XI.] MANURES FOR GRASS LAND 327 



crowd out its neighbours, so that the characteristic 

 vegetation of the field represents the state of equili- 

 brium which has been attained by the various plants 

 under the prevailing conditions of soil and climate. 

 The physical texture of the soil has much to do with 

 the nature of the grasses which will establish themselves 

 under the stress of competition : on the deep, kindly 

 alluvial pastures rye grass becomes prominent; on the 

 thin chalky soils of the Downs sheep's fescue thrives 

 best ; on heavy clays where aeration is deficient the 

 creeping rooted bent grass will cover the surface, while 

 sandy droughty soils often become covered with tufts 

 of cock's foot or brome grass. Just in the same way 

 manuring, by altering the food conditions in the soil, 

 can effect a great change in the character of the herbage 

 of a given field, and the direction which these changes 

 will take must be kept in mind in any discussion of the 

 application of fertilisers to grass land, since in Great 

 Britain we are never dealing with a crop of a pure 

 unmixed grass, like the crops of timothy or blue-grass in 

 America. The best example of the effect of long- 

 continued manuring on the composition of the herbage 

 is afforded by the Rothamsted experiments, where 

 certain plots of old grass land receive the same treat- 

 ment every year and are mown for hay. 



Table XCV. shows the average yield for fifty-three 

 years, and also the character of the resulting herbage, 

 as shown by its separation into grasses, clovers, and 

 weeds in 1902, the forty-seventh year of the experi- 

 ment. 



From this tabic certain facts become apparent. If 

 grass is constantly mown without any return in manure, 

 the resulting impoverishment is shown not only in the 

 small yield but in the preponderance of weeds in the 

 herbage. One - sided manures, which contain only 



