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Estimating Soil Particle Density using Visible Near-infrared Spectroscopy and a Simple, Two-compartment Pedotransfer Function

The average particle density (ρd) is a fundamental soil property, used for calculating the total porosity. Traditional ρd measurement by pycnometer method is tedious and time-consuming. In this study, visible–near-infrared (vis–NIR) spectroscopy and a simple two-compartment linear and curvilinear pedotransfer function only requiring knowledge of soil organic matter content (OM) were tested and compared as alternative, indirect, rapid, and cost-effective methods. Soil ρd was measured by water pycnometer on 179 soils representing a wide range of OM (0.002–0.767 kg kg⁻¹), whereas soil spectra were measured on air-dry samples by vis–NIR spectroscopy. The ρd models were developed using partial least squares regression with leave-one-out-cross-validation using vis–NIR spectral data, and a simple two-compartment pedotransfer function, ρd = A(OM) + B(1 − OM) using the OM content. Predictive abilities of these two methods were tested using three different datasets: (i) minerals soils (OM 0.1 kg kg⁻¹), and (iii) all soils. Calibrating the two-compartment pedotransfer function for the entire dataset gave expected values for the individual particle densities of OM (A = 1.244 g cm⁻³) and mineral particles (B = 2.615 g cm⁻³). The vis–NIR spectroscopy model successfully predicted soil ρd for the entire dataset (R² = 0.87, RMSECV = 0.10 g cm⁻³), with a poorer performance than the two-compartment linear model (R² = 0.96, RMSE = 0.06 g cm⁻³). Using only the mineral soils data did not suffice to obtain realistic and accurate vis–NIR spectroscopy (R² = 0.62, RMSECV = 0.02 g cm⁻³) or OM based (R² = 0.80, RMSE = 0.02 g cm⁻³) models for ρd, illustrating the importance of the wide range of OM content considered in the present study.