Product Manual

31
For maximum accuracy, you may
choose to perform a soil-specific
calibration rather than use either of
the internal (Standard, High Clay,
or Sand) soil calibrations coded into
the TDR 150’s firmware. In these
cases, an independent soil moisture content measurement
is required. A relation can then be developed that relates
the meter’s period reading (see Moisture Type option, p.
12) to actual volumetric water content (VWC). This is
most easily accomplished by doing a regression of one set
of data against another.
VWC data can be obtained with a device such as a neutron
probe, by measuring the weight of a saturated soil column
of known volume as it is gradually dried, or by gradually
wetting a known volume soil with the addition of known
increments of water. In most cases, however, the calibra-
tion will be done with gravimetric sampling. This proce-
dure is briefly described below.
In the field, establish a number of sites to sample. Each
site should be wetted to a different soil moisture content
by adding varying amounts of water. At each site a Field
Scout TDR reading is taken followed by the extraction of
a known volume of soil. Ideally, this would be an undis-
turbed soil core. The wet weight of this soil must be deter-
mined. If the soil cannot be weighed immediately, it
should be stored in a plastic bag to reduce evaporation.
The soil is then oven-dried (105
o
C for 48 hours is a com-
mon requirement) and weighed again. The volumetric wa-
ter content is calculated as follows:
Appendix 1
Soil-Specific
Calibration