34 THE FERTILITY OF THE SOIL [CH. 



gives out, and at present it seems inevitable that it 

 must, mankind will be faced with perhaps the most 

 serious of all catastrophes, a phosphorus famine. 



But there is no immediate cause for alarm, as on 

 the lowest computation the visible supply will last 

 for many years. Further, the phosphates lost from 

 the farm are not destroyed but find their way to the 

 sewage, and thence to the sea. We must therefore 

 look to the ocean for the means of replacing the land 

 deposits of phosphates, and already a fair amount is 

 drawn from this source in guano, fish meal, etc. 



The supplies of potassium compounds already 

 existing in the soil are in general sufficient for 

 ordinary purposes, but additional supplies become 

 necessary as the character of the farming improves 

 and larger crops are grown. Some special soils, 

 such as peats, chalks, and thin sands, need potassium 

 fertilisers in order to yield even small crops, and 

 much labour has been lost through ignorance of this 

 fact. The deposits of potassium salts are extensive 

 and in Germany's keeping; no needless waste is 

 therefore to be apprehended. In the last instance 

 the ocean can be made to give up some of its 

 enormous stock. 



The nature of the plant residues and the ease 

 with which they are decomposed by bacteria depend 

 on climatic factors — the temperature, water supply, 

 etc. — and also on the amount of calcium carbonate 



