82 



SOIL CONDITIONS AND PLANT GROWTH 



The exhaustion of the soil is, therefore, not due to the removal of 

 the crop, but to the cultivation. 



Similar losses take place when heavy dressings of farmyard manure 

 are repeatedly applied to land. One of the Broadbalk wheat plots 

 receives annually 14 tons of farmyard manure per acre, containing 

 200 lb. of nitrogen. Only little drainage can be detected and there 

 is no reason to suppose that any considerable leaching out of nitrates 

 occurs, but the loss of nitrogen is enormous, amounting to nearly 70 

 per cent, of the added quantity. Alongside is a plot receiving no 

 farmyard manure, from which in spite of drainage the loss is only 

 very small. 



Table XXXVI. — Losses of Nitrogen prom Cultivated Soils, Broadbjvlk Wheat 

 Field, Rothamsted, Forty-seven Years, 1865-1912. 



Experiments of this kind have led to the conclusion that some gaseous 

 product is formed in addition to nitrates, and, as no sufficient amount 

 of ammonia can be detected, it is supposed that gaseous nitrogen is 

 given off. The conditions for this decomposition appear to be copious 

 aeration, such as is produced by cultivation, and the presence of large 

 quantities of easily decomposable organic matter. Now these are 

 precisely the conditions of intense farming in old countries and of 

 pioneer farming in new lands, and the result is that the reserves of soil 

 and' manurial nitrogen are everywhere being depleted at an appalling 

 rate. Fortunately there are recuperative actions, but one of the most 

 pressing problems at the present time is to learn how to suppress this 

 gaseous decomposition and to direct the process wholly into the nitrate 

 channel. 



We are now, in a position to explain many of the anomalies and 

 contradictions met with in investigations on the " availability " of organic 

 manures. In these experiments it is usual to supply various nitrogenous 



