276 CAUSES OF FERTILITY AND STERILITY [chap. 



soil in these bacteria and the rate at which it will 

 produce nitrates. 



The condition of land cannot be restored all at once 

 by manuring ; the residues of manures left in the soil 

 after the first season are slow- acting, i.e., only a small 

 proportion of them becomes available year by year, 

 so that there must be a considerable accumulation of 

 such residues before the proportion becoming avail- 

 able during the period of growth is sufficient for the 

 crop. Per contra, the condition can be only too easily 

 destroyed by cropping without manure ; the unexhausted 

 residue left after each year is successively less and less 

 active, the crop falls off rapidly, till at last a sort of 

 stationary condition is reached, and the somewhat inert 

 materials, still left in large quantity, liberate year by year 

 a fairly constant proportion of active plant food. The 

 plots at Rothamsted which have been cropped without 

 manure for more than fifty years show but little less 

 average production during the last twenty years than in 

 the twenty immediately preceding. For example, the 

 unmanured wheat plot shows the following crop in 

 bushels of dressed grain : 



Condition may best be regarded as a state of equi- 

 librium when the land will continue to give a good return 

 in crop for the manure applied ; as a rule, the crop 

 recovered by no means contains the whole of the 

 material applied as manure, a certain portion being 

 retained in a comparatively inactive form. With the 

 land in condition the remaining nutrient material 

 required for a good crop is supplied by the dormant 



