Soil Moisture Instruments

© Brooke Clarke 2012



The factors that effect soil resistance are: moisture, salinity, density and temperature.  Another factor is that when dry the soil contracts and may move away from a fixed electrode.
There are a number of aspects to measuring soil moisture.   The probe can be designed to be buried and stay there or like a spike that can be used in a portable measurement system.  The measurement can be made at a single depth or at multiple depths.  The early probes were a block of Plaster of Paris with a couple of electrodes imbedded.  The moisture in the Plaster of Paris would be the same as the surrounding soil and the resistance between the probes was proportional to the moisture content.

DC systems of measurement suffer from polarization and electrolysis.  AC systems avoid those problems.  I would think that measuring the AC impedance either at a magic frequency or over a band of frequencies would be the best method.

Other things that are similar are measuring the pH (Wiki: pH, pH Meter) or Total Dissolved Solids Wiki: TDS) of a liquid.  The electrical TDS meters depend on the dissolved solids forming ions that increase the conductivity.   So a TDS meter is closely related to soil moisture meters.  pH is measured using a specialized electrode probe that depends on free hydrogen ions.


2941174 Electrical Sensing Unit for Measuring Water in Porous Media, L.A. Richards, Jun 14 1960, 338/35; 73/73; 324/689; 338/34 - AC capacitance
2729099 Aparatus for Measuring Moisture of Soil, R.Rosenthal (Ind Instruments Inc), Jan 3 1956, 73/73; 236/44.00E; 338/34 - resistive bridge
2740032 Electric Hygrometer, G.J. Bouyoucos, Mar 27 1956, 338/34; 73/73; 324/696; 338/331 - talks about plaster of Paris molded probes but they have short life spans.
2636962 Soil Moisture Meter, G. Bouyoucos (Mich. State Board of Agriculture), Apr 28 1953, 338/34; 73/73; 73/335.05; 324/443; 324/696; 338/208; 338/328
- uses AC for resistance meas to eliminate electrolysis and polarization.
2793527 Soil Moisture Testing Apparatus, W.J. Turner Jr. & R.H. Raymond (Rayturn Machine Corp), May 28 1957,
73/73; 324/448; 324/696; 338/34; 338/35; 338/229; 338/333 - driven probe has 4 sample sections at different depths.
2834201 Apparatus for Measuring Humidity Conditions in Materials, C.E. Ohlheiser (American Inst Co), May 13 1958,
73/73; 73/29.02; 324/689; 324/696; 338/35 - grain elevators
2859318 Electrical Humidity Sensing Unit, C.E. Ohlheiser (American Inst Co), Nov 4 1958, 338/35; 73/335.05; 324/696; 338/226 -
5430384 Temperature Compensated Soil Moisture Sensor, L. Hocker (Onset Computer Corp), Jul 4 1995, 324/694; 137/78.3 -
US3882383 Jun 7, 1973 1975
US4132944 Dec 2, 1977 Jan 2, 1979 Simmonds Precision Products, Inc. Apparatus for monitoring electrical properties of a liquid
US4137937 Mar 10, 1977 Feb 6, 1979 ACF Industries, Incorporated Adapter for bottom operable tank car valve
US4197866 Sep 19, 1977 Apr 15, 1980
Soil moisture sampler and controller
US4227151 Aug 28, 1978 Oct 7, 1980 Malcom-Ellis (Liverpool) Limited Measurement and monitoring of the electrical conductivity of liquor samples
US4823087 Jan 16, 1987 Apr 18, 1989 Hideo Sugimori
Merbabu Corporation
US4993640 Jun 12, 1989 Feb 19, 1991
Fluid control system
US5040417 Nov 13, 1989 Aug 20, 1991
Electronic relative humidity/temperature measuring system
US5103179 Mar 5, 1990 Apr 7, 1992 Industrial Chemical Measurement, Inc. Water analyzer with multiple electrodes
US5148826 Sep 3, 1991 Sep 22, 1992
Moisture monitoring and control system
US5179347 Apr 10, 1992 Jan 12, 1993 Irrometer Company, Inc. Electrical sensor for sensing moisture in soils
US5207380 Feb 26, 1992 May 4, 1993
Irrigation control system


Cloud Sensors


Back to Brooke's Products for Sale, PRC68, Alphabetical Index of web pages, Home web page

[an error occurred while processing this directive] page created 9 Mar 2012.