A survey of the pH of samples of Coolup sand under pastures of various ages showed that pastures which had been sown for more than 10 years had pH (CaCl2) values lower than 4.5. This is equivalent to about 5.2 for pH determined with a soil: water ratio of 1: 5. The lowest value recorded was pH (CaCl2) 4.0 for a pasture about 30 years old. The pH (CaCl2) of virgin samples of this soil was found to be about 5.0. Application of 1 ton an acre of ground limestone to old pastures increased total yields by an average of 26 per cent in the year of application. The increased yield was probably not due to increased availability of molybdenum, phosphorus, sulphur, potassium or calcium. Decreased toxicity of manganese or of aluminium are explanations which could not be disproved on the present data but the explanation considered to be most probable was increased availability of soil nitrogen.