vil] NITRATES IN SOIL 195 



they require can be formed in the still unwarmed soil. 

 Nitrates are much more freely formed in the summer 

 than in the winter, and as they are not retained by 

 the soil, they may easily be washed away when the 

 crop has been removed, unless weeds or a catch crop 

 sown to that end are present to take up the nitrates 

 and store them as organic compounds of nitrogen for 

 the future enrichment of the land. 



The need for aeration in connection with the nitrify- 

 ing process has already been alluded to when discussing 

 drainage : all processes of working and cultivating the 

 soil assist nitrification, both by the thorough aeration 

 they effect, and by the mere mechanical distribution 

 of the bacteria into new quarters, where there are fresh 

 food supplies. In some experiments of DeheVain's he 

 found that the drainage water from pots of cultivated 

 soil, which had been sent from a distance, and thus 

 much knocked about in travelling and filling into the 

 pot, contained as much as 466 to 664 parts of nitrogen 

 as nitric acid per million. The drainage water from the 

 Rothamsted wheat plots contains only from 10 to 20 

 parts per million; even the cement tanks at Grignon, 

 2 metres cube, into which the soil had been filled, gave 

 drainage water containing only 39 parts of nitric 

 nitrogen per million. In another experiment a quantity 

 of soil was thrown upon a floor, and worked about 

 daily for six weeks ; on analysis it contained 0-05 1 

 per cent, of nitric nitrogen, as against -002 per cent, of 

 nitric nitrogen in the same soil left in situ. The diagram 

 (Fig. 14), due to King, shows the dependence of nitrate 

 production on temperature and the cultivation of the 

 soil. The lower curve shows the amount of nitrate 

 in parts per million in dry soil in the top foot of land, 

 which was not being cultivated because it carried clover 

 and oats. The upper curve shows the same results 



