PREVENTION AND ERADICATION OF WEEDS 67 



difference in the requirements of the crops under the two 

 methods of cultivation is so great that the eradication of 

 weeds by chemical means has to be regarded from totally 

 different standpoints in the two cases. 



(i) Sprays. On grass-land sprays have only a limited 

 degree of usefulness, because as the herbage consists of a mat 

 of plants which presents in the aggregate a very large area of 

 leaf surface, the amount of spray that is necessary to kill 

 certain obnoxious weeds is sufficient to find a lodgment on the 

 leaves of the herbage and work havoc, particularly among the 

 broader leaved plants, such as clover, which are a most valu- 

 able constituent of the crop. . In the majority of cases it is not 

 possible to apply the spray in such a way that it reaches the 

 weeds without affecting the herbage, therefore, generally speak- 

 ing, this method is of little use on grass-land. 



Iron sulphate has been used in an attempt to eradicate 

 dandelions from lawns, the application being repeated till the 

 treatment had to be discontinued for fear of injuring the grass. 

 The experiment was a failure on account of the great vitality 

 of the dandelion roots, which remain unhurt even though the 

 leaves are destroyed by the spray. Iron sulphate has proved 

 equally ineffective with bracken, and copper sulphate is of 

 little or no use to eradicate thistles. 



Bracken, on account of its tall branching habit, is more 

 amenable to eradication by spraying than any other grass-land 

 weed. The leafy canopy that it forms offers a large surface 

 which catches and retains the liquid and prevents it from fall- 

 ing to the ground in sufficient quantity to injure the underlying 

 herbage. Sulphuric acid has proved effective in clearing bracken 

 from areas of grazing land. A few hours after spraying with 

 a 5 per cent, solution the bracken leaves wilt, and a few days 

 later the stems turn black and wither, as the acid is gradually 

 conducted along the midrib and down the stalk, eventually 

 reaching almost to the underground rhizome. A second 

 spraying after a new crop has sprung up clears off the bracken 

 for the year and weakens it for future years. The grasses 

 underneath are not much affected, the bracken affording them 

 adequate protection, and as in the following year the fern is 

 slower in making its appearance, the grasses are able to get 

 ahead earlier, with the result that the pasture becomes much 

 improved. Arsenite of soda and salt have also been used with 

 good effect on bracken. Arsenite of soda, however, is inadvis- 

 able as a general rule, because it is so very poisonous that there 



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