vil] LOSSES OF SOIL NITROGEN 231 



It will be seen that all the nitrogenous cross-dressings 

 produce an increase of crop when added to the farm- 

 yard manure. When the cross-dressings are used on 

 plots receiving only non -nitrogenous manures, the 

 nitrogen recovered varies between 56 and 78 per cent, 

 of that supplied in the manure; when they are used 

 in conjunction with dung, the recovery of nitrogen in 

 the increased yield above that produced by dung alone 

 varies between 36 and 61 per cent. That the recovery is 

 smaller in the latter cases is due to the fact that with such 

 excessive amounts the yield ceases to be proportional 

 to the supply of nitrogen, being limited by other factors. 

 Denitrification is only likely to cause rapid loss 

 of nitrogen when large quantities of nitrate are 

 applied to undrained or sour land, or when they are 

 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. After a wet winter 

 the wheat crop, especially upon heavy land, often shows 

 patches of lighter colour and poorer growth. These are 



