GSA (Grain Size Analyzer)

Three large groups of recognized texture classes are divided into subclasses for a total of twelve grain size classes according to the U.S.D.A (United States Department of Agriculture)

• Group of sandy classes : In this group there are the soils in which the fraction sandy is more than 70% of the total, and clay less than 15%; it includes two specific subclasses: “sandy frank” and “sandy”.
• Group of clayey classes : A soil is defined as “clayey” when it contains more than 40% clay. In turn it can be defined as “clayey”, “silty clayey” or “sandy clayey”; the last two classes may contain respectively more silt or sand than clay, characteristics that influence the textural properties in a decisive way.
• Group of free classes : It is the group that contains the greatest number of subdivisions . Ideally, a fair soil is given by the balanced mixture of sand, silt and clay; consequently, even the properties that affect the use of the land, such as heaviness or lightness, are present in balanced proportions. Most of the soils of agricultural interest are in some way “free”. The textural class called “franca” lies in an intermediate position between the three granulometric components sand, silt and clay. However, often, a weak prevalence of one of the three fractions requires the use of adjectives that better complete and define the textural classification. Thus, a free soil where sand dominates is called “sandy loam”; in the same way we can have a “loamy loam”, “loamy clay loam”, “loamy sandy loam”, “loamy loam” soil. Even the “silty” class, dominated by silt, is conventionally inserted among the group of the Frankish classes.

The particle size analysis allows us to determine this distribution and is performed with two different techniques: dry if the materials are made up of large granules (up to fractions of a millimeter) and wet if the materials are made up of fine grains ( down to fractions of microns). The operation through which the grains of an earth are separated by sieves is called screening or sieving: it is carried out by hand or with a vibrating screen / sieve shaker.

For very small granules, proceed using sedimentation analysis. The method consists in dispersing the fine parts in water and examining the different sedimentation speed of the solid particles. Stokes’ law is applied which establishes a relationship between the diameter of the suspended granules, the viscosity of the liquid, the rate of fall and the specific weight of the granules. The larger diameter particles (sand) precipitate first, while the finer ones (silt and clay) remain in suspension for a long time.

To disperse the particles before the actual determination, the medium is placed in contact with a dispersing solution of sodium hexametaphosphate (CALGON). These conditions are maintained for a few minutes; then the sediment with the addition of CALGON is placed in a mechanical stirrer kept running for about 10 minutes. The purpose of this operation is to make it act on the aggregates by transforming them into single particles. At the end of this phase, the suspension is placed in a glass cylinder with the addition of distilled water up to a total volume of 1000 cm3. Before reading the density of the fluid, the entire contents of the cylinder are shaken by closing the upper top. At the start of the test, a densimeter is inserted, calibrated at a temperature of 20 ° C in distilled water.

The readings of the density of the fluid at time intervals (measured by chronometer) are thus carried out. The result of the analysis is made more clearly visible through the creation of graphs that show:

• the diameter of the particles on the abscissa
• the percentage by weight that has a diameter indicated in ordinate

You can order the product from our website or in person at the Samaia LLC store.
For details, please E-mail us or call us at our office phone number.