what is dielectric permittivity and dielectric constant?

Electromagnetic methods which measure soil water content do so via measuring the dielectric permittivity of a material. Dielectric permittivity (ε) is grounded in complex physics but in simple terms it can be described as the ability of a substance to hold an electrical charge. The dielectric constant (Ka) is the ratio of the permittivity of a substance to free space. The value of Ka in air is 1 and in water Ka is approximately 80. Soil and ice have values of Ka ranging between 3 and 7. Therefore, there is a gradient in Ka from dry to wet soil and this gradient can be measured via electromagnetic methods. A measured value of Ka can then be converted to % volumetric water content via a calibration equation.