188 AGRICULTURE 



which the heaps rest, with the result that 

 certain spots on the field are over-manured, 

 while the bulk of the field receives a corre- 

 spondingly smaller amount of fertilizing 

 materials. The loss will necessarily be 

 greatest should much rain fall before the 

 heaps have been spread. 



Whenever farmyard manure has been 

 spread over the surface of tillage land it 

 ought to be ploughed in as quickly as pos- 

 sible, because only when it is incorporated 

 with the soil is the loss of ammonia by 

 escape into the atmosphere reduced to a 

 minimum. Moreover, many of the valuable 

 physical effects that dung exerts on the soil 

 cannot operate till the material is ploughed 

 under. 



CHAPTER IX 



ROTATION OF CROPS 



WHENEVER agriculture has advanced be- 

 yond primitive conditions one finds farmers 

 alternating different crops on different areas, 

 in place of growing one single crop-species 

 on the same land year after year. The 

 objects and advantages of a rotation of crops 

 may be summarized as follows. 



