252 SCIENCE PROGRESS 



fluctuates with the seasons. The unmanured wheat plot, for 

 example, has for years given a very constant yield of about 

 twelve bushels to the acre, the annual removal of nitrogen being 

 balanced by recuperative bacterial actions. The dunged plot, 

 on the contrary, has reached another but much higher position 

 of equilibrium, in which the normal annual increments of nitrogen 

 from the manure are disposed of by the accelerated bacterial 

 actions that set nitrogen gas free. Similar wastage takes place 

 when soils rich in residues of previous vegetation, such as the 

 black soils of the prairies or the fens, are put under arable 

 cultivation without recuperative crops. On one of the rotation 

 plots another equilibrium has been reached by which the land 

 is able to yield crops on a level of about twenty-eight bushels 

 of wheat per acre without an extraneous supply of nitrogen, 

 by the growth, once in a rotation of four years, of a crop of 

 clover. An analogous condition must have prevailed under the 

 older system of farming before artificial manures or imported 

 feeding stuffs were available and when the fertility of the farm 

 had to be self-supported. It is necessary that we should be able 

 to draw up a similar balance sheet for various conditions of 

 farming, extensive and intensive, so as to be able to decide, at 

 each level of fertility, how far the necessary nitrogen taken 

 away in the crops or wasted can be supplied by the land itself 

 or must be brought in from without. 



Towards the end of his life indeed Lawes was wont to 

 maintain that the chief problem remaining for solution at the 

 Rothamsted Experimental Station was the fate of the soil 

 under the long-continued treatment it had received. We 

 have seen what complexity the question assumed as regards 

 nitrogen. There are, however, other issues. For example, 

 it has been found that repeated applications of ammonium 

 salts set up an acid condition in the soil ; as this acid con- 

 dition suspends the development of bacteria and substitutes for 

 them another race of micro-organisms, light is thrown upon 

 the special difficulties which are encountered in farming land 

 which is acid by nature. Many of the actions going on in acid 

 soils, as for example the accumulation of peat, can be strictly 

 paralleled and will eventually be explained by some of the 

 Rothamsted plots which have been made acid under known 

 conditions. Again, the fate of other manurial constituents 

 besides nitrogen is important. Investigation of the soil and 



