328 



S YSTEMS OF MANURING CROPS [chap. 



nitrogen or only phosphoric acid, however successful 

 at first, eventually result in increased impoverishment 

 of the land. Nitrogenous fertilisers promote the growth 

 of the grasses at the expense of the clovers. Mineral 

 manures, and particularly potash, promote the growth 

 of leguminous plants and enable them to make headway 

 against the grasses. 



Table XCV.— Yield and Composition of Hay at Rothamsted. 



Another consequence follows from these experi- 

 ments ; since any special combination of fertilisers or 

 any method of treatment encourages particular species, 

 the best results in any given field will always be 

 attained by persisting in the treatment selected. For 

 instance, when a field is laid up for hay certain strong- 

 growing grasses get an advantage ; when the field is 

 grazed other grasses of a dwarfer-spreading habit are 

 more suited by the conditions. It is therefore desirable 

 to keep one field for hay every year and another for 



