Exploits and endeavours in soil water management and conservation using nuclear techniques

Soil water management and conservation efforts have been immensely aided by the advent of nuclear techniques since the 1950s, most notably by the introduction of the neutron moisture meter (NMM). At the end of World War II, the understanding of nuclear physics had increased tremendously, and there was a concerted effort to turn this knowledge to productive and peaceful uses. In 1949, a method for 'the measurement of the moisture content of soil by the slowing of neutrons' was described and subsequent research and development resulted in commercial availability of the NMM by the late 1950s. Research on water management and conservation began replacing gravimetric sampling with the NMM almost immediately, and crop water use data for a wide range of annual and perennial crops in a wide range of soils and climates became available in the 1960s. Although weighing lysimeters remained an important tool for crop water use determination, the wide range of soil and climate effects could not have been investigated without the NMM due to the expense of lysimeters. Advantages of the method included, and still include, a relatively large sampling volume, so that fewer samples are required, and precision better than 0.01 m3 m-3 when correctly calibrated, which exceeds that of gravimetric sampling for equal numbers of samples. The NMM was used for irrigation scheduling in research in the 1950s. But, in the early 1970s, improvements in portability, reliability, and user interface allowed the NMM to become an important tool for irrigation scheduling in production farming. Many of the uses of the NMM require measurement of the change over time of soil water stored within and below a crop or plant stand root zone. Accurate measurement of changes in water content depends on the accuracy of the calibration equation slope. Calibrations are specific to soils and soil horizons, so site-specific calibration is required. Because errors in calibration slope are likely without correct calibration methods, research on calibration methods is discussed and guidelines for accurate calibration are given. (author)