100 



5 lbs. per acre per annum, a little more will come in the form of 

 dust, bird-droppings, and other casual increments, while some 

 may be due to fixation of atmospheric nitrogen by bacteria in the 

 soil not associated with leguminous plants, like the Azotobacter 

 chroococcum of Beyerinck and Winogradsky's Clostridium Pastoria- 

 num. The Azotobacter has been found abundantly in the Rotham- 

 sted soils, and as in the case of grass land like the present the 

 decaying vegetation would supply the carbohydrate which the 

 bacterium must oxidise in order to fix nitrogen, it is quite possible 

 that it may have effected considerable gains of nitrogen. Two 

 other causes may be at work, the absorption of atmospheric 

 ammonia by soil and plant, and the rise of nitrates from the 

 subsoil. To what extent the traces of ammonia in the atmos- 

 phere are absorbed by the soil, as distinct from the washing 

 down of ammonia by the rain, is still a matter of uncertainty, 

 the investigations of Kellner and of Schloesing indicate a com- 

 paratively high figure, about 40 lbs. per acre per ann m as a 

 maximum. But a gain of nitrogen from this source should be 

 even more in evidence on the arable than on grass land, yet 

 the unmanured plots on the arable land do not show any similar 

 amounts of nitrogen either in soil or in crop. Again, though 

 practically no nitrates are found in the drainage water immedi- 

 ately below grass land, both because nitrification is slow and the 

 living plant is active in taking up the nitrates as fast as they are 

 formed, yet nitrates are comparatively abundant in the permanent 

 subsoil water. No data exist on the subject, but it is not unrea- 

 sonable to suppose a certain amount of capillary creep of these 

 nitrates up to the zone where the surface vegetation could reach 

 them. Only by the capillary movements of subsoil nitrates, late- 

 rally or vertically, can one understand how trees in many places 

 continue to obtain the requisite nitrogen for their yearly increase. 

 However, from one cause or other, this Geescroft field during its 

 twenty years of lying in rough natural vegetation does show an 

 increase in fertility which is not entirely easy to account for on 

 ordinary lines. 



EXTRACTS FROM A PAPER ENTITLED "THE 

 EFFECTS OF PLANT GROWTH AND OF MA- 

 NURES UPON THE RETENTION OF BASES BY 

 THE SOIL* 



By A. D. HALL and N. H. MILLER of the Rothamsted Experimenta 

 Station (Laivcs Agricultural Trust). 

 The communication "deals with the changes in the amount of 

 calcium carbonate, the chief substance in the soil acting as a base, 

 which are brought about by natural agencies, by manuring, and 

 particularly by the growth of plants." 



The following summarised statement of results is appended : — 

 " I. Arable soils which contain upwards of I per cent, of cal- 

 cium carbonate are subject to a normal loss of that constituent in 



*Proc. R. Soc. B. Vol. LXXVII, p. 1-32, 1906. 



