How is soil bulk density determined? What is the formula for calculating soil bulk density?

Soil bulk density. Refers to the mass of dry soil per unit volume of undisturbed soil, usually expressed in grams centimeter 3; Porosity is the percentage of pores per unit volume of soil.

That is, the percentage of pores between soil solid particles. The total soil porosity includes capillary pores and non-capillary pores.

Soil bulk density reflects soil structure, air permeability, water permeability and water retention capacity, the general tillage layer soil bulk density of 1 gram cm 3, the deeper the soil layer is the larger the bulk density, up to gram cm 3, the soil bulk density is more ** clear soil structure, better air permeability and permeability. There are many methods to determine the bulk density of soil, and the ring knife method is emphasized.

1. Instruments: ring knife (volume of 100 cm3), balance (inductance gram and gram), oven, ring knife holder, sharpening knife, small shovel, aluminum box, wire saw, dryer, etc.

2. Operation steps: first select the location of the excavation soil profile in the field, then excavate the soil profile, and observe the sun. The excavated soil is placed on both sides of the pit. The depth of digging is generally 1 meter, and if only the bulk density of the soil in the tillage layer is determined, it is not necessary to dig the soil profile.

The soil profile was flattened with a soil trimming knife, and the morphological characteristics of the section were recorded, and the samples were taken in layers according to the profile level, and each layer was repeated 3 times.

Place the ring knife holder on a ring knife of known weight, and the inner wall of the ring knife is lightly coated with petroleum jelly.

Press the ring cutting edge vertically down into the soil until the ring barrel is filled with sample. If the soil layer is solid, you can use a hand hoe to beat it slowly, and the ring knife pressure should be stable and consistent.

Use a soil trimmer to cut the soil sample around the ring blade, take out the ring knife that has been installed, carefully cut off the excess soil at both ends of the ring knife, and wipe the soil outside. At the same time, the natural water content was determined by sampling with an aluminum box at the same layer sampling site.

Cap both ends of the ring knife containing the sample immediately to prevent water evaporation. It is then weighed (to the nearest gram) and recorded.

The aluminium box containing the sample was dried and weighed (to the nearest gram) to determine the soil moisture content. Alternatively, the sample can be taken directly from the ring barrel to determine the soil moisture content.

3. Result calculation: The volume of the ring knife is calculated according to the following formula

v=лr2h

Where: v - ring knife volume (cm3);

r - the inner radius of the ring knife (cm);

h - height of ring knife (cm);

Pi. Calculate the bulk density of the soil according to the following formula:

rs = where: rs - soil bulk density (g cm3);

g - wet sample weight in the ring knife (grams);

v - ring knife volume (cm3);

w - sample moisture content (%)

This method allows parallel absolute errors.

g cm3, take the arithmetic mean.

There are many ways to determine the bulk density of soil, such as wax sealing method, sand filling method, etc.

However, at present, the more widely used measurement method on the market is the instrument method (soil bulk density measuring instrument), and the method used by the soil bulk density measuring instrument is the ring knife method, which is mainly used to cut the soil in the natural state with a certain volume of steel ring knife, so that the soil is just full of the volume of the ring knife, and then weigh and calculate the dry soil weight per unit volume according to the natural moisture content of the soil, that is, the soil bulk density.

Soil bulk density measuring instrument (top).

The formula is: Soil weight = volume bulk density. Soil bulk density is determined by the number of soil pores and soil solids. The weight of any unit of soil can be calculated based on the bulk density of the soil.

Soil bulk density, also known as soil false specific gravity, is the quality of dry soil in the undisturbed soil per unit volume, undisturbed soil is the soil without compression or expansion disturbance, including the volume of air and water, generally use a small hammer to drive the ring knife into the soil, the chain knife with the soil dug out, cut off the excess soil with a knife, seal the two ends of the ring knife, bring it back to the laboratory for analysis, and use the 105 drying method to measure.

Methods for determining soil density:

1. Method selection.

Soil density is usually determined using the ring knife method. In addition, there are wax sealing method, mercury discharge method, sand filling method and ray method. Among them, the wax sealing method and the mercury discharge method mainly determine the density of some hard and brittle soils with irregular shapes; The sand filling method is more complex and time-consuming, unless it is a stony soil, and this method is generally not used for a large number of measurements; The radiographic method requires special instruments and protective equipment, and is not easy to use widely.

2. Basic principles.

Use a ring knife of a certain volume to collect the undisturbed soil with undamaged structure, fill the soil sample with it, and weigh and calculate the quality of the dried soil per unit volume. This law applies to general soils and does not apply to hard and brittle soils.

Soil bulk density. (b) = Drying soil sample quality Ring knife volume.

A ring knife (cylindrical metal quarry) of a certain volume (usually 100cm3) is inserted into the soil to collect soil samples, and the dry soil weight is obtained after drying (105 110 °C, 24h), and the dry soil quality per unit volume is further calculated according to the volume of the quarry. The calculation formula is as follows:

Soil bulk density (b) = quality of dried soil sample and volume of ring knife.

Measurement method: The method of determining the bulk density of soil has the ring knife method.

Wax sealing, mercury.

Discharge method, sand filling method and ray method, digging method, etc. The ring knife method is suitable for general soils, but not for hard and brittle soils, which are most likely to disturb the undisturbed structure of the soil during ring knife sampling.

For this reason, some supplementary test methods are proposed, but they are not all satisfactory, such as the wax sealing method and the mercury discharge method, which mainly determine the bulk density of some hard and fragile soils with irregular shapes, but the work efficiency is very low, and the test results are also extremely unstable; The mercury discharge method is rarely used because mercury vapor is harmful to human health.

The formula is: Soil weight = volume bulk density. Soil bulk density refers to the drying weight of solid soil particles per unit volume (including intergranular pores) in the natural state, and the size of soil bulk density is determined by factors such as texture, structure, and surface content.

Capacity is the basic physical property of the soil.

It is possible to calculate the porosity of the soil and the weight of the soil at a certain depth per unit area, and to estimate the amount of water and nutrients stored in the soil.

For example, the cultivated area is 667 square meters, the cultivated layer is thick, the soil bulk density is cubic centimeters (1340kg cubic meters, cubic meters), and the soil weight is: 667.

There are many methods to determine bulk density, such as ring knife method, wax sealing method, sand filling method, etc., and the ring knife method is used in this experiment.

There are many ways to determine the bulk density of soil, and the measured bulk density of soil is usually determined by the ring knife method. In addition, there are wax sealing method, mercury discharge method, sand filling method and radiation method (double radioactive source) and so on.

The ring knife method is suitable for general soils, but not for hard and brittle soils, because these soils are most likely to disturb the original structure of the soil during ring knife sampling. For this reason, some supplementary test methods are proposed, but they are not all satisfactory, such as the wax sealing method and the mercury discharge method, which mainly determine the bulk density of some hard and fragile soils with irregular shapes, but the work efficiency is very low, and the test results are also extremely unstable;

The mercury discharge method is rarely used because mercury vapor is harmful to human health. The sand filling method is complex and time-consuming, and is often used in stony soils, and is generally not used for mass determination. The radiographic method requires special instruments and protective equipment, and is not easy to use widely.

The pit digging method is suitable for soils with a large root system or gravel content, and it is difficult to use a ring knife to extract soil.

Ring knife method: a certain volume (usually 100cm3) of the ring knife (cylindrical metal soil miner) is inserted into the soil to collect soil samples, and the dry soil weight is obtained after drying (105 110 °C, 24h), and the quality of dry source soil per unit volume is further calculated according to the volume of the soil miner. The calculation formula is as follows:

Soil bulk density (b) = quality of dried soil sample and volume of ring knife.

Soil bulk density should be called dry bulk density, also known as soil false specific gravity, which refers to the ratio of a certain volume of soil (including soil particles and pores between grains) to the volume before drying after drying.