There are some ways to measure humidity in soils, which are different and more difficult than measuring relative humidity or humidity in air. One example is to take the sample, warm it up and measure the time it takes to loss humidity and the weight of the soil before and after the process.
A humidity sensor for soils is implemented with a Microchip dsPIC30F2012 using a piece of paper. The paper has the property of expanding the humidity along it and this effect changes the appearance of the paper, which is detected by an optical mouse. The optical mouse takes around 1500 photographs per second and compares each subsequent photograph to detect movements in any bidirectional axis.
Carlos Lozano-Espinosa, project designer, said that the embedded circuit with the Microchip dsPIC has implemented an algorithm of a PS2 communication with the optical mouse to register the movement of the humidity in the paper and also implements a real time clock to measure the time it takes the humidity to shift in the paper. These two values, space and time, are part of an equation to calculate humidity in percentage of mass of soil over mass of water.
This technique is shown in the next block diagram.
To find the equation of humidity in mycelium a ruler and a chronograph are used. A piece of paper, 3 inch long and 0.2 inch wide is immersed in the mycelium. The humidity expands along the paper for some millimeters in a certain time.
Soil Humidity Meter
Download Project Documentation (zip)