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Laboratory research of soil density (using methods of cutting ring and weighing in water)

Soil density (g/cm3) is the mass of soil of natural composition in the unit of its volume including the pore space. The value of ρ, also called bulk density, depends on the mineralogical and chemical composition of the soil, as well as on the packing of particles, which, in turn, depends on the aggregate state of the soil and factors leading to its compaction or loosening (violations due to mechanical processing, the activity of burrowing fauna, the dynamics of wetting and drying, freezing and thawing, etc.).

Minimum values of ρ are typical for biogenic soils (peat, forest floor, sapropels) – <1.0 g/cm3. Slightly higher values are typical for organomineral soils (humus horizons of soils) with an organic matter content of up to 15% – 1.0-1.3 g/cm3. The average values of ρ, for example, are typical for humus-free soil horizons in the upper part of the profile (eluvial horizons)-1.3-1.5 g/cm3. High values of ρ are typical for mineral soils, overburden and soil-forming rocks - >1.5. Maximum values of ρ are found in gley and salty (saline) soils, where it can reach 2.0 g / cm3.

Why is it needed to measure the soil density?

ρ is an integral physical characteristic that underlies the calculation of a number of indicators of the physical and chemical state of the soil, such as the density of the skeleton (dry) soil and porosity. The values of ρ are widely used in soil science and agriculture: for calculating the reserves of moisture and elements of mineral nutrition of plants (and their balance) in the soil and for calculating irrigation norms. In addition, ρ is an important ecological indicator and characteristic of soil fertility. Optimal ρ values for most crops are 1.1-1.2 g/cm3 on loam and 1.2-1.3 g/cm3 on sandy soils.

Methods for determining soil density in the area of laboratory accreditation.

Определение ρ грунтов в нашей лаборатории осуществляется по стандартной методике двумя методами: методом режущего кольца (ГОСТ 5180 п. 9) и методом взвешивания в воде (ГОСТ 5180 п. 10).

In our laboratory, the determination of ρ soil index is carried out using a standard methodology using two methods: the cutting ring method (GOST 5180 p.9) and the method of weighing in water (GOST 5180 p. 10). The cutting ring method is used to determine the density of soil that is easy to cut or does not retain its shape without a ring. For sampling, a set of steel rings with an anticorrosive coating is used, one side of which is cutting. The characteristics of the cutting rings used must meet the requirements of the methodology. The determination sequence is as follows: each cutting ring is numbered, their heights are measured using the Vernier caliper and internal diameters are measured with an accuracy of 0.1 mm, then the masses of the rings are determined; based on measurements inner volumes of the rings are calculated, the results are recorded in a log; rings are provided with numbered retaining plates (upper and lower), the weight of which are also recorded in the log; on the inner side of the ring a thin layer of vaseline is pre-applied; a soil sample in its natural addition is taken with a smooth indentation of the ring into the ground with the sweep surface flush with the edges of the ring (the sampling is done in duplicate); the ends of the ring plates are covered, fixed, transported to the laboratory and weighed; ρ of the soil is counted according to the standard formula, the results are entered in the log.

The method of weighing in water is used to determine the density of clay unfrozen soils that tend to crumble and are difficult to cut. The determination algorithm is as follows: a sample of soil volume of at least 50 cm3is cut and given a rounded shape; a sample is tied with thread; wax is heated to a temperature of 60 °C, wrapped soil sample is weighed; soil samples are paraffined by sinking them down into the wax to create a water tight shell; a glass of water is set on scales and weighed; the sample tied on a thread to a tripod and lowered into the water, so that it does not touch the walls and bottom of the beaker, weighed again; convinced of the tightness of the paraffin shell, re-weighing the sample; in the case of increasing the sample mass by more than 0.02 g, a second test with a new sample is needed; the density of the soil is calculated by the standard formula, the results are entered in the log.

If necessary, our specialists can calculate for you the skeleton density of dry soil (dry soil density) ρd (g/cm3), which is defined as the mass of absolutely dry soil to the unit of its volume. Determining the soil moisture content is needed to calculate this index.

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