THE EFFECT OF THE CHEMICAL-COMPOSITION OF A FERRALLITIC SOIL ON NEUTRON PROBE CALIBRATION

In a ferrallitic soil in French Guiana, the neutron probe calibration appeared to be problematic: considerable variations in the neutron cou nt rate were observed at very short range and with an almost constant volumetric water content. This local variability of the count rate was explained by the mineralogical heterogeneity of the schist weathering horizon where subverticaly oriented layers are especially rich in bor on, element with large thermal neutron absorption cross section. Vario us calibration methods were carried out and their limits were pointed out. The field gravimetric calibration without taking into account the soil physical and chemical spatial variations appeared to be risky, e ven if different pedological horizons are considered separately. A cal ibration based on the neutron absorption SIGMAa and diffusion SIGMAd c ross sections calculated from chemical analysis led to overestimates o f the volumetric water content. This could be explained by the concent ration of boron atoms in sand-size tourmaline crystals which reduces t heir neutron absorption properties. The direct measurement of thermal neutron absorption and diffusion cross sections on soil samples in a g raphite pile seems to be the best calibration procedure, but it has to be repeated as often as the spatial variability required.