Handbook for Irrigation of Wine Grapes in South Africa

Installation: Holes for installation can be made using a tubular auger similar to those used for taking gravimetric samples. To obtain accurate soil water content measurements, it is essential that DFM ® probes make good contact with the surrounding soil. A soil slurry can be poured down the hole to ensure contact with the soil. Since soil contact is so critical, it is recommended that the suppliers or other experts carry out the installation of DFM ® probes. 8.4.1.7 TENSIOMETERS Soil water matric potential, or the suction by which water is bound by capillary forces in the soil pores, is commonly measured to obtain an indication of soil water status. Tensiometers are used to measure soil water matric potential. They basically consist of a porous ceramic cup connected to a vacuum gauge by means of a clear, ridged plastic or Perspex ® tube (Fig. 8.27). The tube has a small inner diameter to reduce the volume of water inside the tensiometer, and is fitted with a filling cap. The gauge is either mounted on the side of the tube, or in a rubber grommet at the top end of the tube where it also serves as a filling cap. The depth at which measurements are taken depends on the length of the tube, which is usually 30, 60 or 90 cm. Before installation, a tensiometer is filled with water, and the pores saturated as described below. As the soil dries out, water is held in the smaller pores by means of capillary forces. These suction forces cause water to flow from the tensiometer through the pores in the ceramic into the soil around the cup (Fig. 8.27). As water flows from the tensiometer, a vacuum develops within the tensiometer and is registered by the vacuum gauge. The vacuum, or suction, increases as the soil dries out. Following irrigation or rainfall, water flows back into the tensiometer via the pores in the ceramic cup (Fig. 8.27). The vacuum decreases as the soil becomes wetter and will eventually fall to zero at saturation. In reality, tensiometers measure the suction force that roots must exert when they absorb water. Consequently, grapevine water constraints will increase as the matric potential decreases. It must be noted that vacuum or suction is the opposite of pressure, and has a negative value.

260 CHAPTER 8 – PRACTICAL IRRIGATION SCHEDULING