Soil Development under Continuous Agriculture at the Morrow Plots Experimental Fields from X-ray Diffraction Profile Modelling

Abstract

Impact of continuous cropping on clay mineralogy was assessed on a collection of unfertilized soil samples from the Morrow Plots experimental fields covering 110 years of long crop rotations. Evolution of mineralogy was quantitatively determined by fitting X-ray diffraction (XRD) patterns from four size fractions (50–2, 2–0.2, 0.2–0.05 and <0.05 µm) of the surface horizon (0–20 cm). The mineralogy of the three clay subfractions (2–0.2 µm, 0.2–0.05 µm and <0.05 µm) consists mainly of coexisting illite-smectite-chlorite whose compositions range from discrete illite (in the 2–0.2 µm subfraction) to discrete smectite (in the <0.05 µm subfraction). Mixed layers of similar compositions were used to fit XRD data from all clay subfractions. With decreasing size fractions, both the size of the coherent scattering domains and the proportion of illite-rich mixed layers decrease, thus accounting for the higher cation exchange measured in the <0.05 µm subfraction compared to other clay subfractions. The analysis of fine clay subfractions (<0.2 µm or lower) provided key information and constraints to a complete and accurate description of the bulk <2 µm fraction. Additional constraints derived from chemical treatments (K-saturation and heating) proved to be especially useful to propose a reliable structure model for these fine clay subfractions because of their weakly modulated diffraction signature. Mineralogy of all subfractions considered is essentially stable over the studied period (1904–2014), with the relative proportion of the different clay layer types (illite, smectite, kaolinite, chlorite) showing no significant evolution in the bulk <2 µm fraction. A century of continuous cropping thus results essentially in an increase of fine clay particles (<0.05 µm) and a decrease of the 0.2–0.05 µm subfraction, indicative of clay mineral dissolution and consistent with observed increase of cation exchange capacity with time. The relative proportion of the bulk <2 µm fraction is nearly constant over the studied period, indicative of minimal export of clay phases.