CH. xii] Manures for Grass Land 213 



grown, no nitrogen is needed but superphosphate must 

 be appHed to prevent rankness (see p. 145). 



The seeds may receive lime, basic slag or potash ; no 

 nitrogen is usually necessary where the aftermath is 

 folded off. 



The grazing land should periodically receive basic 

 slag alone, or basic slag and kainit, while land laid in 

 for hay should in addition receive an annual dressing 

 of a nitrogenous manure such as sulphate of ammonia, 

 nitrate of soda, etc. : every four years or so, however, 

 dung should be applied instead. Fig. 38 shows the 

 results obtained at Rothamsted. 



No general recipes can be given for the composition 

 of manures. At one time it was supposed that the ideal 

 mixture was that represented by the composition of the 

 ash as showing what the plant had actually taken from 

 the soil. This is now known to be incorrect: the need 

 for manures is determined not by the composition of the 

 plant but by its habit of growth and the conditions 

 under which it lives. On any particular farm the most 

 suitable mixtures can only be discovered by trial ; several 

 recipes can be drawn up on the basis of the information 

 already given, and the most suitable ones tested. The 

 problem is considerably simpHfied in counties where a 

 soil survey has been made or systematic field experi- 

 ments conducted. 



Numerous field experiments have been made to dis- 

 cover the effects of the different fertilisers in various 

 districts: these have been summarised in the writer's 

 Manuring for Higher Crop Production. The following 

 increases have been obtained per cwt. of phosphate and 

 nitrogenous manures respectively: 



