Some problems associated with the use of the neutron probe in swelling/shrinkling clay soils

SUMMARY Comparison of neutron probe count rate ratios measured at 10 cm depth, with bulk soil water contents measured 3–5m away from the access tubes, confirmed that in swelling/ shrinking clay soils, recharge may be over estimated. This is due to the drying soil shrinking form the access tube, creating an air gap and radial cracks which allow preferential re‐writing of the soil near the access tube. Installation of access tubes into dry soil was only a short‐term answer. Comparison of results obtained for bulk soil and for small colds shows that calibrating the neutron probe using small diameter cores may be lead to serious under estimates of changes in water content in shrinking clay soils.