Irrigation scheduling as its name indicates is the application of the right quantity of irrigation water at the right stage, right time, and right method to reduce losses and increase the productivity of crops and water. There are different types of irrigation scheduling i.e. critical crop growth stage-based, leaf water potential based, based on sensitive crop, based on depletion of available soil moisture (DASM), climatological approach (IW/CPE ratio), tensiometry method, etc. can be helpful for achieving the target. Among these methods, farmers generally use critical crop growth stages for irrigation scheduling but for more precise and timely application of irrigation water, the tensiometry method of scheduling can be recommended to progressive farmers.

Tensiometer

Tensiometer is a device that indirectly measures soil moisture status indicating the soil matric potential that plant roots must exert to uptake the available moisture. As they are used for scheduling of irrigation, commonly called irrometer. A tensiometer consists of a long tube that is attached with a vacuum gauge in one end and a porous ceramic tip in another end. There are two types of vacuum gauge i.e. permanent/fixed type and portable/electronic type. Tensiometers mainly suitable for irrigation to horticultural crops because they are of high value and are more susceptible to moisture stress.

Working principle of the tensiometer

The ceramic tip of the tensiometer allows free movement of water inside and outside of the tube. When the soil surrounding the ceramic cup dries, water from the tube moves outward and a vacuum created inside which is the function of water loss and matric suction of soil water. This suction developed is read in the vacuum gauge. Tensiometer doesn’t work in dry soil and it is more suitable for irrigation scheduling in sandy soil. Vacuum gauge in permanent type tensiometer normally calibrated in centi-bar (cb).

• 0-5cb means the soil is saturated.
• 5-10cb means the soil is at field capacity.
• 10-25cb means ideal soil water and aeration for the majority of crops.
• 25-85cb means there is a decrease in the availability of soil water.
• 85-100cb means an excessive quantity of air has entered the tensiometer.
• 0 at dry condition means we have to re-install the tensiometer.

In portable type vacuum gauge, calibration has normally been set in kilopascal (0-100 kPa). The reading of 0 kPa and nearer to it means the soil is at saturation and plants may suffer from lack of oxygen except for some i.e. rice. Optimum plant growth may observe when the soil is at 30-40 kPa in sandy soil and 50-60 kPa in medium to heavy textured soil.

Installation and placement of tensiometer

Tensiometer in any field or orchard should be installed in such an area where there is no bias of moisture movement and the place may be considered as the reference of the total field. The optimum depth of installation is very much important and generally recommended to be placed in the half of the rooting zone i.e. porous cup may coincide with half the depth of rooting depth. Installation steps for tensiometer are given bellow;

1. Tensiometer generally installed just after irrigation or in wet soil.
2. A day before installation, the ceramic tip must be placed in saturated condition for 24 hours.
3. The Tensiometer tube must be filled with water and kept in a container of water until installed.
4. Make a hole with auger up to the required depth of installation and the endpoint should be narrower than the ceramic tip of the tensiometer so that there will be proper contact between soil and tensiometer tip for a non-erroneous result.
5. Fill the soil surrounding the tube up to the soil level.
6. Now the tensiometer is ready and can read the soil moisture potential after 24 hours.

As a thumb rule, apply 1mm of water (i.e. 10000 lit/ha) to reduce the tensiometer reading by 1cb.

Precautions to be taken

Regular attention to be given to avoid excess air entry inside the tube and water must be added when air occupies the top 20-40 mm of the tensiometer. Ideally, the tensiometer should be refilled after each measurement and after each irrigation. Sometimes gauge reading remains zero due to continuously saturated conditions or air leakage from a ceramic cup or bad soil contact with the tip or a fault gauge. If even after irrigation there is higher reading in the tensiometer, then it should be re-placed at a shallow depth within the wetted zone. Tensiometer reading should be taken in the early morning to avoid any fluctuation caused by the heating of the tensiometer.

Conclusion

Tensiometer or irrometer measures the soil-water matric potential which includes both adsorption and capillary effect. It can be used effectively for scheduling irrigation for a wide range of crops and mostly recommended for high-value, orchard crops, and crops that required frequent irrigation. Tensiometer works satisfactorily up to 0.85 bar or 85cb, hence more suitable for sandy soil type. Last but not the least, irrigation scheduling for every crop is the need of the hour because for sustaining food security with the changing climatic scenario and change in the global hydrological cycle, tensiometer may help to enhance water use efficiency and water productivity.