How To Measure Soil Moisture Content

The measurement method of soil moisture

There are many methods for measuring soil moisture, such as 

• titration method,
• Karl Fischer method
• weighing method
• capacitance method
• and resistance method,
• γ Ray method, 
• microwave method,
• neutron method,
• nuclear magnetic resonance method,
• time-domain reflection method (TDR),
• soil tension method, 
• FDR frequency domain reflection,
• ground-penetrating radar method (GPR), 
• separated tracer method (PT), 
• gypsum method 
• infrared sensing method.

Here are some common techniques to measure soil moisture.

1)Time-domain reflection method(FDR):

The speed of high-frequency electromagnetic pulse in soil along the transmission line depends on the dielectric properties of the soil. In a certain electromagnetic wave frequency range (50 m ~ 10 GHz), the dielectric properties of minerals, air and water are constant. Therefore, the dielectric constant of soil mainly depends on the soil water content, so We can build the empirical equation between soil water content and soil dielectric constant, then calculate the water content of the soil.

Pros：

• Fat response time is about 10-20 seconds,
• It is suitable for mobile measurement and fixed-point monitoring.
• Simple operation
• A slight influence by low salinity

Cons:

• When the salinity increases, it will affect the measurement results due to the reflection of the pulse signal from the end of the waveguide rod weakening.
• In some soils with high organic substance and high clay mineral content, calibration is required when the bulk density is too high or too low;
• Complex and expensive.

2)FDR (frequency domain reflectometry):

The probe of FDR is also called a dielectric sensor, composed of a pair of electrodes to produce a capacitor. The soil between them acts as a dielectric, and the capacitor and oscillator produce a harmonic adjustment circuit.

FDR uses a 100MHz sinusoidal signal to reach the dielectric sensor through a specially designed transmission line. The impedance of the dielectric sensor depends on the dielectric constant of the soil matrix. FDR uses the sweep frequency to measure the resonance frequency. The resonance frequency varies with the soil water content.

Pros：

Simple, safe, fast, accurate, continuous fixed-point, automation, wide range, and less calibration.

Cons:

It is greatly affected by the pores of the electrode soild

It is greatly affected by soil salinity under low frequency (≤ 20MHz)

3)Nuclear magnetic resonance (NMR):

Based on this principle, the initial amplitude of the NMR signal emitted by different laser pulse moments is directly proportional to the free water content in the detection range.

Pros:

• It can measure groundwater resources and determine the water content of unfrozen soil in frozen soil areas.
• It can provide qualitative and quantitative information and dynamic soil water content descriptions.

Cons:

• There are uncertain factors in the relationship between NMR information and soil.
• It needs to analyze NMR information to get more accurate results carefully;
• Easy to influence by electromagnetic

4)Neutron method:

It is measured by inserting the neutron source into the soil. The neutron source continuously emits fast neutrons that go through the soil and collide with various atomic ions. The fast neutrons lose energy to slow them down.

When fast neutrons collide with hydrogen atoms, they lose the most energy and are easier to slow down. The higher the water content in the soil, the more hydrogen atoms, and the larger the density of slow neutron. The neutron instrument is to determine the water content in soil by measuring the relationship between the density of slow neutron and water molecules.

Pros:

• It is suitable for the dynamic monitoring of soil moisture in the field.
• It can locate multiple soil layers for a long time and continuously measure and record data.

Cons:

• Calibration is required,
• High equipment cost
• In some cases, a little error in measurement results
• Potential radiation hazards.

5） γ Ray method:

It is similar to the neutron instrument, and the γ ray transmission method uses the radioactive source 137Cs to emit γ Line, receive the energy of rays passing through the soil with probe γ, which can get the result by conversion with the soil moisture content.

Pros:

• fast and accurate
• It can measure and monitor the soil continuously and automatically.

Cons:

• It is affected by soil density and bulk density
• it is only suitable for the laboratory.

6)Separated tracer method (PT):

The non-separated tracer and separated tracer are introduced into the gas phase system. The separated tracer is dissolved in water, so its migration in the gas phase lags behind the non-separated tracer. There is a function between the lag factor of soil water content and the Henry constant.

Pros：

• It can measure soil moisture from a small to large scale,
• No depth limited
• it can determine the distribution of soil moisture at the field and the vertical distribution of soil moisture.

Cons:

• When it is used for large range soil moisture determination, it needs an amount of tracer, resulting in high cost.
• Only high accuracy under ideal conditions.

7)Ground penetrating radar (GPR):

when the high-frequency radar pulse reaches the interface with significantly different dielectric properties, it will reflect part of the received and amplified signal by the receiving device. The size of the reflected signal depends on the difference between the dielectric constants of the two substances and the penetration depth of the radar wave. Soil water content is related to the dielectric constant, and the penetration depth of radar pulse is significantly affected by soil water content.

Pros:

• fast, non-destructive, non-disturbing
• It is suitable for monitoring soil water content in a wide range

Cons:

not maturity

8)Weighing method:

It is also known as the drying method; take soil samples into the oven and dry them to constant weight. At this time, it takes away all the free water in the soil moisture and then weighs to get the soil moisture content. The drying method also includes the infrared method, alcohol combustion method, and oven method.

Pros

• Reliable
• low cost

Cons

• Damage soil during sampling
• It isn’t easy to take a deep sampling
• the operation is complex
• take time

Determination of soil moisture in laboratory

1) Drying method

The drying method is the most commonly used to determine soil water content in the lab. It dried the soil sample at 105 ~ 110 ℃ for 6 ~ 8h.

1. Instruments

Aluminum box; Oven: dryer; Balance (0.0001g)

Steps

1) Take the numbered aluminum box, dry it, cool it, and weigh its constant weight.

2) Weigh 10g (0.0001g) of soil sample with weighing paper and put it into the aluminum box.

3) Open the box cover, place it on the side of the aluminum box, leave a small gap, put it into the oven heated to 105 ~ 110 ℃, and bake for 8h.

4) Take out from the aluminum box, cover it and cool it in the dryer for 20min.

5) Take out the aluminum box from the dryer and weigh it (accurate to 0.0001).

Calculation

W%=(Y1-Y2)/(Y2-Y0)*100

W: water content (%)

Y0 :aluminum box weight (g)

Y1: weight of aluminum box + air dried soil (or wet soil) (g);

Y2 :aluminum box + dry soil weight (g).

2) Dry soil coefficient

The dry soil coefficient refers to the ratio of dry soil weight to air-dried soil weight (or wet soil weight). It is very convenient to use dry soil when converting air-dried soil (or wet soil) into dry soil. The dry soil coefficient can be converted from the water content.

Dry soil coefficient=1/(1+W%) or 100/(100+W%)

2) Alcohol combustion method

It uses alcohol to burn in the sample to evaporate the water to dry quickly. When alcohol burns, the flame is 2 ~ 3cm away from the soil surface, and the sample temperature is about 70 ~ 80 ℃. When the flame is off a few seconds ago, the flame will drop, the soil temperature will rise rapidly to 180 ~ 200 ℃, and then quickly drop to 85 ~ 95 ℃. It will be cooled slowly. Due to the short time in the high-temperature stage, there is little organic substance and salt loss in the sample.

Pros:

It only takes 20 minutes and is very suitable for field measurement.

Cons:

it is not suitable for soil samples with organic matter content higher than 5%)

1. Reagents and instruments

Anhydrous alcohol; Aluminum box (evaporating dish); Balance (sensitivity 0.01 g).

Steps

1) Weigh about 10g of soil sample (accurate to 0.01g) and put it into an evaporation dish or aluminum box with a known weight.

2) Add alcohol into the evaporating dish until the soil surface is immersed (about 10ml).

3) Gently tap the steaming dish to distribute the soil sample evenly. At this time, the sample is easy to be soaked in alcohol.

4) Burn alcohol and wait for it after several minutes. After the soil sample is cooled, add a small amount of alcohol (about 5ml) for the second combustion (there is no need to stir with a handle in the process). Generally, it will reach the constant weight after 2 ~ 3 times of combustion. Then weigh it. (±0.01g)

If you want to simply and quickly measure the moisture content in the soil, you can also use the soil moisture tester.

Soil moisture tester

At present, the basic principle of most moisture testers on the market is similar to TDR and FDR methods. They measure the moisture content in soil by using the relationship between electromagnetic waves or current and soil dielectric constant.

From the perspective of electromagnetism, the soil is composed of four kinds of dielectric materials: air, soil, solid material, bound water, and free water. At radio frequency and standard state (20 ℃, 1 atmospheric pressure), the dielectric constant of pure water is 80.4, the solid substance of soil is about 3 ~ 7, and the air is 1.

The salt in the water will directly affect its dielectric properties, especially for low frequency (< 30MHz). Many experiments show a nonlinear relationship between apparent soil dielectric constant and volumetric water content. When the frequency is > 1GHz, the nonlinear reason is mainly due to the soil-bound water, which is related to the specific surface of the soil. Therefore, under a particular frequency (100mhz-200mhz), we can use FDR and TDR instruments to measure water content in saline-alkali land.