly impact how soils behave. Outcomes of most agricultural as well
as engineering projects are often defined by the properties of the soil involved. Soils are made
of soil solids and pore space; the soil solids are made up mostly of minerals as well as organic
matter while the pore space is made up of air and water. Ideally, these two portions are in a
50/50 ratio (Figure 1). Soil physical properties describe the soil particles and the manner in
which they aggregate and are arranged. The following exercise will focus on soil texture, soil
density, and soil porosity.

Figure 1. Ideal soil composition (Text Figure 1.18)

Soil Texture
Soil texture is the proportion of the different sized particles in soil. Only the fine earth fraction
of sand, silt, and clay are included. There are two methods for determining texture in soils by
feel and mechanistically using particle size analysis. Neither the coarse fraction greater than
2mm in diameter nor organic matter are included in textural analysis. In the previous lab
exercise, soil texture was estimated by feel. The particle size analysis procedure via mechanical
means is accomplished using a Bouyoucos hydrometer and calculated using Stokes Law. Stokes
law establishes a relationship between particle size and sedimentation. The velocity by which a
particle fall through a liquid is proportional to the gravitational force and the square of the
effective particle diameter. In other words, ‘the bigger they are, the faster they fall’. When
the soil is dispersed, the larger, sand particles will settle or fall to the bottom of a liquid faster
than silts or clays.

When conducting this experiment in the lab, the first task it to remove the coarse fraction from
the soil sample which is generally done by sieving (2mm). Soil particles want to stay together;
the soil separates and their aggregates do not easily separate. In order to achieve separation
both mechanical and chemical intervention is needed. Sieving removed large portions of the
organic matter, but it still is a significant agent in the binding of soil particles together, so
hydrogen peroxide is also added to oxidize or destroy the remaining organic matter. A
dispersing chemical agent, commonly hexametaphosphate, is also added in addition to water to
create a soil solution and then and stirred in mixer for 4 minutes. The dispersing agent causes
an exchange of sodium with polyvanet cations like calcium and magnesium in the clays causing
them to disperse. Depending on the need for accuracy, the soils can be left to sit overnight to
allow for complete dispersal or moved onto mechanical mixing. After approximately five
minutes on a mixer, the soil solution is placed in a settling column.

The soil particles will settle based on Stoke’s Law.

Equation 1: V = d2 g (Ds – Df)
18 ŋ

Where: V = velocity (cm/s)
d = diameter of the particles (cm)
g = gravity (acceleration 980 m/s2)

Ds = density of the soil
Df = density of the liquid
ŋ = viscosity of the liquid (g/cm s)

To simplify the equation, several assumptions are made. The particles are assumed to be
spherical with particle densities of 2.65 g/cm3. Also, the density and viscosity of the liquid are
assumed to be constant, at a given temperature which simplifies Stoke’s Law to:

Equation 2: V = k*d2

Where: V = velocity (cm/s)
k = constant
d = diameter of the particles (cm)

A hydrometer can be used to measure the amount of particles in suspension. Hydrometers are
generally used to measure density or specific gravity and can be calibrated for many different
functions such measuring the density of milk and its constituents or even alcoholic content in
brewing activities. Bouyoucos developed a hydrometer calibrated in grams per liter (g/L) that
allows direct measurement of the concentration of mineral particles in a suspension.
Bouyoucos also established that sand-sized particles (2.0 to 0.05 mm) settle out of suspension
in 40 seconds while silt-sized particles (0.5 to 0.002 mm) require approximately 2 hours to
settle out of suspension. After 2 hours, it is assumed that only clay-sized particles (<0.002 mm) remain in suspension. After the soil sedimentation column has been constructed, the entire column is gently shaken to suspend all of the soil particles. After 40 seconds, the hydrometer is read. The sand in the column has settled to the bottom leaving silt and clays still in suspension. Then the column is allowed to continue to settle for 2 hours and a second reading is collected. At this point, the silt has also settled to the bottom leaving only clay in suspension. Percentages of sand, silt, and clay can then be determined. Temperature also influences the density and viscosity of water. If the temperature deviates from 20°C (68°F), the hydrometer reading must be corrected. For every degree over or under 20°C, 0.36 g/L should be added or subtracted from the hydrometer reading. If the temperature is above 20°C the value will increase, and if it is below 20°C the value will decrease. (Note these values are in the metric Celsius temperature scale not the traditional Fahrenheit.) Equation 3:

Corrected reading (g/L) = hydrometer reading + 0.36 (Recorded Temperature – 20°C)
(Note: For this exercise, round to the nearest tenth or one decimal place)

The corrected readings can then be used with the following equations to calculate the
percentage of sand, silt, and clay in a sample. The textural triangle can then be utilized to
identify the textural class of the soil.

Equation 4: % Sand + % Silt + %Clay = 100%

Equation 5: % Silt + % Clay

(Corrected 40 second hydrometer reading/Mass of Dry Soil) * 100

Equation 6: %Clay
(Corrected 2 hour hydrometer reading/Mass of Dry Soil) *100

Equation 7: % Sand
100 – (% silt + % clay)

Equation 8: %Silt
100 – (%sand + %clay)

Next, using the percetages of sand, silt, and clay, the textural triangle can be utilized to classify
the soils into a textural class. First, find the percentage clay on the left side of the triangle, then
from the bottom, locate the intersection of the percentage clay, and finally out from there on
the right side of the triangle the percentage silt. The textural class is the interesection of the
three percetages (Figure 2, Text Figure 4.6).

Figure 2: Soil Textural Triangle (Courtesy of NRCS)

Note: For this exercise, soil weight, hydrometer readings and temperatures will be provided to
calculate percentage of sand, silt, and clay, followed by determination of the soil textural class
using the soil textural triangle.

Soil Density and Porosity
Soils are a mixture of solids, liquids, and gases. The liquids in soil solution and soil gases for air
exchange are located in between the soil solid particles. The density and porosity of soils
greatly effects the ability of that soil to support a range of activities from plant growth to
engineering.

Density generally is defined as mass per unit of volume. There are two types of soil density
measurements: particle density and bulk density. Particle density (Dp) is the mass per unit
volume of soil solids not including pore space. Most of the major soil minerals are quarts,
feldspars, micas, and silicates, so the range of particle densities seen in most mineral soils
ranges only from 2.6 to 2.75 Mg/m3. Soil organic matter as well as organic soils are much less
dense with ranges in the 0.9 to 1.4 Mg/m3 range. Bulk density (Db) includes the pore space and
is the mass per unit volume of dry soil representing the density of the soil as a whole. The
relationship between bulk density and particle density are illustrated in Figure 3 (Figure 4.32 in
the text).

Figure 3. Relationship between bulk density and particle density (Text Figure 4.32).

Soil porosity is directly related to density. Porosity represents the ‘other half’ of the ideal soil
composition from the soil mineral portion and is the portion of soil containing air and water.

Bulk density and particle density predict total porosity. Pore size determines the role they play
in soils. Macropores are the largest in size with effective diameters greater than 0.08 mm and
are responsible for mass flow of water and air through the soil profile. Micropores are smaller
and are responsible for water and nutrient retention thru capillary action. Interconnectivity of
the soil pore network is important in how water, gases, and nutrients move through the soil
profile.

Soil bulk density varies with soil depth, soil texture, and structure. The surface horizons where
higher organic matter occur tend to have lower bulk densities than subsurface horizons where
organic matter is decreased and there is there is less aggregation and root penetration. Soil
texture plays a significant role in bulk density as well. Finer textured soils like clays, silts, and
loams tend to generally have lower bulk densities than sands. Sands also tend to have a greater
abundance of macropores while finer textured soils with have more micropores. In soils with
strong structure, macropores occurring between peds allow for greater water infiltration rates.
Bulk densities for mineral soils typically range from 1.0 to 1.8 Mg/m3 (Text Figure 4.34).

Managing bulk density is an important factor in soil productivity from an agricultural
standpoint. How soils are managed can alter these properties and either decrease or increase
their long term effectiveness. Two management operations that negatively impact bulk density
as well as overall soil structure are tillage and compaction. Generally, long term tillage leads to
higher bulk densities, lower overall pore space, and less organic matter over a wide range of
soil textures (See text example Table 4.5). Water infiltration rates are decreased, soil rooting
zones are not easily accessible and gas exchange is decreased. Compaction is also a concern as
continual passes with heavy equipment decreases pore size, collapses established soil
aggregates and overall causes a loss of useful soil structure. Other factors that can increase
bulk density include limiting crop rotations with varying rooting depths and structures,
incorporating or removing crop residues, and overgrazing livestock which leads to trails and
loafing areas where compaction can occur.

Bulk density and porosity in soils can be measured by using a double-cylinder drop hammer
sampler which contains and inner cylinder of known volume to remove an undisturbed soil
sample, then determining the dry weight of the sample (measuring a volume of moist soil then
drying it in an oven overnight to determine the percentage moisture) and using the equations
below to determine bulk density, particle density, and soil porosity. Bulk density is the weight
of total soil per unit of volume. Soil particle density can be determined by placing a known
amount of soil into a volume of water and determining the volume of water it displaces. Water
is removed from the sample with drying and air will leave the soil pores when placed in water,
the volume of the water displace can be used to determine the particle density.

Note: For this exercise, the cylinder dimensions will be provided to calculate the total volume
of soil collected, oven dry soil weights, and volume of water displaced to be used for the
determination of bulk density, particle density, and pore space.

Equation 9: Volume of Cylinder (cm3) = π r2 * height of the cylinder

Where = r is the radius of the cylinder
Height = height or depth of the sampling ring

Equation 10: Db (Bulk Density, g/cm3) = Weight of oven dry soil (g)
Volume (cm3)

Equation 11: Dp (Particle Density, g/cm3) = Weight of oven dry soil (g)
Volume of water displaced (mL or cm3)

Equation 12: % Pore Space = 100% – (Db/Dp) *100)

Where Db = Calculated Bulk Density

Dp = Calculated where possible or if not, assumed particle density of 2.65
g/cm3

Recommended reference from NRCS:
http://www.nrcs.usda.gov/Internet/FSE_DOCUMENTS/nrcs142p2_053256

(Lab End – Assignment Questions on Next Page)

http://www.nrcs.usda.gov/Internet/FSE_DOCUMENTS/nrcs142p2_053256

Intro Soils – Lab 2 Assignment Questions
Soil Texture, Density, and Porosity

o Utilize lab as well as lecture materials: Soil Architecture and Physical Properties (Ch 4)
o Note for this and future assignments: For full credit, always show your work. These are

relatively simple equations so “/” for divide or “*” for multiply is just fine. As long as I
can recreate how you came to your answers you will get full credit, if correct. Also, if
you use a source or reference other than your text or this lab material, it should be
cited. It does not have to be in any specific format, just be sure to give the person or
group the proper credit.

o Read the lab carefully, the information you need to complete the calculations below is
readily available.

o Submission closes with Module 1, February 7, 2016

1.) Soil Texture: For each of the following soils, calculate the percentage of sand, silt, and
clay and then classify the soil into a textural class using the soil textural triangle. A total
of 50 grams of dry soil was used to conduct the experiment, and the temperature for all
of the hydrometer readings was 72°F. (8 points total)

Hint: First, utilize equation 3 to correct each reading for temperature, then utilize
equations 3 thru 12 to determine percent sand, silt, and clay, finally utilize the textural
triangle to determine soil textural class.

a.) Soil A: 40 second hydrometer reading: 35 g/L

2 hour hydrometer reading: 15 g/L

Percent Clay:
Percent Sand:
Percent Silt:

Soil Textural Class:

b.) Soil B: 40 second hydrometer reading: 10 g/L
2 hour hydrometer reading: 2.5 g/L

Percent Clay:
Percent Sand:
Percent Silt:

Soil Textural Class:

2.) Soil Bulk Density and Porosity
For each of the following soils calculate the bulk density, particle density, and porosity.
Each of the soil samples was collected using a standard double-cylinder drop hammer
sampler which has a height of 6 cm and a diameter of 5 cm. (6 points total)

Hint: First utilize equation 9 to determine the volume of the cylinder, then utilize
equation 10 to determine bulk density, finally utilize equations 11 and 12 to calculate
particle density and total porosity respectively.

a.) Soil C: The soil has an oven dry weight of 152 g. When that core was ground and

placed in 100ml of water, the final volume of was 157 ml.

Bulk Density:
Particle Density:

% Total Porosity:

b.) Soil D: The soil has an oven dry weight of 188 g. When that core was ground and
placed in 100ml of water, the final volume of was 170 ml.

Bulk Density:
Particle Density:
% Total Porosity:

3.) In your own words, describe how Stoke’s Law is used to estimate particle size and
subsequently soil texture. Include in your discussion why there are two separate
hydrometer readings taken and which particles are being measured at each step. (3
points)

4.) Practically speaking discuss some of the soil physical properties that might be different
between soils A and B based on just their soil textural class. (Think soil structure and
generalized properties. (2 points)

5.) Identify the soil textural class for the following using the textural triangle. (3 points)

a. Clay 40%, Sand 50%, Silt 10%
b. Clay 20%, Sand 40%, Silt 40%
c. Clay 10%, Sand 25%, Silt 70%

6.) Practically speaking discuss some of physical differences that one might find in Soil C

and Soil D based on the density and porosity values you calculated. What might some
potential issues be with Soil D based on those same values? (2 points)

7.) How are the density properties and porosity in soils related? (2 points)

8.) Explain why surface soil horizons might have higher bulk densities than subsurface soils.

(2 points)

9.) Would one expect to find a greater preponderance of macropores in a sandy loam or a
silty clay and why? (3 points)

10.) The idea of a ‘heavy soil’ is based on textural properties of a clay soil and its ease
or lack thereof of tilling while sandy soils are considered ‘light’ based on that same
principle. How does this idea change when viewed from the standpoint of bulk density?
( points)

11.) How might continuous cropping with a single crop (e.g., continuous corn or
cotton vs a corn, wheat, bean rotation) effect bulk density? (3 points)

12.) In your own words, give a brief description of the data shown in Table 4.5 of the

text and discuss how it brings together the ideas of soil texture, soil management, bulk
density, and porosity. (3 points)