Handbook for Irrigation of Wine Grapes in South Africa

Figure 8.30 contact with the surrounding soil. The part of the tensiometer protruding above the soil surface should be covered with a metal can to prevent damage by the sun over time. Installation of most instruments is difficult in soils with a high stone content (Fig. 8.30A). Therefore, it is advisable to dig a profile pit in the grapevine row. Roots should be disturbed as little as possible (Fig. 8.30B). While the tensiometer is held in place with the ceramic tip at the desired depth on the grapevine row side of the soil pit, soil is carefully packed around the tip and the tube. Once the tensiometer is secure in place, the rest of the soil and stones can be carefully filled back.

A

B

FIGURE 8.30. In stoney soils (A), profile pits must be dug for the installation of tensiometers or other probes and (B) the root system should remain as intact as is practically possible.

Maintenance: Tensiometers slowly lose water after a number of wetting and drying cycles, depending on the soil texture and level of soil water depletion. Therefore, it is recommended that tensiometers should be re-refilled in situ after irrigation or heavy rainfall (Fig. 8.31). Hold the protruding tensiometer tube with one hand while removing the filling cap to prevent the tensiometer from turning. If the tensiometer turns, contact between the ceramic cup and the soil will be disturbed. Before re- filling, push the tube of the water bottle down into the ceramic cup to make sure that air is not entrapped inside the tensiometer. When the tensiometer is full, hold the protuding tensiometer with one hand and replace the cap.

FIGURE 8.31. Re-filling Bourdon gauge tensiometers in the field using a squeeze bottle fitted with a long, thin plastic tube.

264 CHAPTER 8 – PRACTICAL IRRIGATION SCHEDULING