Using the Dual Energy Gamma-Ray Transmission Technique to Measure Soil Bulk Density and Water Content of Central Southwestern Nigerian Soils

In this work, a radiological technique which simultaneously measures soil bulk density and water contents accurately and quickly in a non-destructive manner at different depths of the soil profile of the major soil series of Central Southwestern Nigeria is developed. Undisturbed samples from Iwo, Ondo, Egbeda, Itagunmodi, Okemessi, Mamu, Origo and Jago soil series of Southwestern Nigeria were collected on monthly intervals from June 2006 to May 2007. Using gamma-ray energy pairs of 122 and 1112 keV; and 344 and 1408 keV obtained from europium-152 (¹⁵²Eu) radionuclide, the attenuation coefficients for soil and water, μ_s, μ_w and consequently ρ_s and θ_v at varying soil depths of these series were calculated. Comparative gravimetric measurements of these soil parameters were carried out. Using the XCOM computer algorithm with the soil elemental concentrations obtained using the Energy Dispersive X-Ray Fluorescence analysis (EDXRF) technique, theoretical estimates of μ_s for the various soil series were obtained and compared with the experimental values. The result of the developed dual energy gamma-ray transmission technique was compared with the gravimetric measurement method and Pearson correlation coefficient (0.987, p < 0.001) was obtained for ρ_s and (0.996, p < 0.001) for θ_v. The results of the measurements showed that θ_v minima with values 0.1931, 0.1987, 0.2377, 0.2111, 0.1738, 0.1701, 0.2334, 0.2341 and minima ρ_s values of 1.21 g/cm³, 1.20 g/cm³, 1.02 g/cm³, 1.38 g/cm³, 1.12 g/cm³, 1.14 g/cm³, 0.64 g/cm³, 1.33 g/cm3 were obtained respectively for the soil series stated above in January. The maxima θ_v with values 0.3621, 0.4212, 0.3962, 0.3794, 0.3049, 0.2811, 0.4415, 0.4416 and maxima ρ_s values of 1.52 g/cm³, 1.67 g/cm³, 1.74 g/cm³, 1.69 g/cm³, 1.70 g/cm³, 1.68 g/cm³, 1.08 g/cm³, 1.54 g/cm³ occurred for these soil series respectively in July except for Mamu soil series which occurred in May. These correlated very well with the occurrence of the Dry and Rainy seasons in the study area. The results of the EDXRF analysis showed that maxima Iron, Fe concentrations of 5.0890 ppm at the depth of 90 - 120 cm into the soil profile, was obtained for Iwo series, 4.4691 ppm at 50 - 70 cm for Ondo, 6.3438 ppm at 15 -