Author: Mr. Guy Sela, CEO of SMART! Fertilizer Management software and an international expert in plant nutrition and irrigation.
The tensiometer should be placed properly in so that the readings will correctly represent the soil moisture. Correct readings will help you reach correct conclusions for irrigation scheduling.
At each monitoring station two tensiometers should be placed, at two different depths- one in the center of the active root system, and one below rooting depth. For most vegetable crops, for example, this means one tensiometer at a depth of 20-30cm and one at 40-60cm.
It is important that the ceramic cup of the tensiometer comes in good contact with the surrounding soil. Drill a hole in the soil, in the diameter of the tensiometer tube and at the appropriate depth. The hole should be drenched with water and the tensiometer should be then inserted into the hole. Put soil around the tensiometer and gently compress it to its natural density, but not tighter, or this will interfere with the readings.
The shallow tensiometer is used for determining when irrigation is needed and the deeper tensiometer is used for adjusting the amount of water to apply.
When the reading of the shallow tensiometer reaches a certain value, irrigation is needed. One way to determine this value is to use the soil water retention curve. Another way is as follows: record the readings of the shallow tensiometer on a daily basis, at the same time of day. At first, the readings will slowly go up, until the day in which the reading will increase sharply. The value from the day prior to this sharp increase should be used for irrigation scheduling.
Each time the tensiometer reading reaches this value, irrigation should be applied. The interval between irrigations will change according the water consumption of your crop.
The deep tensiometer indicates whether the irrigation water reached below the rooting zone. If the irrigation water reached the deeper tensiometer, its reading will go down to zero, but if the irrigation water amount was too low, the deeper tensiometer will show a higher reading than the shallow tensiometer. In this case, there is no leaching of salts below the rooting zone and salts might accumulate and damage the crop.
If the active root zone was thoroughly wet, both tensiometers will give a near zero reading soon after irrigation, and as the soil dries up, the readings will gradually increase.