the neutron moisture meter (nmm)

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87 Module 4. Determining Plant Available Water Capacity The neutron moisture meter (NMM) Calibrating and using the NMM in soil characterisation A neutron moisture meter emits fast neutrons which are reflected back at slow speeds to a detector in proportion to the concentration of hydrogen atoms in the soil; this hydrogen is found predominantly in water molecules. The detection of the neutrons is measured by the meter as a count rate. To provide a reference (to allow for instrument variabil- ity etc), the count rate is usually expressed relative to the count rate measured in water (using a drum of water with an access tube installed). The calibration enables the result- ing count rate ratio to be converted to volumetric water content. A neutron moisture meter (NMM) must be calibrated to measure the actual soil water content for a particular soil type. To calibrate the NMM, volumetric soil water values (at varying moisture contents) for a particular soil type are related to the NMM readings at the same soil depth lay- ers. This calibration allows a regression equation to be calculated for the particular soil type. In most cases, this information is not available when a soil is characterised for the first time; the characterisation exercise pro- vides some of the information necessary to calibrate the neutron moisture meter. Even so, the NNM can still indicate wetting up (or drainage) as the changes in the count rate, over time, reflect relative changes in water content. This should be started be- fore the initial wetting up of the characterisation site so that all water movement can be tracked (as shown in Figure 4.10). When calibrating the neutron moisture meter for a particular soil type, NMM readings and corresponding volumetric water contents should be determined when the soil profile is dry (generally at tube installation) and when wet (at final sampling drained up- per limit). Additional readings and volumetric data for intermediate water contents will improve the accuracy of the calibration func- tion. A calibration equation is then derived for that particular soil type. Figure 4.10. The sequential wetting-up of a heavy clay soil monitored by neutron moisture meter. The wetting front moves progressively deeper into the profile of a heavy-textured clay soil under water ponded for an extended period; in a lighter- textured soil, or where trickle irrigation is used, wetting-up would normally occur more quickly.

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