Electrical Resistance Blocks
Soil moisture meters and sensors may rely on a portable sensing device that is pushed into the soil or into an access tube that's placed in the soil for monitoring purposes. These devices measure some physical property that is correlated to the level of moisture in the soil. One common type of portable soil moisture meter is the electrical resistance block. This sensor works on the basis that water conducts electricity; soil with higher moisture levels will have a lower electrical resistance. Two types of commercially available electrical resistance blocks are gypsum blocks and granular matrix sensors. The blocks work when they are buried in tightly packed soil with no air pockets in a location that's representative of the field you are testing. The blocks absorb moisture from the soil and the findings may be read with a data logger or a portable, hand-held meter.
With data loggers, it is not necessary to visit every buried block to take a reading by hand. Data loggers are mounted on a post and hard wired to each of your electrical resistance blocks. At specified intervals, the data logger sends an electric current to the blocks. The resulting measurements are converted into soil moisture readings. The data logger stores the readings in memory and may offer a graphical display of the moisture levels based over several days or several weeks. This information allows crop producers to see recent trends in soil moisture levels.
Thermal Dissipation Blocks
Thermal dissipation blocks are porous ceramic blocks that contain temperature sensors and small heaters. They work on the basis that wet objects heat up slower than dry objects. Like the electrical resistance blocks, thermal dissipation blocks are buried tightly in the soil and read with hand held meters.
Tensiometers are water filled tubes that have a porous ceramic tip that's placed in the soil. The airtight containers have a vacuum gauge on the opposite end from the ceramic tip which protrudes above the ground. Tensiometers measure soil water tension and work best in coarse soils between 0 and 80 centibars. The vacuum gauge displays the water tension measurement in centibars. Tensiometers are simple to install and remove, but require regular servicing in order to fill the tube with water. This type of soil moisture meter is often used in orchards, cultivated fields and fields with annual crops.