Agriculture Reference
In-Depth Information
Box 2.1
Particle Size Analysis in the Laboratory
2-mm sieve fraction of air-dry soil. This is
soil of very low water content that has been dried in the air at 40°C. The soil
aggregates must be completely disrupted and the clay particles dispersed. Methods
used include treatment with hydrogen peroxide (H
2
O
2
) to oxidize organic matter
that binds particles together, mechanical agitation, ultrasonic vibration, and
dispersion of the clay in “Calgon” (sodium hexametaphosphate). Details are given
in specialist topics such as Klute (1986).
Once a soil suspension has been made, the particles are allowed to settle in a
special cylinder, and the density of the suspension is measured at certain times.
The coarse and fine sand particles settle out very rapidly, and they are measured by
subsequent sieving. The settling velocity
v
is directly proportional to the particle
radius
r
squared (assuming the particles are spheres), according to the equation
The analysis is carried out on the
h
Ar
2
(B2.1.1)
t
A
is a coefficient that depends on the acceleration due to gravity, the viscosity
of water (which changes with temperature), and the densities of the particle and of
water. Clay particles with an upper size limit of 2
m will fall slowly at a particular
velocity, from which their depth
h
in the suspension can be estimated after time
t.
By measuring the density (mass/volume) of the suspension at that time, the quantity
of clay can be calculated. Density is measured by placing a Bouyoucos hydrometer
in the suspension or by calculating the loss in weight of a bulb of known volume
when immersed in the suspension—the
plummet balance method
.
Constant temperature should be maintained. Two assumptions simplify the
calculation of settling velocity using equation B2.1.1:
1. The clay and silt particles are smooth spheres, even though they have irregular
flat shapes.
2. The particle density does not vary with mineralogy.
In practice, we use an average particle density of 2.65 g/cm
3
, and we speak of
the “equivalent spherical diameter” of the particle size being measured. The results
of particle size analysis are expressed as the mass of the individual fractions per 100
g of oven-dry soil (
2 mm). Oven-dry soil is soil dried to a constant weight at 105°C.
By combining the coarse and fine sand fractions, the soil may be represented by one
point on the textural triangle of figure 2.3.
•
Silt
grains cannot be detected by feel, but their presence makes the soil feel
smooth and silky, and only slightly sticky.
•
Clay
is characteristically sticky, although dry clays, especially the expanding-
lattice types (section 2.2.4.2), require much moistening and kneading
before they develop maximum stickiness.
• A high organic matter content reduces stickiness in clays and makes sandy
soils feel more silty.
• Finely divided calcium carbonate also gives a silty feeling to a soil.
Depending on the estimated proportions of sand, silt, and clay, the soil is assigned
to a textural class according to a textural triangle (fig. 2.3).
