Changes in the clay mineralogy of soil samples collected from the Rothamsted Classical Experiments over the past ~ 150 years were examined. Comparisons were undertaken to assess the impact of (i) different fertiliser practices on the top soil and sub soil of the Park Grass Experiment and (ii) the effects of changing land use from agriculture to woodland where soil pH either remained close to pH 7 or became acidic. Analyses were undertaken on the < 0.2 µm clay fraction and measurements included cation exchange capacity (CEC), surface area and X-ray diffraction (XRD). The fine clay fraction of all the samples is composed of illite/smectite (I/S), illite and kaolinite minerals. Decomposition modelling of XRD spectra identified three I/S phases including (i) a high smectite I/S phase (d(001) ~ 15 Å), (ii) a low smectite I/S phase (d(001) ~ 12.5 Å) and (iii) an illite phase (d(001) ~ 11.2 Å). The I/S phase is typically made up of ~ 60% of high smectite I/S, ~ 20% low smectite I/S and ~ 20% illite. Attempts to account for changes in clay CEC and surface area with time proved to be inconclusive, possibly because of the differing proportions of the three I/S phases in each sample. Some temporal changes in the d(001) spacing of I/S mineral phases from the decomposed XRD spectra (> 0.4 Å) are reported in both top soil (0–23 cm) and sub soil (46–69 cm) and are likely related to (i) changes in soil K+ or NH4+ status and/or (ii) increasing soil acidity. The greatest change was found in the Park Grass Experiment (NH4)2SO4 plot where soil pH fell below 3.7. It is believed that solubilisation of hydroxyl-Al in smectite interlayers enabled renewed access to K+ or NH4+ ions, leading to increased collapse in d(001) spacing. However, the results show that over a diverse range of soil conditions, I/S minerals of the Batcombe series soils showed considerable resilience to major change. This is considered to be a result of competition for sorption sites by (i) the presence of competing ions added as fertiliser or liming materials, (ii) an increase in H+ ions as soil acidity increases, (iii) the possible role of organic carbon in protecting I/S minerals and (iv) the role of the I/S minerals as K+ reservoirs. However, the impact of decreasing soil pH should be considered when land is re-forested as this may lead to decreases in the d(001) spacing of I/S minerals, thus compromising useful clay properties such as CEC
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