Dataset: Park Grass Soil pH 1876-2023

Since being set up in 1856 the Park Grass plots have received different fertiliser, manure and lime treatments, many of which have had marked effects on soil pH, which are reflected in species composition, yield, microbial biomass and various soil factors, particularly soil pH. Atmospheric deposition has also impacted on the vegetation and soil. On some treated plots the soil became highly acidified and mats of undecomposed plant material have built up. This dataset reflects these various effects, consisting of soil pH(in water) data for Park Grass plots, 1876- 2023 and also for the mats on the soil surface since 1967:
<b>
- pH of soil 0-23 cm deep, 1876-2023.
- pH of mat samples, 1967-2023.
- pH of 0 - 23 cm soil including mat, 1984 - 2023.
- pH of soil 23 - 46 cm and 46 - 69 cm depth samples, 1876 - 2023. </b>

These soil pH data are discussed in Poulton et al. (2026). Soil acidification occurs through the loss of base cations (Ca, Mg, K, Na) via crop removal or leaching as bicarbonate, chloride, nitrate, and sulphate, leading to hydrogen and aluminium ion accumulation. Acidifying anions derive from fertilisers, organic matter oxidation, and nitrification of ammonium-based fertilisers. Leaching losses depend on rainfall and drainage. Studies on Park Grass experiment examined these processes and developed the RothLime model to predict lime requirements. Optimum grassland soil pH is around 6.0.

Continuous nitrogen in the form of mixed ammonium salts (until 1916) and ammonium sulphate since, caused strong acidification, with topsoil pH near 4 by 1959 despite regular liming. In contrast, applications of sodium nitrate did not acidify the soil. From 1903 onwards, systematic liming trials tested CaCO3 rates and methods. A new scheme was introduced on most plots in 1965. Most plots were divided into four sub-plots which were to receive chalk (if necessary) to achieve and maintain a soil pH of 7, 6 and 5, together with an un-limed sub-plot. Large, but somewhat irregular, chalk applications resulted in considerable fluctuations in pH in some sub-plots but eventually achieved the target pHs, though the sub-plots which had become very acid this took decades (see figure above for illustration). Since 1991, lime applications have followed a three-year adjustment cycle and fluctuations in pH have been minimal. Declining atmospheric acid deposition has since raised rainfall pH and soil pH slightly, moderating previous acidification trends.