How To Measure Soil Porosity

Soil porosity refers to the ratio of the except part of the soil occupied to the whole space. The size of porosity is related to soil type, texture, and organic substance content, which impacts soil water conservation, nutrient conversion, and plant root growth.
The research shows that deep loosening of the soil is beneficial for improving the porosity, water permeability, and air permeability of the soil, resulting in less soil erosion.

There are many methods to measure soil porosity, such as air pressure specific gravity, water absorption, and bulk density conversion.

Water absorption method

Take the sample soil with the ring knife, and place them in the water container together. Keep the water level not higher than the height of the ring knife. After soaking for 24 hours, weigh it and put it in the 105 ℃ ovens for 24 hours.

They dried and weighed it again to get the saturated moisture content. Because the specific gravity of water is 1, the volumetric saturated moisture content equals the porosity.

The bulk density conversion algorithm

porosity = (1-soil bulk density / soil particle density) * 100%. This method will take time to determine soil particle density.

The barometric-specific gravity method

It is based on the Boyle Mariotte Shen theorem. After collecting the soil sample with a standard ring knife, place it in a 105 ℃ drying oven for 24 hours, and then send them to the airbag of the barohydrometer for air extraction. The difference between the extracted gas and the atmospheric pressure is directly displayed in the glass tube, which can be converted into porosity.

The disadvantages of the above three methods:

Firstly, getting soil samples with tools such as ring knives will destroy the original state of the solid to a certain extent, especially for the deep solid.

Secondly, long the measurement cycle and not good at the real-time performance.

These will affect the accuracy of the test results.

How to improve it?

Resistivity is the physical property used to express the resistance characteristics of various substances. The unit is ω• m.

We can measure each test point’s soil resistivity before and after subsoiling by the Wenner resistivity method.

The research shows that the soil resistivity itself contains rich information reflecting soil quality and physical properties. We can build a mathematical relationship between resistivity and soil porosity. The current conduction in the soil is generated by the moisture passing through the small holes between soil particles. The larger the porosity of the soil, the smaller the resistivity;

Soils with high clay content are higher porosity than that of sandy soils. Using the quantitative relationship between resistivity and soil physical state parameters to characterize and measure a physical property is a non-destructive, multi-dimensional, and multi-scale real-time measurement method.

Soil porosity meter

The soil porosity meter is based on Boyle Marriott’s Law, consisting of an external sample chamber, a built-in pressure chamber connected with the sample chamber, and a digital barometer. Before measurement, it is necessary to calibrate the pressure chamber’s volume and sample chamber’s volume.

First, measure the pressure change before and after the connection and balance between the pressure chamber and the sample chamber, and then put the calibration block with known volume into the sample chamber to re-measure the pressure change before and after the connection and balance between the pressure chamber and the sample chamber.

After the soil sample is dried and weighed, put it into the sample chamber, open the connecting valve between the pressure chamber and the sample chamber, and measure the pressure change before and after gas balance.

Finally, through the attached excel document, we can get the soil porosity, the soil void or field water capacity when PF is equal to 1.8, the soil saturated water capacity, etc.

and we can get the soil three-phase ratio according to the following formula:

Vs＝VL＋VF＋Vsw

• VS is the volume of soil sample (equal to the volume of soil sample ring), and VL is the volume of soil void.
• VF is the volume of solids in a soil sample (excluding liquid soil moisture and soil air raid volume)
• VSW is the volume of soil moisture (which can be obtained from the weight difference before and after drying of soil sample).

Wrap up

Soil pore is the space containing water and air, which is also the space for the storage, exchange, growth, and respiration of water and solutes, roots, and microorganisms in the soil. Soil pore is closely related to soil water balance, soil respiration, water, and nutrient transport. So, the determination of Soil pore is essential.