Applications of neutron methods in agronomic research

Calculation of soil-water content from neutron moisture gauge measurements requires that the instrument's calibration curve should be known. This curve represents graphically, for a given site, the relation between the moisture gauge count and the water content averaged over the soil volume considered for the measurement. The calibration curve depends on the measuring instrument - the characteristics of its source, its detector and measuring geometry, especially the diameter of the access tube - and on the type of soil, i.e. its dry bulk density and chemical composition. The parameters associated with the instrument are taken into account by comparison with a reference moisture gauge. The influence of the soil's chemical composition is expressed by its neutron constants, which are determined directly in the laboratory. The thermal neutron diffusion and absorption constants for the soil can be used to plot the moisture gauge calibration curve by determining dry bulk density in addition. This quick method has a wide range of applications, including for mineral and organic soils. Apart from the moisture gauge, the neutron method has other uses in agronomy -simultaneous detection of thermal and epithermal neutrons, by which water content can be determined independently of dry bulk density; measurement of photoneutrons from the γ-n reaction in deuterated water, so that the latter can be used as a tracer in field experiments; and lastly, in-situ neutron radiography of roots. (author)