Methods of measuring soil moisture

Soil moisture is the water in the soil. Quick and accurate measurement of soil moisture is important for farm management. Since the middle of the last century, people have been using a variety of methods to measure soil moisture, such as the drying method, the dielectric class method, the neutron method and the negative pressure meter method, but each method has its own characteristics.

Today, we will introduce them to you.

Drying method: it dries the soil sample first and then weighs it before and after to calculate the moisture content. This is the standard method commonly used at home and abroad, and other methods are compared with this method. There are several drying methods, among which the constant temperature oven drying method is the most classic and accurate.

The main characteristics of the drying method are:

It destroys the soil and cannot be observed for a long period of time.

Each time the sample is taken from a different location, the error will be higher.

Sampling is labour intensive and not economical.

The constant temperature oven drying method takes more than 8 hours and requires a drying box and power supply, not suitable for the field. Alcohol combustion method is not suitable for fine-grained and organic soil, and the error is also large. Infrared drying method has high accuracy, but it needs specialised instruments.

Therefore, nowadays the drying method is seldom used to observe soil moisture for a long time, unless it is used for some special research or calibration of instruments.

Dielectric type method is a method to determine the moisture content by measuring the difference of dielectric constant in soil. It consists of the time-domain method and the frequency-domain method. The time-domain method is to measure the water content by the propagation speed of electromagnetic waves in the soil, and there are two methods, TDR and TDT. Frequency domain method is to measure soil water content by frequency domain measurement technique, including methods such as FDR and SWR.

Compared with the time-domain method, the frequency-domain method has an operating frequency of 20~150MHz, which makes the equipment simpler and the measurement more convenient, and the price is lower. However, the electromagnetic wave of 100MHz is greatly affected by soil temperature and salinity, so the measurement accuracy is not as high as the time domain method.

Time domain reflectometry (TDR) is the development direction for long-term monitoring of soil moisture.

The dielectric constant of water is much larger than that of other soil media, so the dielectric constant of the soil is mainly dependent on the moisture content. tdr measures the moisture content of the soil by transmitting high-frequency electromagnetic waves, which propagate through a special probe and are reflected back to the transmitting source. By analysing the changes in speed and amplitude of the electromagnetic waves, the moisture content of the soil can be calculated.