CHAPTER Yl. 

 MANURES. 



For a soil to possess fertility — i.e., to be able to properly sup- 

 port the growth of plants — certain conditions are necessary. 

 The following may be mentioned as being perhaps the most 

 important : 



1. Its mechanical or physical condition must be suitable. 



2. It must contain sufficient plant food in a form which is 

 readily available to the crop. 



8. It must not contain any appreciable quantity of poisonous 

 or injurious substances. 



4. It must not contain injurious insects, fungi, or other 

 organisms which are destructive to the crop. 



5. The temperature, sunshine, rainfall and other climatic 

 conditions must be suitable. 



Of these, the second and third, and to a less extent, the 

 first, are matters in which chemistry may be of service. 



Every crop removed from a soil robs the latter of materials 

 which have been used in building up the former's tissues. Soil 

 which annually bears a crop must, in time, become exhausted of 

 its store of plant food and unfitted to bear further crops. 

 Generally, one constituent of plant food becomes exhausted 

 first, and in many cases restoration of this constituent would 

 renew the fertility for some time longer. Substances which 

 are added to a soil in order to replace the ingredients which 

 have been removed by previous crops are called manures. 



All constituents of plants present in a soil, except the 

 carbon, are diminished by the growth of crops upon it, but 

 the substances which usually first become deficient are com- 

 bined nitrogen, phosphates, calcium carbonate and potash. 



96 



