Soil moisture measurement method

Soil moisture is the moisture contained in the soil. It is the main source of crop water, the main component of the soil, and also an important part of the water cycle. Soil water can be divided into adsorbed water, capillary water, and gravity water. When water in soil can be absorbed by plants, it is called effective water; when it is not absorbed by plants, it is called ineffective water. The effective water is a value between the field water holding capacity and the wilting coefficient. See the table below for details.

Surface soil texture and maximum content of available water

Now that we know the concept of soil moisture, let's take a look at soil moisture determination methods. Common soil moisture determination methods are: titration, Karl Fischer method, weighing method, capacitance method, resistance method, γ-ray method, microwave method, neutron method, nuclear magnetic resonance method, time domain reflectometry (TDR), soil tension Method, soil moisture meter method, soil moisture sensor method, gypsum method and infrared remote sensing method.

The method for determining soil moisture in the laboratory is described below. Although the process is a bit complicated, it is a good method for determining soil moisture.

The specific steps for measuring soil moisture are as follows:

1, the scope of application

This standard is used to determine the moisture content of all types of soil except gypsum soil and organic soil (a soil containing more than 20% organic matter).

2, the principle of measurement

The weight loss of a soil sample when baked at a constant weight of 105±2°C is the quality of the moisture content of the soil sample.

3, instruments, equipment

Soil drill; soil sieve: aperture 1mm; aluminum box: small diameter of about 40mm, height of about 20mm; large diameter of about 55mm, height of about 28mm; Analytical balance: 0.001g and 0.01g; small electric oven; drying Device: Within the color change silica gel or anhydrous calcium chloride.

4, sample selection and preparation

Air-dried soil sample: Select a representative sample of air-dried soil, crushed, passed through a 1mm sieve, and mixed until ready for use.

Fresh soil samples: Take a representative fresh soil sample with a soil drill in the field, scrape off the upper floating soil in the soil drill, and add about 20 g of soil to the required depth in the middle of the soil driller. Crush and quickly load the known accurate mass. Inside a large aluminum box, tightly cover, put into a wooden box or other container, bring it back to the room, wipe the outside of the aluminum box clean, immediately weigh, determine the moisture as soon as possible.

5, the measurement step

Determination of moisture in dried soil sample: Take a small aluminum box and bake it in a thermostatic oven at 105°C for about 2 hours. Transfer it to a desiccator and cool it to room temperature. Weigh it to the nearest 0.001g. The dried soil sample was well-mixed with a corner scoop, and about 5 g was dipped, evenly tiled in an aluminum box, covered, and weighed to the nearest 0.001 g. The aluminum lid was peeled off, placed under the box, and baked in an oven preheated to 105±2°C for 6 hours. Take out, cover it, move it into a desiccator and cool to room temperature (approximately 20 minutes). Weigh it immediately. The determination of moisture in air-dried soil samples should be performed in two parallel assays.

Determination of fresh soil moisture: A large aluminum box containing fresh soil samples was weighed on an analytical balance to the nearest 0.01 g. Uncover the lid, place it under the box, and bake it in a baking oven that has been preheated to 105±2°C for 12 hours. Remove, cover, cool to room temperature in a desiccator (approximately 30 minutes) and immediately weigh. Determination of fresh soil moisture should be measured in triplicate.

6. Calculation of measurement results

Calculation formula

Moisture (analytical basis), %=[(m1-m2)/(m1-m0)]×100..............................(1)

Moisture (dry basis), %=[(m1-m2)/(m2-m0)]×100....................................(2)

In the formula: m0 is the quality of the dried aluminum box, g; m1 is the quality of the aluminum box and soil sample before drying, g; m2 is the quality of the aluminum box and soil sample after drying, g.

The results of the parallel determination are expressed as an arithmetic mean, one decimal place. The difference between the parallel determination results is that the dry soil sample with moisture less than 5% should not exceed 0.2%, the moist soil sample with 5-25% moisture content should not exceed 0.3%, and the large grain (particle size about 10mm) with heavy water content greater than 15% should be thick and moist soil. The sample must not exceed 0.7% (equivalent to a relative difference of no more than 5%).