It is a rapid

simple and reliable method to measure soil moisture content using dielectric property of soil.Based on the principle of frequency domain reflectometry（FDR）

the soil dielectric constant can be tested by the travel frequency of electromagnetic waves in soil

and the soil volumetric water content will be obtained.A set of an automatic irrigation system is designed based on FDR.The theory of FDR is briefly introduced

the hardware and software components of the automatic irrigation system are explained.The soil water content information measured by the soil moisture sensor converts to 0

5 V voltage signal by signal processing

after analog to digital（A/D） conversion

sent to single chip AT89S52

control on-off time of solenoid valve ac

cording to output value to realize automatic irrigation and water-saving irrigation.Experiment shows that this system is stable

accurate and sensitive

satisfies the requirements of automatic irrigation.