Soil Particle Size Analysis Methods

There are three basic classifications of soil particle size: clay, silt and sand, from smallest to largest, respectively. These three basic classifications are subdivided into more groups due to the large range, and are often called soil separates. There are several different methods to determining how much clay, silt and sand is in a sample of soil. Methods may be mechanical, chemical, or both with many different innovations on these methods to fit unique situations being utilized by researchers. Please redirect to the Loss on Ignition test, for a method which determines the amount of organic matter in the soil.

Basic Analysis

Soil has three basic classifications, but there are further definitions within those classifications to clarify analysis. These can vary a small amount across soil classification systems, but for the purposes of soil ecology, the utilization of the USDA classification is a standardized classification, as well as the World Reference Base (WRB), an international standard for soil classification system endorsed by the International Union of Soil Sciences. The USDA classification is based off the grade scale from Wentworth (1922).

Soil particle names	Diameter Ranges (mm) USDA classification	Diameter Ranges (mm) WRB classification
Clay	less than 0.002	less than 0.002
Silt	0.002 - 0.05	0.002 - 0.063
Very fine sand	0.05 - 0.10	0.063 - 0.125
Fine sand	0.10 - 0.25	0.125 - 0.20
Medium sand	0.25 - 0.50	0.20 - 0.63
Coarse sand	0.50 - 1.00	0.63 - 1.25
Very coarse sand	1.00 - 2.00	1.25 - 2.00

There is also soil material classification which is based off the percentage of clay, silt, and sand within the sampled material, which can vary across soil classification systems. For the purposes of soil ecology, soil classifications are defined by the USDA.

See also: Soil Textures

Methods

US Standard Sieve

Sieving

There are different methods of sieving, such as dry or wet sieving, with different innovations on the basic process.

For dry, flat sieving, the soil must be dried to a constant weight before the sample is actually put through any sieves. All moisture held in the soil should be eliminated either by exposing the sample to 120˚F for at least 24 hours or air drying. Once this is accomplished, the sample is put through a series of sieves, which should be arranged with a larger size mesh on top to the smaller size mesh on the bottom. The size of these screens is dependent on which particles are to be isolated for the experiment.

Stacked Sieve: largest screen size at the top, smallest screen sieve at the bottom

There are U.S.A. standard size sieves which are often utilized for this process.

The sieve with the larger holes will screen out any larger particles present in the soil; this could include gravel, defined as a particle larger than 2mm in diameter. The subsequent sieves will act in the same way for progressively smaller soil particles. For example, silt particles ranges from 0.05mm - 0.002 mm, therefore a sieve with a corresponding screen hole size of 0.002 mm would be necessary to filter for silt particles.

Once the stack of sieves have been set up, the dry soil sample can be sifted. Sieves can be shaken mechanically or manually; this can be done horizontally, vertically, or both. There is also no standard time for shaking flat sieves, the duration being dependent on the type of soil and the determined point of stopping the shaking which differs among researchers. Regardless, by the end of the sieving, the sample should be roughly separated into gravel, sand, silt and clay. The proportions of different particle amounts to the entire soil sample can then be calculated with basic division. A simple example is, 26g sand/50g total soil = 52% sand. Dry flat soil sieving is a purely mechanical process to determine soil particle size in a sample of soil, lacking a clear standardization of method and so likely to have some level of error in measurements. It is also not effective for all types of soil all the time, especially for clay particles. This is because clay particles are made up of either three or four charged ions, which leads these charged particles to cling together. This tendency is called flocculation, defined as the aggregation or clumping together of smaller particles to form larger particles due to different physical, chemical, and biological interactions.

Hydrometer

Using a hydrometer is a second method to determine the proportion of different particles in a soil sample, orginally developed in 1927. To address the issue of the ionic bonds between clay particles, sodium hexametaphosphate is added to the water as a deflocculant. The sodium hexametaphosphate acts as a dispersing agent, interacting with the charged particles on clay particles to prevent the clay particles from coming together into a clump.

To use the hydrometer method, a solution of water mixed with sodium hexametaphosphate is prepared. The accuracy of particle percentages is dependent on a constant temperature, adequate particle dispersal, and correct timing of the density observations. Once the solution is prepared, it must be placed into an orbital shaker overnight. If it is not placed into an orbital shaker, it should be shaken with a mixing stone for about 5 minutes, the stone removed, and placed in a centrifuge for 15 minutes. The solution must then be transferred to 1000 milliliters or 1 liter graduated cylinder, where water is added to fill the graduated cylinders totally. The particles should separate based on their size and sink, with the largest flaling to the bottom and the smallest particles remaining higher. The soil hydrometer is then used to measure the relative density of the solution. The hydrometer will need to be placed into a water filled graduated cylinder to allow for proper calibration before measurements can be taken. Record the value of the hydrometer for this “blank” solution as a baseline. For accurate measurements, the hydrometer should be placed into the graduated cylinder with the soil mixture at different time frames. The number visible on the hydrometer is the value to be recorded.

For sand percent composition, the hydrometer should be put in and measurement should be read between 30 to 60 seconds, with general recommendations to mark the measurement at 40-45 seconds. Once this has been recorded, the hydrometer should be removed, and the solution is to be stirred or shaken again. For further accuracy, the hydrometer can be put in again and the measurement can be recorded at 40 seconds again to take the average of the two readings of the sample.

For silt percentage composition, the hydrometer should be read between 1 to 1.5 hours.

For clay percentage composition, the hydrometer should be read between 6 to 24 hours, though there is variation depending on the time that the hydrometer is read.

With these values the percentages of sand, silt, and clay can be calculated as follows:

%Silt = (dried soil mass - (sand hydrometer value – blank hydrometer value)/ (dried soil mass) * 100

%Clay = (clay hydrometer value – blank hydrometer value)/ (dried soil mass) * 100

%Sand = 100 – (%Clay + %Silt)

The sand and silt proportions may be similar between the dry flat sieving and hydrometer tests, however a more accurate proportion of clay particle may be obtained from a hydrometer reading due to the nature of the sodium hexametaphosphate acting as a deflocculant.