HOFFMANN: SOIL PRODUCTIVITY. 89 



some experimentation, the paraffin block method 17 was devised 

 and subsequently used in all work herein reported. 



Several preliminary experiments were performed in an en- 

 deavor to ascertain whether bacterial growth would modify 

 the soluble chemical content of the soil solution as determined 

 by the effect produced upon the growth of seedlings in the 

 respective soil extracts. The general procedure employed was 

 as follows : 



For any given soil, 1000-gram portions of air-dry soil in 

 large glass containers, were sterilized in the autoclave. In all 

 cases one 1000-gram portion was left unsterilized to secure a 

 control on the action of the normal soil flora. After cooling, 

 the respective containers were inoculated with equal quantities 

 of suspensions of the organisms employed, and, in addition, 

 one with a suspension of the normal soil. The unsterilized 

 soil sample received the same quantity of liquid used above, 

 but in a sterile condition. Sufficient water was always added 

 to give a moderately moist condition. The per cent employed 

 was, of course, dependent upon the nature of the soil, relatively 

 small quantities being employed for the sandy soils whereas 

 the marsh and peat soils received a comparatively large per 

 cent. 



In addition to the above a sterile control for each soil was 

 employed. This was made by adding to one container of steril- 

 ized soil, the same quantity of sterilized water which was 

 employed in the preparation of the inoculating suspensions for 

 the other containers. 



The soil cultures thus prepared were incubated at room 

 temperature for not less than seven days and usually for two to 

 four weeks. To the cultures 1000 cc. of distilled water were 

 then added. In the earlier work the mass was thoroughly and 

 vigorously shaken and then filtered through a clean Berkefeld 

 filter. In filtering it was found more expedient to add the soil- 

 water mixture directly, rather than to attempt to siphon off 

 the more or less clear supernatant liquid which collected on 

 standing, and then filtering. Filtration was far more rapid. 

 The soil added in the first method appeared to aid in the 

 filtration and to retard the clogging-up of the Berkefeld. In 

 the later work, however, it was found just as efficient to filter 

 through ordinary filter paper, using Schleicher and Schiill's 



17. C. Hoffmann, Botanical Gazette 55, No. 3, Mar. 1913. Centbl. Bact.. 2 Abt.. 

 Bd. 34, 1912. 



