Soil texture

Soil texture focuses on the particles that are less than two millimeters in diameter which include sand, silt, and clay.

The first classification, the International system, was first proposed by Albert Atterberg in 1905 and was based on his studies in southern Sweden.

Atterberg chose 20 μm for the upper limit of silt fraction because particles smaller than that size were not visible to the naked eye, the suspension could be coagulated by salts, capillary rise within 24 hours was most rapid in this fraction, and the pores between compacted particles were so small as to prevent the entry of root hairs.

[3] Australia adopted this system, and its equal logarithmic intervals are an attractive feature worth maintaining.

Furthermore, large sand particles can be described as coarse, intermediate as medium, and the smaller as fine.

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.

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.

[8] The texture by feel method involves taking a small sample of soil and making a ribbon.

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.

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.

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.

The hydrometer method requires the use of sodium hexametaphosphate, which acts as a dispersing agent to separate soil aggregates.

[12] 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.

The stable position during settling of particles with such shapes is with the maximum cross-sectional area being perpendicular to the direction of motion.

Therefore, with lower velocity, the calculated diameter also decreases determining an overestimation of the fine size fraction.

Laser diffraction is a measurement technique for determining the particle size distribution of samples, either dispersed in a liquid or as a dry powder.

Compared to other techniques laser diffraction is a fast and cost-effective method to measure particle size and quickly analyze soil samples.

Moreover, since the sample can be dispersed properly, there is no need to combine two different measurement techniques to obtain the full range of the particle size distribution, including the silt and clay content.

This requires precise knowledge of the complex refractive index of the particles’ material, including their absorption coefficient.

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.