Evaluation of the occurrence of soils with pH higher than 8.4 observed within the Countryside Survey. Project SP1304 Final Report

Soil pH values in the UK have generally increased during the last three decades but this increase has been particularly pronounced in the South-East of England. There has also been a progressive increase, since the first Countryside Survey in 1978, in soils with pH values of 8.3 and above, indicating that pH may not be controlled by the presence of calcite alone. The maximum pH value in solution when calcite reaches saturation at atmospheric conditions is 8.3, but higher values can be reached when, in the presence of calcite, monovalent cations, most commonly sodium, are accumulated. The combination of high pH and the presence of sodium has detrimental effects on soil properties with implications for ecosystem function and services. This combination is associated with colloid dispersion, loss of organic carbon, decrease in soil permeability, and increase in run-off and erosion. Using Countryside Survey (CS) data collected in 1978, 1998, and 2007 we found that the mean soil pH for locations with pH>8.3 and sampled in all three surveys had experienced an increase of ~1 pH unit from 1978 to 2007. The majority of samples were from calcareous soil types or over calcareous parent material, and predominantly in the Arable Broad Habitat type. Visual observation of samples revealed the presence of abundant calcite crystals in most of the soils indicating that farming practices may be bringing calcareous minerals to the surface via deeper ploughing. In addition, the region where soil pH is increasing is coincident with the greatest decrease in acid atmospheric deposition over the last 20 years. Relatively low Na/Ca ratios indicated a system still controlled mostly by calcite rather than sodic salts. However, when the soils were plotted on a stability diagram that considered the variables affecting soil colloid stability, more than half of the soils were located in the area where a decrease in permeability is expected. This has broad implications for erosion and runoff. Since the preservation of aggregate stability is crucial for soil function it is imperative that soils susceptible to degradation are identified and specific studies are made to mitigate soil structural damage.