92 NITROGENOUS MANURES [chap. 



this action is speedy in normal soils, whereas the 

 organic compounds of nitrogen have to undergo 

 several successive processes of bacterial breaking down 

 before they reach the plant, so that some of them, like 

 straw and the residues of protein digestion, may remain 

 for a very long period in the soil before their nitrogen 

 becomes converted into nitrate. 



It is important for the farmer to know what return 

 he may expect from a given nitrogenous manure in the 

 year of its application, and whether the nitrogen which 

 is not recovered by the first crop may be expected to 

 become available in the next or following seasons. It 

 is necessary even to put a money value upon the 

 residues left behind in the soil after the first crop has 

 been grown with the manure, because a tenant leaving 

 his farm is entitled to compensation for the unexhausted 

 fertility he has thus added to the soil but has had no 

 opportunity of cropping out. 



A large number of investigations have been made 

 as to the relative value of nitrogen combined as nitrate 

 of soda and sulphate of ammonia ; but it has already 

 been explained in the preceding chapter that the 

 comparative effect of nitrogen from these two sources 

 will be determined by a variety of external conditions, 

 such as the crop under consideration, the amount of 

 calcium carbonate in the soil, the supply of potash, 

 dormant or available, and the effect of the manures 

 upon the tilth of the soil ; in consequence, no general 

 answer is possible that will apply to all cases. From 

 the Rothamsted experiments it is found that nitrate 

 of soda affords the better source of nitrogen for wheat, 

 grass, and mangolds, the superiority amounting on the 

 average to about ten per cent. ; but that, for barley, 

 potatoes, and turnips, the two manures are of equal 

 value, nitrogen for nitrogen. 



