Types of Soil Moisture Sensors

1. Volumetric Sensors

Volumetric Sensors directly measure the volume means quantity of water in the soil. Volumetric water content is the volume of liquid water per volume of soil. It is usually expressed as a percentage. For example, 25% volumetric water content (VWC) means 0.25 cubic inch of water per cubic inch of soil. These sensors are expensive and can be divided in to, TDR - Time Domain Refractometry, TDT - Time Domain Transmissiometry and FDR - Frequency Domain Refractometry sensors. These sensors are very expensive and can range anywhere from Rs. 70,000 to 3,20,000 per unit. Readings are extremely accurate and fast. Calibration of sensors as per soil type is a must to do thing, and installation is quite difficult. Mostly used in research and development activities.

All these sensors do not measure Electrical condutivity and TDS of soil water (moisture).

Capacitance Type Sensors

Capacitance type sensors are electromagnetic sensors and are very fast & accurate. They require on site calibration which is a technical job. Readings are affected by clay content,soil salitnity, temparature and bulk density. Price ranges from Rs. 70,000 to 2,40,000 depending upon the manufacturer and calibrations required.

Capacitance and FDR sensors sends electric voltage pulse which is stored by soil,water and air present in soil.Since soil water, air and soil particles have different capacity to store or reflect voltage pulse, this property is used to measure soil moisture.

TDR & FDR Type Sensors

Very accurate electromagnetic type sensors. Site specific calibration is not required, though saline soils require calibration. Its oneof the most expensive technology. Price rangee between Rs. 1,10,000 to 3,20,000 depending upon manufacturer.

TDR sensors may contain 2 or 3 probes acting as waveguides which sends defined voltage pulse. When this pulse reaches end of waveguide and reflects back. More water present in soil, more the dielectric resistance, and rapid the speed of voltage pulse.

Soil Tension Meters

Most economical and simple tool to measure soil moisture. A tension meter is a vertical, water-filled tube with a porous tip that is inserted into the soil at recommended depths; the soil draws water out of the porous tip of the sealed tube, creating a vacuum. Drier soils create a stronger vacuum since water molecules are harder to pull off soil particles. Has limitations for use on drier soils, and require to be maintained regularly. Response time is slow and do not work properly on fine textured soils where clay percentage is more.

Solid State electrical resistance Sensors

It works on basic principal of electrodes which are given specific voltage current and measure movement of electrical charge between these two electrodes. More the dry soil greater the resistance, thus lower the water present.

Solid state sensor based on Gypsum method, have electrodes placed in a gypsum tube, which has limitations for use on saline and alkaline soils, further since gypsum dissipates it needs to be replaced from time to time. In more coarse soil such as sandy soils, these sensors are slightly less accurate. Gypsum tube based sensors requires more time to get readings.

Another solid state sensor which are most commonly found do not have gypsum tubes and able to give readings instantly.