Soil pH is an important property and may determine land-use and quality. Few crops are well adapted to acid soils and so amendments and management are sought to alkalinize or limit further acidification. This chapter reviews the various processes that influence soil acidity. The effects of industrial activity and farming are considered. Two important industrial processes are acid drainage following pyrite oxidation and acid precipitation, best known as ‘acid rain’. The cycling of C (carbon), N (nitrogen) and S (sulfur) induce changes in acidity and each of these is examined. The measurement of soil pH is briefly discussed. The adverse effects of soil acidity and countermeasures are also considered.