As with any consumer walking into a local supermarket, irrigation professionals today face an overwhelming number of choices when it comes to the variety of sensors used for making better irrigation decisions. All three types of sensors continue to be extensively used today. Gypsum blocks work on the simple principle of electrical resistance. As the soil dries, resistance increases. Using a common AC ohmmeter, many sensors can be read manually by moving through the field, or they can be multiplexed and read on a continual basis.
Gypsum blocks are essentially the least expensive method of soil moisture measurement.
However, they respond slowly to changes in soil water status, potentially suffer from hysteresis and have a rather narrow range of soil tension measurement. They are also known to breakdown due to soil chemistry such that fertilizers could dissolve around the block and accelerate deterioration. Tensiometers have been around about as long as gypsum blocks. Their advantage is simplicity. The ceramic cup inserted into the soil establishes an equilibrium with the soil water tension.