February, 1912. 



139 



Scientific Agriculture. 



practical importance and in this connec- 

 tion the long-continued application of 

 particular manures to the field plots 

 afforded some interesting data. For ex- 

 ample, ic is found that when nitrogen is 

 supplied as manure to the mangold crop, 

 as much as 78 percent, is recovered when 

 the nitrogen is pat on in the form of nit- 

 rate of soda, whereas the recovery from 

 the nitrogen of ammonium salts is only 57 

 per cent, and from farmyard manure as 

 little as 32 per cent. Nor is the deficit 

 left behind in the soil. On the wheat 

 field, for example, of the nitrogen sup- 

 plied as farmyard manure year after 

 year until the soil has become exceed- 

 ingly rich, only about one quarter has 

 been recovered in the crop and another 

 quarter accumulated in the soil, whilst 

 at least half has been lost by bacterial 

 conversion into nitrogen gas. How to 

 tune up this conversion factor and 

 obviate these losses is one of the most 

 important questions before the soil 

 chemist. 



Allusion has already been made to the 

 part played by bacteria in the soil in 

 bringing nitrogen into combination and 

 as a consequence of such discoveries the 

 whole point of view of the function of 

 the soil has been changed during the 

 history of the Rothamsted experiments. 

 It is no longer regarded as simply a 

 vehicle for the transference of nutrients 

 to the crop but as an active laboratory, 

 sometimes enriching, sometimes merely 

 circulating, but at other times impover- 

 ishing the capital embarked in the land ; 

 the relative magnitude of these processes 

 will become more and more under con- 

 trol as the conditions under which they 

 operate are better known. 



Another important outcome of this 

 vital view of the soil is the question of 

 the conservation of its fertility under 

 different systems of farming. We see 

 from the long-continued trials at 

 Rothamsted that under any scheme of 

 treatment the soil tends to arrive at a 

 state of equilibrium and to yield a crop 

 which only fluctuates with the seasons. 

 The unmanured wheat plot, for ex- 

 ample, has for years given a very cons- 

 tant yield of about twelve bushels to 

 the acre, the annual removal of nitrogen 

 being balanced by recuperative bacte- 

 rial actions. The dunged plot, on the 

 contrary, has reached another but much 

 higher position of equilibrium, in which 

 the normal annual increments of nitro- 

 gen 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 vegetations, such as the black 

 soils of the prairies or the fens, are put 



under arable cultivation without recu- 

 perative 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 condi- 

 tion must have prevailed under the old 

 system of farming before artificial ma- 

 nures or imported feeding stuffs were 

 available and when the fertility cf the 

 farm had to be self-supported. It is neces- 

 sary that we should be able to draw up a 

 similar balance sheet for various condi- 

 tions of farming, extensive and inten- 

 sive, 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 

 condition 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 soil, as for example the 

 accumulation of peat, can be strictly 

 paralleled and will eventually be ex- 

 plained 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 of the drainage waters of some of 

 the Rothamsted plots which have been 

 continuously receiving soluble com- 

 pounds of phosphoric acid shows that 

 the phosphoric acid is immediately ar- 

 rested and accumulates in the surface 

 layers of the soil, where indeed it re- 

 mains in a condition continuously avail- 

 able for the needs of the crop. Potash 

 compouuds, on the contrary, are washed 

 rather deeper into the soil and are not 

 so thoroughly arrested ; in both cases 

 the absorption is so complete that the 

 farmer may apply these manures in the 

 autumn and winter and need not fear 

 the loss of whatever is not taken up by 

 the first crop which occupies the land, 



