SOIL TEXTURE AND TEXTURAL CLASS
Laboratory Exercise # 3
OBJECTIVES:
To learn and understand the significance of the following concepts related to soil texture:
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The definition of soil texture, soil separate, and textural class.
The relative size of particles defined as sand, silt, and clay.
How to determine a soil's textural class using the textural triangle when the
soil texture is known.
How to estimate a soil's textural class by the "feel" method.
How soil textures and textural classes relate to soil management and land use.
INTRODUCTION
Soil texture is one of the most important properties of a soil, and it greatly affects land use and
management. It affects the amount of water and nutrients that a soil can hold and supply to
plants. Soil physical properties such as structure, and movement of air and water through the
soil are affected by texture. The parent material, as modified by weathering and soil- forming
processes, determines a soil horizon’s texture. Texture is a relatively permanent physical
property under natural conditions, and, for most agricultural or forestry uses, it is not practical
to change soil texture. In intensely managed systems texture can be changed by adding
coarser or finer mineral material.
Soil texture is defined as the relative proportion of sand, silt and clay. The ranges of diameters
of the three separates are: sand (2.0- 0.05 mm), silt (0.05-.002 mm), and clay (<0.002 mm).
Soil textures can be determined in laboratory analyses where the amount of sand, silt, and clay
are measured quantitatively. Once the soil texture is determined, the soil textural class is
determined using the textural triangle (Figure 3.1). In the field, the soil's texture may be
determined by the "feel" method.
Soil textural classes are based on the relative proportions of the various soil separates (sand,
silt, and clay. There are 12 different soil textural classes. The percentage units (0-100%) of
sand, silt, and clay are listed along the sides of the triangle. Also notice that the relative
proportion of sand, silt, and clay always adds up to 100%. These 3 separates are the only
particles used to determine soil texture. Soil texture refers only to the mineral fraction of the
soil. Organic matter is not considered when determining texture or textural class. A precise
analysis of soil texture requires that organic matter be removed.
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Table 3.1 Diameter ranges of the soil separates (sand, silt, and clay).
Soil Fraction
Sand
Silt
Clay
Soil Separate
Very coarse sand
Coarse sand
Medium sand
Fine sand
Very fine sand
Silt
Clay
Figure 3.1 Textural triangle.
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Particle
Diameter in mm
1 to 2
0.5 to 1
0.25 to 0.5
0.10 to 0.25
0.05 to 0.10
0.002 to 0.05
< 0.002
Laboratory Activity
Materials
Soil (including both
known and unknown samples)
Textural triangle
Texture by feel flow chart
Hydrometer
Electric stirrer
Plunger
Sedimentation cylinder, 1000 ml
Metal dispersing cups
Sodium hexametaphosphate (HMP)
Wire mesh sieve with 0.053 mm openings (No.
270)
QUALITATIVE ANALYSIS (Texture by feel)
A. Particle Size Separates
Determining the texture of a soil sample in the field is a useful skill for most on-site investigations.
The goal is to estimate the relative amounts of sand, silt, and clay by considering how each feels
when rubbed between your fingers. Before examining a mixture of these particles in a particular
soil, begin by separately examining some pure sand, silt, and clay.
For each of the samples labeled sand, silt, and clay, moisten a small amount (about a tablespoon) of
material. Rub the sample firmly between your thumb and index finger. Answer the following
questions.
Which separate is sticky and plastic?
Which is gritty?
In which can you see individual particles?
Which makes a grinding noise when
rubbed?
When squeezed into a ball, which falls apart
when handled?
Name something familiar that feels like silt.
(a)
(b)
(c)
(d)
(e)
(f)
B. Texture by Feel
The typical method of estimating soil texture by feel involves only a few spoonfuls of soil and
enough water to moisten the sample.
1. If necessary, before adding water, use your fingers to break up any large aggregates. It helps to
have the soil as crushed as possible before adding water.
2. Add enough water to moisten the sample.
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3. Firmly knead the soil in your hand to uniformly mix the sample. The sample should have the
consistence of putty or cookie dough. If it is too dry, add more water; if it is too wet, add more
dry soil.
4. Continue to knead the soil in your hand, noting the grittiness or smoothness of the soil, and
make a ribbon (by squeezing the soil between your thumb and index finger), which you will use
to assess the stickiness and plasticity of the sample.
5. Note the quality or length of the ribbon, as well as the relative grittiness, smoothness, or
stickiness of the sample.
6. Use the attached flow chart (Fig. 3.2), which highlights the important differences among the
textural classes, to aid in your decision making.
7. Note the following about each sample:
Sample
Known Textural class
Ribbon quality/length
Gritty, smooth, or sticky
1
2
3
4
At the extreme, a soil with a sand textural class will not even stick together to form a ball when
squeezed in your hand. A soil that is high in sand is not sticky and does not form a good ribbon. As
the clay content increases, the length and strength of the ribbon increase. However, the only way to
develop an accurate sense of texture by feel is to practice on samples of various textures to
“calibrate” yourself through practice and experience.
After practicing on several known samples, you will “test” yourself on some unknown samples.
Sample
Ribbon quality/length
Gritty, smooth, or sticky
1
2
3
4**
**Use a sample from your own lawn or garden
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Textural class
QUANTITATIVE ANALYSIS DEMONSTRATION
C. Blank (No Soil) Sample (Completed by Instructor)
Prepare a blank, which is used to correct for the presence of the dispersing agent in the water. The
blank consists of 950 ml of distilled water and 50 ml of 1 N HMP (Sodium hexametaphosphate).
The HMP is added first to the sedimentation cylinder and then distilled water is added to the 1000
ml mark. The blank solution is thoroughly mixed for 1 minute and labeled appropriately.
D. Soil Sample Preparation (Completed by Instructor)
1. Weigh out 30 g of soil for each sample and record the weight on the data sheet (line 2).
2. Transfer the sample to a beaker or flask, and then add 10 ml of concentrated HMP solution and
100 ml of distilled water. Mix the HMP-soil slurry until the soil is fully wetted (no clumps on
the bottom). Rinse down any soil adhered to the sides of the glass, cover, and allow to set
overnight or longer.
3. Quantitatively transfer the soil slurry to a stainless steel dispersing cup. If necessary, add
distilled water to the cup until it is about one-third full. Mix using an electric stirrer for
approximately five minutes.
4. Continue with either step 5 for determination of clay content, or step 11 for determination of
sand content.
E. Sand & Clay Determination (Hydrometer Method)
Following soil sample preparation (steps 1 through 4):
5. Quantitatively transfer the soil slurry to a labeled sedimentation cylinder using as little distilled
water as possible. Add distilled water to the cylinder to bring the total volume to 1000 ml.
6. Carefully insert the plunger and mix the solution with the plunger for 30 seconds.
7. Carefully remove the plunger from the cylinder and immediately record the time as the START
TIME.
8. Immediately insert the hydrometer slowly and carefully. Do not drop it into the cylinder.
9. At exactly 40 seconds, read the value of the suspension density (in g/L) from the stem of the
hydrometer.
10. Carefully remove the hydrometer and thoroughly rinse it with distilled water. Carefully wipe the
water off the hydrometer with laboratory wipes after rinsing.
11. Repeat steps 6 through 10 until the readings are within 0.5 g/L of each other. Record this value
on your data sheet.(Record on line 4 of next page)
12. Immediately following measurement of the soil sample, measure and record the density of the
blank.(Record on line 5 of next page)
13. Shortly before seven hours, insert the hydrometer, allowing 10-20 seconds prior to the
hydrometer reading for the hydrometer to equilibrate in the suspension. Measure and record the
suspension density (in g/L). (Record on line 10 of next page)
14. Again, measure and record the density of the blank.(Record on line 11 of next page)
15. Follow the steps below to calculate the sand, silt, and clay content of the sample, and then
determine the soil texture using a textural triangle.
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Hydrometer readings for Distance Education students: 40 seconds = 20 g/l
Blank
2 g/l
7 hours
11 g/l
Blank
0 g/l
1.
Sample identification
2.
Soil weight [g]
3.
Start time
4.
Average 40-second hydrometer reading, soil [g]
5.
40-second hydrometer reading, blank
6.
Corrected reading (line 4 – line 5)
7.
Grams of silt+clay (line 6)
8.
Grams of sand (line 2- line 7)
9.
Percent sand [%] (line 8 / line 2) x 100
10. 7-hour hydrometer reading, soil [g]
11. 7-hour hydrometer reading, blank
12. Corrected reading (line 10 – line 11) [g]
13. Grams of clay (line 12)
14. Percent clay [%] (line 13/ line 2) x 100
15. Percent silt [%] (100- line 14 – line 9)
16. SOIL TEXTURAL CLASS
If you would like to try a simple method to estimate the percent sand silt and clay of soil from your
garden or lawn, click on the link below for instructions.
http://www.todayshomeowner.com/diy-soil-texture-test-for-your-yard/
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Figure 3.2 Flow chart for determining textural class.
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STUDY QUESTIONS
1. Distinguish between soil texture and soil textural class.
2. What material may be added to a soil to change it’s texture? How will organic
materials (e.g. peat, leaves) added to the soil, change a soil’s texture and textural
class.
3. Distinguish between soil texture and soil structure.
4. Sand grains consist primarily of one type of mineral, which mineral is it?
5. List the physical properties that texture may affect in a soil.
6. Why were textural classes developed?
7. List the size fractions for each of the soil separates.
8. If you wanted to change the sand content from 20% sand to 50% for1000 lbs of
soil, how many lbs of sand would have to be added?
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