From measurements and calculations using a depth probe, the relative response is estimated for soil layers of different thicknesses. For a 10 cm thick layer of soil with the instrument centre in the middle, about 40 to 70 per cent of the response is caused by this particular layer at a water content of 8 to 40 per cent by volume. For a 20 cm layer the response for similar conditions is about 65 to 85 per cent.
A special calibration function for the upper (0–20 cm) soil layer is demonstrated for the neutron depth probe. It is found that the water content in this soil layer can be estimated in this way with about the same degree of accuracy as can be expected by conventional methods.