*2.4.1 Type of soil moisture sensors: Soil tension sensors*

Soil moisture can be estimated by electrical resistance instruments. As the soil moisture content changes, the electrical properties of the soil also change. Electrical resistance sensing devices are sensitive to salts and fertilizer in the soil. A typical soil tension sensor is a solidstate electrical resistance sensing device that also measures soil matric potential (SMP). This type of sensor reads the resistance changes as the soil tension changes, which depends on the soil moisture content. A common soil tension sensor is WATERMARK, manufactured by the IRROMETER company (Riverside, CA, USA). The sensor measures from 0 to 239 kPa. The value of 0 kPa indicates that the soil reached saturation. The measurement of 239 kPa indicates that the soil is dry. Based on the tension (kPa) values, depletion in water holding capacity can be estimated. An example of depletion in water holding capacity for different soil types is described in **Table 2**. From the depletion values, irrigation recommendations and timing can be estimated to ensure the soil moisture level is at optimal condition for plant growth.
