42 An Account of the Soil [pt. i 



Nevertheless the results are of much value to the agri- 

 cultural chemist in investigating soil fertihty problems. 



We must now turn to the changes undergone by the 

 organic matter. During the process of cultivation the 

 organic matter becomes oxidised and some of it dis- 

 appears as gas ; it thus suffers much more rapid changes 

 than the mineral particles. Illustrations can be seen in 

 parts of North America where the original prairie soil 

 was fairly rich in organic matter but after some years 

 of wheat cultivation it has lost much of its stock. In 

 Minnesota Snyder found that an amount containing 50 

 per cent, of the nitrogen was lost in twenty years' culti- 

 vation: in Saskatchewan Shutt observed a loss of an 

 amount containing 30 per cent, of the nitrogen after a 

 similar period. With the organic matter is lost also the 

 advantages it conferred : the soil becomes impoverished, 

 and, if much clay is present, it becomes difficult and 

 expensive to cultivate. Hence such soils tend to be 

 thrown out of cultivation and to become derehct. 

 Similar losses occur in market gardens and wherever 

 large dressings of farmyard manure are appUed: they 

 necessitate the use of more manure than is really needed 

 by the plant and they add to the expense of production. 



A closer analysis of the loss shows that the carbon of 

 the organic matter goes off as carbon dioxide and that 

 some at any rate of the nitrogen is changed to ammonia, 

 and there is evidence that some is lost as gaseous nitro- 

 gen. The question is under investigation at Rothamsted; 

 it is of enormous agricultural importance because of the 

 seriousness of the loss on rich soils and the necessity for 

 reducing all wastage nowadays: it is discussed more 

 fully in Chapter VI. 



The ammonia remaining in the soil is at once seized 



