Renovation of phalaris (Phalaris tuberosa L.) pastures by mouldboard ploughing resulted in an increased yield of phalaris but a decreased yield of other species. Only at one of three sites was there a net increase in yield. The increased yield of phalaris may have been due to its greater ability to survive the fairly drastic ploughing treatment. The phalaris then had a competitive advantage when the pasture grew again. From data for soil ionic nitrogen, plant response to nitrogen and sulphur, and chemical analysis of plant material, it was concluded that the increase in yield was due to greater availability of nitrogen and sulphur, indicating accelerated breakdown of organic matter. This effect was considered to be due to the temporary prevention of plant growth, since the continual removal of ionic nitrogen by plants would tend to reduce the nitrogen available to microorganisms, thus widening the carbon: Nitrogen ratio. A similar competition for sulphur may also be important. Recurring sulphur deficiency due to infrequent applications of fertilizer may be important in the decline of pastures on sulphur-deficient soils. The clover fraction would be particularly affected since grasses are considered to compete strongly with clovers for sulphate. The data for ionic nitrogen in soil showed that high exchangeable ammonium occurred during the winter months.