THE ROTHAMSTED EXPERIMENT 405 



croft field), the land became specially difficult to manage, remain- 

 ing persistently wet, and then drying out with an excessively hard 

 crust." 



From any estimates that can be based upon the percentages of 

 nitrogen found in samples of soil from these fields, very large in- 

 crease is shown; in fact, much larger than can be accounted for 

 by any existing knowledge concerning nitrogen fixation. It is 

 questioned if the samples collected in 1904 are strictly comparable 

 with those taken 20 years before, because of the increasing porosity 

 and looseness of the soil. Thus, the 9-inch stratum of 1881 might 

 occupy 10 inches or more in 1904. 



Director Hall estimates that Geescroft field (even without 

 legume plants) has gained a quantity of nitrogen " which at the 

 lowest reckoning amounts to about 25 pounds per acre per year," 

 and adds: 



"The nitrogen brought down in the rain would account for perhaps 5 Ibs. 

 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 Beijerinck and Winogradsky's Clostridium pasto- 

 rianum. Two other causes may be at work, the absorption of atmospheric am- 

 monia by soil and plant, and the rise of nitrates from the subsoil." 



In the author's opinion, the two most important factors involved 

 are the difficulty of securing comparable samples and the mechani- 

 cal addition of foreign substances, especially the dust of summer, and 

 the dirty, drifting snow of winter, light trash (leaves, weeds, etc.), 

 which blow about until they find a lodging place in such a small 

 " wilderness " as the abandoned portions of these fields furnish. 

 An extreme illustration of this is found in a Rothamsted note 

 concerning the potato tops on Hoos field in 1877: 



"Tops withered, not weighed, each lot spread on its own plot, but high wind 

 (October 14) blew all off before plowing." 



One experienced in farm practice will easily recall conditions 

 under which field dust is drifted by the wind. The extent varies 

 from the cloud which follows the harrow to the dust storm, during 

 which a field, even of clay loam, in certain mechanical condition, 

 may lose very appreciable amounts of its best soil, which requires 



