Handbook for Irrigation of Wine Grapes in South Africa

Chapter 8

9. Place the tensiometer in water until gauge reads zero. If the needle is slightly below or above zero, it can be adjusted using a small screw driver while the tensiometer is still standing in the water. 10. Place the ceramic cup in a small plastic bag containing 10 m  of water and secure the bag around the tube using an elastic band to prevent it from drying when transported to the field for installation. 11. The water in the tensiometer exerts a pressure in the ceramic cup, depending on the height of the water column and height above sea level. In order to obtain accurate values of soil water matric potential, a correction (kPa) must be calculated as follows: Correction ≈ -(h + d) ÷ 1000 x 101.3 Eq. 8.7 where h is the height (cm) between the soil surface and the gauge and d is the depth (cm) to the ceramic cup (Fig. 8.29). It must be noted that the correction is negative and needs to be subtracted from the tensiometer reading. In practice, it means that readings must be reduced by approximately 3 kPa for every 30 cm of water column. For example, if a 60 cm tensiometer reads 40 kPa, the actual soil water matric potential is ca. 34 kPa (40 - 6 kPa). Figure 8.29

h (cm)

Surface

d (cm)

FIGURE 8.29. Dimensions needed for the correction of tensiometer readings where h is the height between the soil surface and the gauge and d is the depth to the ceramic cup.

Figure 8.30 Installation: A 20 mm diameter metal tube can be used to make holes for installation of tensiometers in the soil. The tube s d iven into the soil to the desired depth, and the tensiometer is carefully inserted. The tensiometer tube should fit snug into the holes. If the tensiometers are forced into the holes, the ceramic cups might break. It is advisable to mark the depth on the tensiometer tube before insertion to make sure that it is installed at the correct depth. If the tube fits loosely in the hole, soil slurry can be washed in with water to ensure that the ceramic cup makes good

IRRIGATION OF WINE GRAPES 263