SUMMARY: The changes in botanical composition of closely cut turf (such as that of lawns) which result from the periodic applications of heavy dressings of ammonium sulphate have been followed at five centres during a period of 3 years. These dressings were applied at the rate of 3 lb./1000 sq. ft. (1–2 cwt. per acre) every 2 weeks during the spring and summer months. The applications were “watered in” to avoid injury from a solution of high concentration.A method of botanical analysis by means of randomly selected quadrats was employed. The changes in botanical composition are expressed as changes in the area covered by each individual species. The data have been treated statistically where possible.As a result of the treatment the areas covered by the weeds (i.e. all species other than the Gramineae) were greatly diminished. Some species such as Trifolium repens and Achillea Millefolium were more susceptible than others. As an illustration of the order of magnitude of this effect it was found at one centre that there was a decrease in “total weeds” (i.e. the sum of the areas covered by individual species) from 17‐0 to 3–7 per cent., between April and September, of the total area examined. On the control plots the changes were slight.A reduction in the “total weeds” was associated with an increase in the area covered by the grasses, the dominant species in each case showing the greatest expansion.In addition to trials with ammonium sulphate, two experiments were carried out, one with ferrous ammonium sulphate and the other with an equivalent mixture of ferrous and ammonium sulphates. It was found that both these treatments produced greater reductions than ammonium sulphate alone. In one case ferrous ammonium sulphate at 10 lb./1000 ft.2 reduced the “weed area” from 12‐5 to 0–3 per cent., while ammonium sulphate applied at an equivalent nitrogen rate, reduced the area from 10‐6 to 5‐0 per cent, during the same period.The changes of soil reaction show that the view that soil acidity is a pre‐requisite for weed diminution by ammonium sulphate is untenable.It is suggested that the effect of the ammonium sulphate is due to a differential action of the ammonium ions, which are toxic to the majority of the weeds while increasing the growth of the grasses.The writer wishes to thank the Secretaries of the Bishop's Stortford, Royal Worlington, Gog and Magog and Stoke Poges Golf Clubs for their help, and also to express his gratitude to Sir Frederick Keeble and Mr Page for their suggestions. Finally, he is indebted to Mr Marmoy of Jealott's Hill for the bulk of the pH determinations.