DISTRIBUTION OF NITRIFYING ORGANISMS 221 



tendency to increase and is but little higher than the proportion 

 in the soil of other adjoining meadows which have only been 

 laid down to grass for thirty years or so. Again, in the Broad- 

 balk wheat field, the plot which receives farmyard manure is 

 supplied annually with far more nitrogen than is removed in 

 the crop. During the earlier years of the experiments there 

 was in consequence a rapid rise in the proportion of nitrogen 

 in the soil, but this rise has diminished, and has been latterly 

 by no means equal to the annual increment of nitrogen. A 

 state of equilibrium is eventually attained, when the destructive 

 agencies find the conditions so favourable for their develop- 

 ment that the quantity of nitrogen compounds broken down 

 to the state of gas becomes equal to the surplus of combined 

 nitrogen that is added year by year. 



III. NITRATES IN CULTIVATED SOILS. 



The nitrifying organisms are in the main present only in the 

 surface soil which is subject to cultivation ; at depths greater 

 than 9 inches from the surface the organisms become more 

 scanty and less effective in inducing nitrification in a suitable 

 medium. During the sampling of several of the Rothamsted 

 soils Warington took advantage of the pits dug into the 

 subsoil to obtain small samples of the undisturbed subsoil, 

 portions of which were then introduced into solutions capable 

 of undergoing nitrification. It was found that the nitri- 

 fying organisms were present in all the samples down to 

 3 feet from the surface ; at 6 feet, where the subsoil was clay, 

 half the samples failed to induce nitrification, at 8 feet the clay 

 subsoil showed no evidence of the presence of nitrifying 

 organisms. Whenever the subsoil passed into the chalk rock, 

 which in one case extended to within 5 feet of the surface, no 

 nitrifying organisms were found. Practically the whole of the 

 nitrification going on in a comparatively close soil like that of 

 Rothamsted takes place in the first 9 inches which gets 

 stirred about and aerated by the action of the plough. 



It will now be realised that the most favourable conditions 



