The Cheap Vegetable Gardner created this soil moisture sensor using nothing more than picture wall hanging hooks, a soda straw, hot glue, and some Plaster of Paris.
After a little research there was the classic science class method of creating a homemade sensor by simply putting two galvanized nails in the soil and measuring the resistance between them. A major problem with this solution is the soil composition can significantly vary the resistance and keeping the spacing between the nails could be troublesome.
After a little more research I came across the method that has been working well for over 50 years. This method includes taking two metal probes and inserting them into a gypsum block and measure the resistance between them. The gypsum absorbs the water and provides a decent range of resistance and moisture measurement.
Unfortunately I was fresh out of gypsum, so I looked around the garage and found a good substitute, Plaster of Paris…