1890.] On the Question of the Fixation of Free Nitrogen. 87 



Accordingly, as stated in a postscript to our paper, dated October, 

 1888, it had been decided to institute experiments at Rothamsted on 

 similar lines, and a preliminary series was then in progress. A 

 second and more extended series has been conducted in the past 

 season, 1889. It is proposed to give, on the present occasion, a 

 description, and some of the numerical results, of the experiments 

 made in 1888, and a description, and some illustrations, of the growth 

 in those of 1889. 



It was in 1883 that Hellriegel commenced a comprehensive series 

 of vegetation experiments in pots, in which he grew agricultural 

 plants of various families in washed quartz sand. To all of the pots 

 nutritive solutions containing no nitrogen were added ; to one series 

 nothing else was supplied ; to a second a fixed quantity of nitrogen as 

 sodium nitrate ; to a third twice as much ; and to a fourth four times 

 as much was added. The result was that, in the case of the Graminea?, 

 and some other plants, the growth was largely proportional to the 

 nitrogen supplied, whilst in that of the Papilionaceas it was not so. 

 In the case of these plants, that of peas for example, it was observed, 

 however, that in- a series of pots to which no nitrogen was added, 

 most of the plants were apparently limited in their growth by the 

 amount of nitrogen which the seed supplied; whilst here and there 

 a plant growing ostensibly under the same conditions would develop 

 very luxuriantly ; and, on examination, it was found that whilst no 

 nodules were developed on the roots of the plants of limited growth, 

 they were abundant on those of the plants that grew luxuriantly. 



In view of this result, Hellriegel instituted experiments to deter- 

 mine whether, by the supply of the organisms, the formation of the 

 root-nodules, and luxuriant growth, could be induced, and whether by 

 their exclusion the result could be prevented. To this end, he added 

 to some of a series of experimental pots 25 c.c, or sometimes 50 c.c, 

 of the turbid extract of a fertile soil, made by shaking a given 

 quantity of it with five times its weight of distilled water. In some 

 cases, however, the extract was sterilised. In those in which it was 

 not sterilised there was almost uniformly luxuriant growth, and abun- 

 dant formation of root-nodules ; but with sterilisation there was no 

 such result. Consistent results were obtained with peas, vetches, 

 and some other Papilionaceae ; but the application of the same soil- 

 extract had no effect in the case of lupins, serradella, and some other 

 plants of the family which are known to grow more favourably on 

 sandy than on loamy or rich humus soils. Accordingly, he made a 

 similar extract from a diluvial sandy soil where lupins were growino- 

 well, in which it might be supposed that the organisms peculiar to 

 such a soil would be present; and, on the application of this to 

 nitrogen-free soil, lupins grew in it luxuriantly, and nodules were 

 abundantly developed on their roots. 



