Handbook for Irrigation of Wine Grapes in South Africa

Chapter 8

Equation 8.5 can be used to calculate the SWC as follows: SWC = 0.15 x 1.6 x 2 x 100 = 48 mm. Unfortunately, the sampling is labour intensive and time consuming, particularly if the soil is dry. Consequently, the gravimetric method is not suitable for on-farm irrigation scheduling. 8.4.1.3 NEUTRON PROBES The neutron scattering technique is widely used to measure soil water content (Fig. 8.19). The instrument has a probe which contains a radio-active source that radiates neutrons. The probe is lowered to a predetermined depth in a 50 mm diameter access tube installed in the soil. The radiated neutrons lose energy when they collide with water molecules in the soil, and become “slow” neutrons (Fig. 8.20). Some of the slow neutrons are reflected back to the probe which also contains a counter specifically for “slow” neutrons which have collided with water and were reflected back. In practice, higher counts mean more water, and vice versa when the soil is drier. The meter displays neutron counts that need to be converted to soil water content (mm). Neutron probes have the advantage that they can measure over the full range of plant available water content, i.e. from field capacity to permanent wilting point. Since the neutrons are scattered though the soil in a sphere of ca. 30 cm in diameter, it measures in a relatively large soil volume compared to gravimetric samples.

FIGURE 8.19. A neutron probe is being used to measure soil water content in a vineyard.

IRRIGATION OF WINE GRAPES 253