Handbook for Irrigation of Wine Grapes in South Africa

Figure 8.20

Meter

Soil surface

Counter

RA source

FIGURE 8.20. Schematic illustration where red arrows indicate neutrons leaving the radio- active (RA) source, whereas black, dashed arrows indicate the reflected “slow” neutrons registered by the counter after colliding with water molecules.

Since the neutron probe counts can be affected by environmental conditions, particularly external radio-active radiation, it is better to work with neutron count ratios rather than actual neutron counts. In such a case, the count ratio must be calibrated against gravimetric soil water content. The count ratio is calculated as follows. CR = C act ÷ C std Eq. 8.6 where C act is the actual neutron counts at a given point and depth, whereas C std is a standard count. The latter is the mean of 10 counts taken on the day that the soil water content measurements are carried out in the field. Take the standard counts with the probe still inside the protective housing and make sure that there are no other water sources nearby. The neutron probe can be placed on its box when the standard counts are taken. Most of the commercially available neutron probes have set count times, e.g. 4, 8, 16 and 32 seconds. Longer count times will be time consuming if a large number of measurements have to be carried out. Experience has shown that 8 seconds are adequate to obtain reliable measurements.

254 CHAPTER 8 – PRACTICAL IRRIGATION SCHEDULING