146 COMMON WEEDS 



tility, no doubt because the freely-growing leguminous 

 plants (24 per cent of the herbage on the average) 

 have provided an ample supply of nitrogen. There is, 

 however, " a rather large proportion of Sorrel/' and 

 Yarrow is very abundant. 



Where potash was omitted the average crop was less 

 by about one-fourth, and little more than half of that 

 on the plot to which potash was applied. The legu- 

 minous plants were much fewer than on the plot last 

 mentioned, and there was a proportionate increase in 

 weeds, the characteristic species being the Buttercup, 

 Black Knapweed, Plantain, and Yarrow. 



When superphosphate only was continuously applied 

 the result has been disastrous, and Mr. Hall even goes 

 so far as to say that " the aspect of this plot, where the 

 most abundant grass is Quaking Grass, and where 

 weeds, chiefly Hawkbit, Burnet, and Plantain, are un- 

 usually prominent, would seem to indicate that the land 

 is more exhausted here than on the unmanured plot." 

 Again, a nitrogenous manure alone is often thought 

 exhausting, but probably the phosphatic manures used 

 singly will even more quickly impoverish the soil. 



We now come to consider the effect of a complete 

 artificial manure. Where the complete phosphate- 

 potash-ammonium manure was given, on two plots the 

 average yields have been no less than 54 cwt. (ammonium 

 salts = 86 Ib. of nitrogen) and 65 cwt. (ammonium 

 salts =129 Ib. of nitrogen) respectively. Yet the hay 

 was not so good as on the plot receiving a complete 

 mineral manure, i.e. without nitrogen only, because in 

 the former case " the large amounts of nitrogen have 

 so stimulated the development of the grasses that legu- 

 minous plants have disappeared entirely, and even the 

 weeds are crowded out." On the plot receiving the 

 complete artificial manure an excess of the nitrogenous 



