238 CAUSES OF FERTILITY AND STERILITY [chap. 



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 

 residues in the soil which have become active : at the 

 same time, these reserves are protected from depletion 

 by renewal from the inactive portions of the current 

 manuring. On the other hand, if the land is in poor 

 condition the crop gets little or no assistance from 

 the soil, but is grown from the active part only 

 of the manure : the rest of it accumulates and begins 

 to build up condition, which, however, does not tell 

 on the yield for some time. As a practical conse- 

 quence, it is noticed that only land in good condition 

 gives a paying return year after year for the application 

 of manure : yet if the experiment be made of omitting 

 the manure on a portion of the land for one year, 

 there is little corresponding reduction of yield, as 



