Texture indicates the relative content of particles of various
sizes, such as sand, silt and clay in the soil. Texture influences
the ease with which soil can be worked, the amount of water and
air it holds, and the rate at which water can enter and move through
To find the texture of a soil sample, first separate the fine
earth* , all particles less than 2 mm, from larger particles
such as gravel and stones. Fine earth is a mixture of sand, silt
and clay. You must be sure to use only fine earth to perform
the following field tests.
6.1 Quick field tests to determine soil texture
For fish-pond construction, it is better to have a soil with a high proportion
of silt and/or clay which will hold water well. To check quickly on the
texture of the soil at different depths, here are two very simple tests
you can perform.
Take a handful of moist soil and squeeze it into a ball;
Throw the ball into the air about 50 cm and then catch it
If the ball falls apart, it is poor soil with
too much sand;
If the ball sticks together, it is probably good
soil with enough clay in it.
Take a handful of soil and wet it, so that it begins to stick
together without sticking to your hand;
Squeeze it hard, then open your hand ...
If the soil retains the shape of your hand, there
is probably enough clay in it to build a fish pond;
If the soil does not retain the shape of your hand,
there is too much sand in it.
6.2 How to find the approximate proportions of sand,
silt and clay
This is a simple test which will give you a general idea of the proportions
of sand, silt and clay present in the soil.
The bottle test
Put 5 cm of soil in a bottle and fill it with water;
Stir the water and soil well, put the bottle down, and do
not touch it for an hour. At the end of an hour, the water will
have cleared and you will see that the larger particles have
At the bottom is a layer of sand;
In the middle is a layer of silt;
On the top is a layer of clay. If the water is still not clear,
it is because some of the finest clay is still mixed with the
On the surface of the water there may be bits of organic matter
Measure the depth of the sand, silt and clay and estimate
the approximate proportion of each.
6.3 How to rate soil texture from fine to coarse
Soil texture may be rated from fine to coarse. A fine texture
indicates a high proportion of finer particles such as silt and clay.
A coarse texture indicates a high proportion of sand. More
precise definitions may be obtained from Table 4. The
simple test below will help you to rate the soil texture from coarse to
The mud-ball test
Take a handful of soil, wet it, and work it to the consistency
Continue to work it between thumb and forefinger and make
a mud ball about 3 cm in diameter;
Soil texture can be determined by the way the ball acts when
you throw it at a hard surface, such as a wall or a tree ...
If the soil is good only for splatter shots (C) when either
wet or dry, it has a coarse texture;
If there is a "shotgun" pattern (D) when dry and it
holds its shape against a medium-range target when wet, it has
a moderately coarse texture;
If the ball shatters on impact (E) when dry and clings together
when moist but does not stick to the target, it has a medium
If the ball holds its shape for long-range shots (F) when wet
and sticks to the target but is fairly easy to remove, it has
a moderately fine texture;
If the ball sticks well to the target (G) when wet and becomes
a very hard missile when dry, it has a fine texture.
6.4 Soil textural classes and field tests for their determination
A more accurate determination of soil texture
Soils may be assigned to textural classes depending on the
proportions of sand, silt and clay-size particles. These textural classes
are defined in Table 4 and they are represented
in Table 6. In the field, there are several ways by
which you can find the textural class of the fine-earth portion
of a particular soil sample.
The ball-shaking test
Take a handful of soil and wet it;
Make a ball about 3-5 cm in diameter;
Place the ball on the palm of your hand: it appears shiny;
Shake it from side to side rapidly while watching the surface
of the ball ...
If the surface of the ball becomes rapidly dull and you can
easily break the ball between your fingers, it is sand
or loamy sand;
If the surface of the ball becomes dull more slowly and you
feel some resistance when breaking the ball between your fingers,
it is silt or clay loam;
If the surface of the ball does not change and you feel resistance
when breaking the ball, it is clay or silty
The dry crushing test
Take a small sample of dry soil in your hand;
Crush it between your fingers
If there is little resistance and the sample falls into dust,
it is fine sand or fine loamy sand
or there is very little clay present;
If there is medium resistance, it is silty clay
or sandy clay;
If there is great resistance, it is clay.
The manipulative test
The manipulative test gives you a better idea of the soil texture. This
test must be performed exactly in the sequence described below because,
to be successful, each step requires progressively more silt and more
Take a handful of soil and wet it so that it begins to stick
together, but without sticking to your hand;
Roll the soil sample into a ball about 3 cm in diameter;
Put the ball down...
If it falls apart, it is sand;
If it sticks together, go on to the next step.
Roll the ball into a sausage shape, 6-7 cm long ...
If it does not remain in this form, it is loamy sand;
If it remains in this shape, go on to the next step.
Continue to roll the sausage until it reaches 15-16 cm long
If it does not remain in this shape, it is sandy loam;
If it remains in this shape, go on to the next step.
Try to bend the sausage into a half circle ...
If you cannot, it is loam;
If you can, go on to the next step.
Continue to bend the sausage to form a full circle ...
If you cannot, it is heavy loam;
If you can, with slight cracks in the sausage, it is light
If you can, with no cracks in the sausage, it is clay.
The shaking test: how to differentiate clay from silt
Both silt and clay soils have a very smooth texture. It is very important
to be able to tell the difference between these two soils because they
may behave very differently when used as construction material for dams
or dikes where the silt may not have enough plasticity. Silty soils when wet may become
very unstable, while clay is a very stable construction material. plasticity.
Silty soils when wet may become very unstable, while clay is a very stable
Take a sample of soil and wet it;
Form a patty about 8 cm in diameter and about 1.5 cm thick;
Place the patty in the palm of your hand: it appears dull;
Shake the patty from side to side while watching its surface
. . .
If its surface appears shiny, it is silt;
If its surface appears dull, it is clay.
Confirm this result by bending the patty between your fingers
. . .
If its surface becomes dull again, it is silt;
Put the patty aside and let it dry completely
If it is brittle and dust comes off when rubbing it with your
fingers, it is silt;
If it is firm and dust does not come off when rubbing it with
your fingers, it is clay.
Note: record the results of the shaking test - rapid, slow,
very slow, not at all - according to the speed with which the surface of
the patty becomes shiny when you shake it.
4 USDA textural classes of soils1
Common names of soils (General
Sandy soils (Coarse texture)
Loamy soils (Moderately coarse texture)
Loamy soils (Medium texture)
Loamy soils (Moderately fine
Sandy clay loam
Silty clay loam
Clayey soils (Fine texture)
1 Based on the USDA particle-size classification,
as defined in Table 2.
6.5 Laboratory tests for textural classes
If you need to define the textural class of your soil more accurately,
you should take disturbed soil samples to a testing laboratory for a quantitative
determination of the particle sizes. This is called a mechanical
soil analysis. The following are some of the things which may be
done in the soil laboratory:
Your soil sample will be dried;
Particles greater than 2 mm, such as gravel and stones, will be removed;
The remaining part of the sample, the fine earth, will be finely ground
to free all the separate particles;
The total weight of the fine earth will be accurately measured;
The fine earth will be passed through a series of sieves*
with mesh of different sizes, down to about 0.1 mm in diameter;
The weight of the contents of each sieve will be calculated separately
and expressed as a percent of the total initial weight of the fine earth;
The weights of the very small particles of silt and clay which have
passed through the finest sieve will be measured by sedimentation. They
will also be expressed as a percent of the total initial weight of the
The results of a mechanical soil analysis made in the laboratory may
be given to you in one of the following forms:
Note: it is important to know which system of particle-size
classification (Table 2) is being used by the soil laboratory for testing.
If it is the one used by the US Department of Agriculture (USDA)
which defines silt from 0.05 to 0.002 mm, follow the method
described. However, if the laboratory uses another system such as the
international system which defines silt from 0.02 to
0.002 mm, you should request an additional quantitative determination
of particle sizes of 0.05 to 0.02 mm in diameter (coarse silt). This will
allow you to modify the results given to you, to adjust them to the USDA
system, and to use the following textural triangle method.
Usually, a complete mechanical analysis of your soil sample is not necessary.
For your requirements, a simple particle-size analysis may be sufficient.
This gives you the percentage of soil particles with a size equal
to or larger than 0.075 mm in diameter. If the percentage is less
than 50 percent, it is a fine grained soil
(fine texture). If the percentage is more than 50 percent, it
is a coarse grained soil (coarse texture). With this information you
can then judge the soil quality as described in Sections 11.2 and
Note: 0.075 mm is the opening size of US Standard Sieve
No. 200. For engineers, this particular size represents the separation
limit between sand and silt + clay (see Table
2, line 6).
TABLE 5 Mechanical soil analysis - particle-size analyses: textural
classes and pH for selected soil samples
Sandy clay loam
Sandy clay loam
Sandy clay loam
6.6 The textural triangle method to determine the
basic textural classes
The textural triangle method is based on the USDA system of particle
size where the following classification is used:
Silt: all particles within the size range of 0.002-0.05
Send your soil sample to a soil laboratory for mechanical analysis;
When you receive the results of this analysis, find, if necessary,
the relative percentages of sand, silt and clay, as defined above, within
the total size range of 0.002-2 mm.
For each soil sample, determine its textural class using the triangular
diagram shown in Table 6; as follows:
Find the percentage of sand along the base of the triangle
and follow a line, going up toward the left;
Find the percentage of clay along the left side of the
triangle and follow to the right the horizontal line until you meet
the previous line for sand (point o). This point shows
the soil sample texture;
Check that this point corresponds to the percentage of silt
of your analysis by following a line from point 0 up to the right, until
you reach the percent silt scale on the right side of the triangle;
If the value agrees for silt, your soil sample texture is determined
by the area of the triangle in which point 0 falls, as
indicated in the example below.
6 Triangular diagram of the basic soil textural classes according
to USDA particle sizes
NOTE: The soil textural classes shown
in the red portion of the large triangle are best for fish-pond
< 0.002 mm
6.7 The particle-size frequency curve
The usual mechanical analysis provides percentages for the three
particle-size classes of sand, silt and clay, such as the one for
clay loam shown in the example.
If this is not sufficient, some soil laboratories can provide a much
more detailed analysis, with a further breakdown giving the relative amounts
of soil particles for more size classes. The results of this kind of analysis
may be given in the form of a simple table where the weight for each particle
size is given as a percentage of the total dry weight of the fine earth
of the soil sample, such as the one shown below.
= Clay loam
Particle size (mm)
Percent total dry weight
It may also be given as a particle-size frequency curve (PSF-curve),
as described and shown in the next paragraph.
Note: for very small particles (less than 0.1 mm in diameter),
soil technicians often use the measurement unit called micron(m)
to avoid too many decimals.
1 micron (m ) = 0.001 mm (or one thousandth of a millimetre)
1 mm = 1 000 m
What is a PSF-curve?
A particle-size frequency curve is plotted on a graph where the
logarithms of the particle size are shown on the
horizontal axis, with the size decreasing toward the right,
and the cumulative percentages of occurrence of the
particle size are shown on the vertical axis.
Note: generally, two scales are shown on the vertical
axis. To the left, the percentages relate to particles passing
through sieves of a particular size. Here, the percentages
increase from bottom to top. To the right, the percentages relate
to particles not passing through sieves of a particular
size. Here, the percentages increase from top to bottom.
What does a PSF-curve show?
If you look at the examples of particle-size frequency curves in Table
7, you will note the following:
The inflexion point (IP) of the curve shows you the most
frequent particle size by weight; in some cases, there may be more than
one inflexion point as, for example, if the sample (a composite sample)
contains more than one type of soil (see Table 7, curves d and e);
The more vertical the curve or part of the curve, the
more uniform the particle size; a vertical line represents a perfectly
The more inclined the curve or part of the curve, the
greater the difference between the particle sizes, the smaller the pores
between the particles, and the more compact the soil;
The total quantity of soil particles within
a particular range of particle sizes is defined as the area below
the PSF- curve which lies between these two particle sizes, as, for
example, from 0.08 mm to 0.3 mm (shaded area) (see Table 7, curve c).
To find this quantity as a percentage of the total dry weight of the
soil sample, transfer the points which correspond to 0.08 mm and 0.3
mm on the PSF-curve to one of the vertical scales and calculate the
percent difference: in this case, read on the left vertical scale, 68
percent and 75 percent. The difference is 7 percent.
TABLE 7 Typical particle-size frequency curves
Note: Table 8 shows five PSF-curves
for five types of soil, varying from gravel/sand to heavy clay. Study
each carefully to observe its relative position in the graph, its inflexion
point and its inclination.
TABLE 8 Particle-size frequency curves for selected soils showing
mechanical analysis results down to very small particles of clay
1 Gravel and sand (old alluvium), 2 Sand, 3 Silt, 4 Calcareous
clayey soil (marl), 5 Heavy clay
How do you get a PSF-curve?
Some laboratories provide a PSF-curve for soil samples and some do not.
When you receive the results of a mechanical soil analysis, you may also
get a PSF-curve. For each soil sample, you will receive a graph showing
one PSF-curve. Table 10 shows a PSF-curve
prepared by a soil laboratory for one soil sample, see example below.
If your soil laboratory does not give you a PSF-curve, you will receive
the results in the form of a table giving the frequency of occurrence
(in percent of total dry weight) for a certain number of particle sizes.
You can use this table to prepare a PSF-curve yourself. Table 9 is a blank graph that you can use
to prepare a PSF-curve. If possible, use a photocopy of Table 9 for each
curve you plot. You will then be able to use the blank graph over and
over again, to make more photocopies.
How to draw a PSF-curve
To draw a PSF-curve, proceed as follows:
Calculate the cumulative percentages of occurrence for
each given particle size, starting with the largest size;
Enter the cumulative percentages in pencil on a photocopy of the blank
graph in Table 9, using the right vertical scale;
Join these points by drawing a continuous curve: this is a PSF-curve.
Note: remember that cumulative percentages represent weights
of particles that have not passed through a particular size
sieve. Therefore, use the right vertical scale of the graph
(0-line at the top) for plotting cumulative percentages.
YOU RECEIVE THIS
YOU CALCULATE THIS
Particle size (mm)
Percent total dry weight
TABLE 9 Blank scale for drawing particle-size frequency curves
How to use a PSF-curve to obtain particle-size frequency percentages
To obtain the percentages of occurrence of certain particle sizes using
a PSF-curve, such as, for example, to find the textural class using the
textural triangle method, proceed as follows:
Using the right vertical scale (0-line at the top), read
from the given PSF-curve the cumulative percentages corresponding to
selected particle sizes, such as 0.05 mm (limit sand-silt) and 0.002
mm (limit silt-clay);
Write these readings in a two-way table which gives the cumulative
percentage for each particle size, starting with the largest;
Calculate the frequency of occurrence of each range of particle size.
Particle size (mm)
You have already been shown
how to calculate the total quantity of soil particles
(frequency of occurrence) within a particular range of particle
sizes. Now, calculate in the same way the frequencies of occurrence
for sand, silt, and clay (in that order) for the PSF-curve
in Table 10. They are as follows:
28 - 0 = 28
70 - 28 = 42
Clay less than 0.002
100 - 70 = 30
Introduce these values 28-42-30 into the textural triangle
(see Table 6); the soil is a clay loam, a
moderately fine-textured soil.
From the 0.075-mm particle-size reading, you conclude that the
sample contains 19 percent of particles larger than
TABLE 10 A typical particle-size frequency curve from a soil laboratory
Further uses of the PSF-curve: effective size and uniformity coefficient
Another important use of the PSF-curve is to express the characteristics
of the particle-size distribution of a soil by numerical values
so that the results of a great number of soil samples may be easily compared.
Engineers frequently use Hazen's method which defines two
particular values which are most suitable for sands. These
The effective size or D10 of
a soil is the diameter in millimetres of the sieve through which 10
percent (by weight) of the sample passes;
Note: this value gives an estimate of the most important
particle sizes by weight: 10 percent of the soil consists of particles
smaller than D10 , 90 percent of the soil consists of particles
larger than D10
The uniformity coefficient or U of
a soil is the ratio of the diameter (in mm) of the sieve hole through
which 60 percent (by weight) of the sample passes (D60) to
the effective size (D10) or U = D60 � D10
Note: when the PSF-curve is a vertical line (U = 1), the
particles of the soil sample are perfectly uniform in size. Usually, U
is not equal to 1 and the more difference there is, the more the particle
size varies within the soil sample.
To obtain D10 and D60 , find the points where the
PSF-curve intersects the horizontal lines which correspond on the left
vertical scale to the cumulative percentages of 10 and 60 percent
Note: the more vertical the PSF-curve (U closer to 1), the
more uniform the soil sample.
TABLE 11 Calculation of effective sizes and uniformity coefficients
from particle-size frequency curves