In this paper, an unilateral sensitive capacitive soil moisture
sensor was presented which has advantages of high accuracy,
simple calibration process (two-step calibration method) and
noninvasive test. The ration of plate space and plate width
(a/b=0.8∼1) of probe was proposed more appropriately
according to the change of probe sensitivity, and sensitive
depth with a/b respectively. Laboratory tests were consisted
of 4 selected grown media with various known amounts
of water content, compacting to a particular bulk density,
and measuring the resulting voltages. Results showed that
(1) β parameter model appropriately described the relationship
between the output signal and the soil volumetric water
content, the minimum correlation coefficient R2 = 0.9911 in
the soil, peat, and vinegar residue. (2) The sensor accuracy
reached the ‘fair’ rank even in perlite which swelling character
leaded to poor correlation coefficient R2 = 0.9554. (3) The
two-step calibration β method could be considered as a quick,
easy, and robust method as validation analysis illustrated that
the maximum error was less than 0.025 units excepted in
perlite. (4) The error caused by the change of bulk density was
less than 0.025 units in peat and vinegar residue. Therefore,
this sensor and its two-step calibration β method have a broad
practical application in agricultural production.