11. I>. II1TCIIIN8ON 9S 



residues might to some extent subserve the process of nitrogen iixation. 

 This view accords with the conclusions formed by Henry (3), who was 

 able to demonstrate that when leaves are kept in a moist but aerated 

 condition a slight assimilation of atmospheric nitrogen takes place. This 

 was in the first instance disputed, but subsequently continued, by Horn- 

 bcrger(i). 



The experiments discussed in the following pages were originally 

 undertaken with the object of ascertaining in the first place whether 

 and in what manner these residues become available for the assimilation 

 of atmospheric nitrogen, and how far the conditions in the field are 

 favourable for the operation of the process to any appreciable extent. 



Since the discovery of such nitrogen fixing organisms as Closlrliliuin 

 Paslorianum by Winogradsky (5) in 1893, and more particularly Azolo- 

 baclcr chroococcuin by Beijerinck(<i) in 1901, a considerable amount of 

 work has been carried out, but chiefly with reference to the cultivation 

 of these organisms under laboratory conditions and with synthetic media. 

 Such work as that of Freudenreich("), C4crlach and Vogel(f), Remy(9), 

 Lohnis and his scholars (lO), Heiuze(ii), Lipman(i2), Haselhoilt and Brede- 

 mann(i3), Warmbold(i4), Stoklasa(i5), Schneider(iC) and many others 

 sufficed to show not only that these bacteria are widely distributed, but 

 that under certain conditions of temperature, aeration and food supply, 

 an energetic assimilation may be induced. 



Much of this work, though valuable from a physiological standpoint, 

 was somewhat lacking in general applicability, and it is mainly due to the 

 investigations of Koch and his collaborators (17) that we possess some 

 conception of the intensity of the process under more natural conditions 

 in the soil itself. 



In these experiments the addition of sugar to the soil resulted in gains 

 of 3-10 mgrms. of nitrogen per gram of carbohydrate supplied, but later 

 work showed that when calculated on the amounts of sugar actually 

 oxidised these increments were of a much higher order. The best 

 utilisation of the sugar occurred when dextrose was applied at the rate 

 of 2-0 per cent, of the weight of the soil, or when 0-2 per cent, was added 

 eight times, or 0-5 per cent, five or eight times. A limit to the efficiency of 

 these repeated applications was however soon reached, and the interest- 

 ing observation was made that when the lower proportions were more 

 frequently used or when the higher doses such as 1-0. 1-.5 or 2-0 per cent. 

 were applied five times, a proportionate decrease in activity ensued. 

 Part of this effect was, no doubt, due to the high concentration of sugar 

 in the soil water, but it may also be attributed, with a fair degree of 



