One common type is the capacitance probe, which measures the dielectric constant of the soil. Volumetric water content, which represents the percentage of water volume to soil volume, can then be calculated.
Another type is the tensiometer, which measures soil water tension. Water tension is a measure of how tightly water molecules are held to soil particles. The higher the tension, the drier the soil.
Soil moisture meters can provide valuable data, but it's important to understand their limitations. The accuracy of soil moisture meters can be affected by a variety of factors, including soil type, probe depth, and temperature. It is recommended to use multiple sensors and calibrate them regularly.