1 78 SOIL CONDITIONS AND PLANT GROWTH 



gators : it passes through phases in its life history that give 

 the appearance of wholly distinct organisms. 



The Nitrogen Cycle. 



The chemical investigations of the nitrogen cycle in soils 

 have usually been confined to changes in the percentage of 

 nitrogen and in the amount of nitrate present, and conse- 

 quently they throw little light on the actual reactions taking 

 place. Incomplete as they are, however, they have served a 

 useful purpose by indicating the nature of the problem and 

 furnishing material that has helped in unravelling the rather 

 complex changes going on. Four cases have been studied : 



1. The simplest is that of an ordinary arable loam kept 

 moist, aerated, and at 10 to 15 C. these being normal con- 

 ditions free from vegetation and from the washing action 

 of rain these being abnormal conditions. A considerable 

 formation of nitrate then takes place, about 3 per cent, per 

 annum of the nitrogen being converted, and generally there is 

 a small loss of nitrogen, presumably in the free state. How 

 far the accumulation of nitrate would go under these circum- 

 stances has never been ascertained, because the experiment is 

 necessarily very slow. Boussingault (48^) stated that in eleven 

 years one-third of the nitrogen of a rich soil changed to nitrate, 

 and about one-half of the carbon to carbon dioxide. 



2. If the conditions are made more normal by exposing 

 the soil (still kept free from vegetation) out-of-doors to the 

 action of ^rain and weather generally, the nitrates do not 

 accumulate but wash out, and can be detected in the drain- 

 age water. The soil thus loses nitrogen compounds, and in 

 course of time the loss becomes very considerable. At 

 Rothamsted a little plot of arable land T oVo acre in extent has 

 been kept free from vegetation by hoeing, but not otherwise 

 disturbed, since 1870 ; it has now lost one-third of its original 

 stock of nitrogen. The plot has been converted into a lysimeter 

 by isolating it from the surrounding ground by cement 

 partitions and then underdraining : the drainage water is all 

 collected and analysed. At the end of forty-seven years the 



