44 An Account of the Soil [pt. i 



Under our climatic conditions the nitrates do not get 

 the opportunity of persisting long but are either washed 

 out by rain or taken up by plants. Once the stock is re- 

 duced a further quantity begins to be formed and so far 

 no hmit has been reached to the amount of nitrate a 

 soil can be made to yield. One of the Rothamsted plots 

 which has been cropped with wheat every year since 

 1843 and has had no manure since 1839 still goes on 

 yielding nitrate and in September 1913 contained nearly 

 35 lbs. of nitrogen as nitrate, equivalent to 210 lbs. of 

 nitrate of soda in the top 18 inches of soil per acre. 

 Another piece of land is kept bare of all vegetation and 

 is undermined in such a way that the whole of the 

 drainage-water can be collected for analysis. Ever since 

 1870 when the experiment began the land has yielded a 

 large supply of nitrate, the amount being equivalent to 

 300 lbs. of nitrate of soda per acre every year for the first 

 20 or 30 years, and to some 170 lbs. in more recent years. 



A further change goes on in certain circumstances. 

 When all air is excluded from the soil by flooding it for a 

 long time with water, the nitrates are hable to decompose 

 yielding nitrites and subsequently gaseous nitrogen. This 

 change, kno"WTi as denitrification, only goes on slowly in 

 cold weather and probably is of rare occurrence under 

 British agricultural conditions where land would only 

 be waterlogged in winter, if at all. But it seems to go on 

 in the wet rice fields of the East and in these circum- 

 stances nitrates are not used as manure. 



All these changes result in loss of nitrogen : fortunately 

 there are others that bring about gains, chief among 

 them being the fixation of gaseous nitrogen by the 

 organisms in the root nodules of leguminous plants. 

 Some also is fixed by certain free Mving bacteria called 



