296 



SCIENCE PROGRESS 



carefully studied in several countries. It is satisfactorily 

 established that the exhaustion is not due to the removal of 

 material by the crop, but rather to bacterial or erosion changes 

 that set in as soon as the soil is ploughed. So long as ground 

 remains uncultivated and covered with grass or other vegetation 

 that is not removed, the tendency of the bacterial change is in 

 the direction of accumulating nitrogenous organic matter in the 

 soil. Percolation of rain water and consequently loss of soluble 

 matter is reduced to a minimum, and there is little surface 

 erosion. But as soon as the land is cleared and ploughed 

 erosion and leaching take place, and aeration becomes more 

 thorough. The balance of bacterial change therefore shifts, 

 the decomposition processes predominate, and the tendency 

 now is for nitrogenous organic matter to decompose, giving 

 off carbonic acid, ammonia, and, curiously enough, gaseous 

 nitrogen. The ammonia is converted into nitrates, part of 

 which is used by the plant, but part is lost. The annual losses 

 of nitrogen from two virgin soils and the proportion recovered 

 in the crop are as follows : 



Total loss of nitrogen 

 per annum, lb. per acre. 



Nitrogen in 

 crop. 



Difference, being dead 

 loss, lb. per acre. 



Minnesota ' 



Indian Head, Sask.^ 



170 



ICX) 



37h 



68 



Thus the cultivation of a virgin soil is a most extravagant 

 process ; it involves a great depletion of the stock of plant 

 food, only one-third or one-quarter of which is utilised. This 

 loss of nitrogenous organic matter reacts on the physical pro- 

 perties of the soil, and in course of time marked deterioration 

 sets in. It is important to point out that no method of reducing 

 these losses is known ; they go on in any rich soil — in gardens, 

 market gardens, heavily dunged fields, etc., and are the result 

 of bacterial processes, not of crop growth. Of course as soon 

 as cultivation ceases and the land is left in permanent grass 

 and clover mixtures for a few years, the accumulation processes 

 once more predominate and the soil is restored ; the process 

 may often be hastened by adding calcium phosphate. Thus it 

 happens that soil deterioration ceases when, in the development 



' Investigated by Snyder. 



' Investigated by Shutt. 



