Soil moisture tension

DETERMINATION OF SOIL MOISTURE Tension by Tensiometers (Irrometers)

·         A tensiometer measures soil moisture. It is an instrument designed to measure the tension or suction that plants’ roots must exert to extract water from the soil. This tension is a direct measure of the availability of water to a plant.

·         Tensiometers are most useful when a crop’s water requirements are high and when any stress due to water shortage is likely to damage crop potential.

·         Tensiometers measure how tightly water is held to the soil particles and not how much water is left in the soil. A sandy soil will reach a high tension sooner than a clay loam because sandy soils cannot supply as much water to the plant and it is used up more quickly.

·         Tensiometers do not operate in dry soil because the pores in the ceramic tip drain and air is sucked in through them breaking the vacuum seal between the soil and the gauge on top of the tensiometer.

Principle

         When the tensiometer is installed in soil, water moves into and out of the ceramic cup in relation to the soil water tension. When water moves out of the cup into the soil, it creates a vacuum in the tube which is measured in the gauge. When desired tension is reached the field is irrigated.

Parts of a tensiometer

·         Tensiometers are used to measure the soil moisture tension.

·         It consists of a 7.5 cm long  porcelain clay cup, a protective metallic tube, a vacuum gauge and a hollow metallic tube holding all parts together.

·         At the time of installation the system should be filled with water from the opening at the top and closed with rubber cork.

Installation of a tensiometer

         Materials required: A hollow pipe with a sharp edge, hammer, tensiometer and coloured stakes.

Procedure

1.      Select the spot for installation.

2.      Bore a hole by driving a hollow pipe with sharp cutting edge in the soil by hammering it to the desired depth.

3.      Insert the tensiometer into the hole.

4.      Compact the soil around the sten of the tensiometer.

5.      Make a small heap so that water will not stagnate near the tensiometer

6.      Take readings in the morning at 8.00 A.M.

7.      Record the readings frequently so that the difference between two consecutive readings is not more than 10 centibars.

8.      Plot the readings on a paper against days.

Recording the readings in a tensiometer

v  Vacuum gauges are normally calibrated in kilopascals (from 0 to 100 kPa).

v  Tensiometers operate successfully up to approximately 75 kPa. A reading of 0 kPa indicates saturated soil in which plants will suffer from lack of oxygen.

v  Optimum plant growth occurs when the soil is kept wetter than 30 to 40 kPa for coarse textured soils (sands) and 50 to 60 kPa for medium-textured and heavy-textured soils.

v  Readings in excess of 70 kPa indicate that the soil is dry enough to reduce growth .

v  Usually a tensiometer installed in moist soil can be measured 24 hours later.

v  Tensiometers need regular attention. Some air does enter through the ceramic tip and sometimes through joins, particularly at high tension.

v  The amount of air entering depends on the quality of the ceramic tip.

v  At each reading the water level in the tensiometer must be checked; when air occupies the top 20 - 40 mm of the tensiometer, water must be added.

v  Ideally tensiometers are refilled after each measurement.