Dielectric Properties of Soil to the Moisture Meter of Resistance Type

Abstract

The most widely used method for determining soil moisture in the field is that of Bouyoucos and Mick or Colman utilizing buried plaster blocks.
These blocks are adequately sensitive to moisture changes under dry condition or at high pF, but not sufficiently sensitive under wet condition or in the neighborhood of field capacity.
For the purpose of researching the cause of decrease; sensitivity measurement of the electrical properties of plaster blocks and surrounding soil was made at a fixed frequency of 1, 000c/s.
1. The curves (A) and (B) in Fig. 5 show that the capacitance of plaster block in soil depends on the dielectric behavior of surrounding soil.
2. Relation of the apparent dielectric constant (at 1, 000c/s) and pF of soil depends not only on moisture content but on soil structure (Figs. 7, 8 and 9).
3. Identical values of C for different pF values (Fig.3) leads to identical values of impedance for different moisture contents. Therefore, impedance will not serve as a reliable indicator of soil moisture content at low pF.
4. The signal source of moisture meter is affected by the anomalous increase in the capacitance such as 0.5 to 0.1 microfarad obtained for plaster blocks in wet soil at low pF (Fig. 6).