Handbook for Irrigation of Wine Grapes in South Africa

Figure 8.28

A

B

FIGURE 8.28. Measuring soil water matric potential (A) at three depths using mercury manometer tensiometers and (B) a tensiometer fitted with a vacuum transducer for automatic recording.

Preparation and calibration: To ensure reliable measurements up to -80 kPa, tensiometers should be prepared as follows: 1. Do not touch the ceramic cup with bare hands to prevent clogging of the pores by skin oils. Always use tissue paper or a clean cloth to handle the cup. 2. Make sure the ceramic cup is tightly screwed against the O-ring in the bottom end of the tube. 3. Fill the tensiometer with tap water. If available, de-aerated water will speed up the preparation process. Use a squeeze bottle fitted with a long, thin plastic tube that fits inside the tensiometer. The plastic tube must be long enough so that it reaches the ceramic cup. Before filling, push the plastic tube down into the ceramic cup to ensure that air is not entrapped inside the tensiometer. 4. Screw the cap back firmly and hang the tensiometer outside in a sunny place. The gauge will slowly begin to rise, depending on the ambient air temperature and wind speed. As the vacuum increases air bubbles will form inside the tube, and the water level will decrease. On the first filling, the gauge will usually rise to ca. -40 to -50 kPa. 5. Place the tensiometer in a bucket containing clean water so that the lower half of the ceramic cup is covered. 6. Allow the tensiometer to absorb water until the gauge is back to zero. This step is critical, so do not refill the tensiometer before it is placed in the water. 7. Once the gauge is back to zero, refill the tensiometer and hang out to dry. 8. Repeat steps 5 to 7 until the gauge reads at least -70 kPa.

262 CHAPTER 8 – PRACTICAL IRRIGATION SCHEDULING