144 



PRODUCTION 



and by the transport of rich sediments to the sea, both of which 

 may be considerable in regions subject to a heavy rainfall l ; 

 second, through the physical energy expended by animals and 

 human beings not used in the cultivation of the soil ; third, by the 

 disappearance of sewage material from towns into the sea or into 

 large bodies of inland water 2 ; fourth, by the loss of fertilising 

 elements in returned manure and organic refuse through undue 

 concentration and exposure to the air ; and fifth, especially in 

 new countries, by the destruction through fires of accumulated 

 humus and of all except the mineral constituents of grasses and 

 other vegetation. 



Against those losses must be set certain gains : first, through 

 the natural disintegration of rock material into soil and the action 

 of bacteria in making the soil constituents available as plant food ; 

 second, through the natural accumulation of humus from the decay 

 of wild flora ; third, through the artificial and hastened accumula- 

 tion of humus from the ploughing in of green crops ; and fourth, 

 through what reaches the land as fertilising material from the sea 

 in the form of fish and fish refuse, and seaweed. 



Under the more intensive methods of agriculture, even when the 

 rearing and maintenance of live-stock constitutes an important 

 part of the whole business, a deficit is apt to arise in soil fertility 

 unless artificial fertilisers are used. These tend more and more to 

 become the chief means of maintaining the balance of fertility in 

 the more densely populated agricultural areas, and the supplies 

 of them require, therefore, to be examined in some detail. The 

 following table shows the world's output 3 of the more important 

 artificial fertilisers in the period preceding the year 1914 in millions 

 of metric tons. 4 



1 Experiments conducted at Rothamsted show that the loss of soluble 

 elements by seepage and drainage is often serious. With regard to soil 

 erosion, it is said that 11 million acres of farm lands in the United States 

 have been made valueless owing to this cause. In this connection, see also 

 Revue Economique International?., Feb., 1912, pp. 391407. 



3 An average adult annually excretes 12 Ibs. of nitrogen, 7 Ibs. of phosphoric 

 acid, and 5 Ibs. of potash, Journal Board of Agriculture, May, 1916, p. 129. 



3 Not including stocks held in reserve. 



4 The figures are those given by the International Agricultural Institute. 



