vil] LOSSES OF SOIL NITROGEN 203 



used with excessive amounts of fresh dung, which has 

 not been rotted and so deprived of much of its soluble 

 organic matter. Of course, a steady loss of nitrogen 

 due to such causes as have been enumerated above must 

 also be expected wherever large quantities of organic 

 nitrogenous manures are accumulating in the land. If, 

 for example, we compare 2 and 3 of the Broadbalk wheat 

 plots at Rothamsted, the latter of which is unmanured 

 and the former receives dung containing 200 lbs. of 

 nitrogen per acre every year, we find that at the end 

 of the fifty years, 1844-93, the dunged plot contained 

 in the top 18 inches about 2680 lbs. more nitrogen than 

 the unmanured plot, or a mean annual accumulation of 

 50 lbs. The extra crop grown on the dunged plot 

 would remove a further 31 lbs., thus leaving 119 lbs. 

 per annum to be accounted for, either as nitrogen 

 washed away in the drainage water or lost as gaseous 

 nitrogen by denitrification processes. 



Iron Bacteria. 



Another series of bacteria playing an interesting 

 part in certain soils, consists of those which secrete 

 hydrated ferric oxide or bog-iron ore in undrained soils, 

 where the soil water contains ferrous bicarbonate in 

 solution. Winogradsky investigated four of these 

 organisms, to whose vital processes he considered the 

 presence of soluble ferrous salts was essential. Molisch, 

 however, regards the secretion of ferric hydrate as, in 

 a sense, an accidental accompaniment of their growth, 

 much as the separation of large quantities of silica, so 

 characteristic of the straw of cereals, is unessential to 

 their development. It has already been noted that 

 these iron earths do not form in soils containing calcium 

 carbonate, which seems to prevent the formation of any 

 soluble ferrous compounds. 



