Texture by feel Hand analysis is a simple and effective means to rapidly assess and classify a soil's physical condition. Correctly executed, the procedure allows for rapid and frequent assessment of soil characteristics with little or no equipment. It is thus a useful tool for identifying spatial variation both within and between fields as well as identifying progressive changes and boundaries between
soil map units (
soil series). Texture by feel is a qualitative method, as it does not provide exact values of sand, silt, and clay. Although qualitative, the texture by feel flowchart can be an accurate way for a scientist or interested individual to analyze the relative proportions of sand, silt, and clay. The texture by feel method involves taking a small sample of soil and making a ribbon. A ribbon can be made by taking a ball of soil and pushing the soil between the thumb and forefinger and squeezing it upward into a ribbon. Allow the ribbon to emerge and extend over the forefinger, breaking from its own weight. Measuring the length of the ribbon can help determine the amount of clay in the sample. After making a ribbon, excessively wet a small pinch of soil in the palm of the hand and rub in with the forefinger to determine the amount of sand in the sample. Soils that have a high percentage of sand, such as sandy loam or sandy clay, have a gritty texture. The international soil classification system
World Reference Base for Soil Resources (WRB) uses an alternative method to determine texture by feel, offering another flow chart.
Sieving Sieving is a long-established but still widely used soil analysis technique. In sieving, a known weight of sample material passes through finer sieves. The amount collected on each sieve is weighted to determine the percentage weight in each size fraction. The method is used to determine the grain size distribution of soils that are greater than 75 μm in diameter, as sieving has a strong disadvantage in the lower measurement border. In fact, in case of finer fraction at high content of clay and silt (below 60 μm), the dispersion becomes challenging because of the high cohesiveness of particles, stickiness of powder to the sieve, and
electrostatic charges. Moreover, in the sieving particles pass with the smallest side through the mesh opening, which means that the plate-shaped clay and silt particles might be sieved as well. In all this generally leads to a massive underestimation of the fine fraction. In order to measure silt and clay (with a particle size below 60 μm), a second, independent sizing method (most often
hydrometer or
pipette technique) is used on the sample taken from the bottom sieve. Particle size distribution obtained from sieve analysis should be combined with the data from a
sedimentation analysis to establish a complete
particle size distribution of the sample.
Hydrometer method Sedimentation analysis (e.g. pipette method, hydrometer) is commonly used in the soil industry or in geology to classify sediments.The hydrometer method was developed in 1927 and is still widely used today. The hydrometer method of determining soil texture is a quantitative measurement providing estimates of the percent sand, clay, and silt in the soil based on
Stokes' law, which expresses the relationship between the settling velocity and particle size. According to this law the particles settle down because of the weight and gravity action. However, there are two additional forces acting in the opposite direction of particles's motion which determines the equilibrium condition at which the particle falls at a constant velocity called
terminal velocity. The hydrometer method requires the use of
sodium hexametaphosphate, which acts as a dispersing agent to separate soil aggregates. The soil is mixed with the sodium hexametaphosphate solution on an
orbital shaker overnight. The solution is transferred to one liter graduated cylinders and filled with water. The soil solution is mixed with a metal plunger to disperse the soil particles. This is the reason why the sedimentation analysis applies well when assuming that particles are spherical, have similar densities, have negligible interactions and are small enough to ensure that the fluid flow stays laminar. Deviations from Stokes' equation are to be expected in case of irregularly shaped particles, such as clay particles which are mostly platy or tubular. The stable position during settling of particles with such shapes is with the maximum
cross-sectional area being perpendicular to the direction of motion. Sedimentation analysis can be operated continuously with a high degree of accuracy and repeatability. The particle size distribution of soil containing a significant number of finer particles (silt and clay) cannot be performed by sieve analysis solely, therefore sedimentation analysis is used to determine the lower range of the particle size distribution.
Laser Diffraction Laser diffraction is a measurement technique for determining the particle size distribution of samples, either dispersed in a liquid or as a dry powder. The technique is based on light waves getting bent when encountering particles in a sample. The measured
equivalent spherical diameter is the diameter of a sphere having on the cross-sectional area the same diffraction pattern as the investigated particle. The angle of diffraction depends on the particle size, hence the pattern of diffraction depends on the relative amounts of different particle sizes present in that sample. This diffraction pattern is then detected and analyzed by means of
Mie and
Fraunhofer diffraction models. The outcome of the measurement is a
particle size distribution (PSD). The particle diameter in the laser diffraction method is determined in relation to their potential volume, which is calculated on the basis of an optical diffraction image at the edges of the particle cross-section. The volume of clay particles is the diameter of the plate’s cross-section, which is treated in the calculations as the diameter of the sphere. Therefore, their dimensions are usually overestimated in comparison to those measured via sedimentation analysis. Because these parameters are often difficult to retrieve, especially the
light absorption coefficients for various particles and soil grains,
Fraunhofer theory, which only takes into account the light diffraction phenomena at the edge of the particles, is often recommended for natural soils.
X-ray sedimentation The
X-ray sedimentation technique is a hybrid technique which combines sedimentation and
X-ray absorption. The particle size is calculated from the terminal settling velocities of particles by applying
Stokes' law. The adsorption of the X-radiation is used to determine the relative
mass concentration for each size class by applying the
Beer-Lambert-Bouguer law. ==See also==