30 The Rothamsted Wheat Experiments. 



twenty years and the second twenty years so far as to 

 determine which of the two periods was on the whole the 

 more favourable for the growth of wheat. Nevertheless, 

 the foregoing illustration will serve to convey an idea of the 

 lines on which such an inquiry would be conducted. 



EFFECTS OF THE UNEXHAUSTED RESIDUE OF 

 MANURES. 



Having, in the report of the first twenty years, discussed 

 at length the amount and character of the produce obtained 

 in different seasons, the authors proceed in a second section 

 to inquire into the effects of the unexhausted residue from 

 previous manuring (both nitrogenous and mineral) upon 

 succeeding crops. The questions of the permanency of 

 effect of different manures, and of the tendency to exhaus- 

 tion which partial manuring may induce, are of great 

 practical importance, and are frequently discussed by prac- 

 tical men. By means of a series of tables and coloured 

 diagrams, a number of valuable conclusions are established, 

 of which it is only possible to give here a summary. 



It appears, then, that a somewhat heavy loam, of fair 

 average wheat-producing quality, taken at the end of a five- 

 course rotation since manuring, gave scarcely any increased 

 produce of wheat in the year of the application, when 

 manured with a mixture of silicate of potash and super- 

 phosphate of lime ; but it gave a very considerable, though 

 progressively diminishing, amount of increase, when after- 

 wards manured for nineteen consecutive years with ammonia 

 salts alone. Though, at the outset, the soil still contained an 

 excess of annually available mineral constituents, as compared 

 with the annually available nitrogen supplied by soil and 

 season without manure, yet the annual application of large 

 quantities of ammonia-salts made apparent the relative 

 deficiency of mineral constituents, even as early as the fourth 



