294 ANNUAL KEPORTS OF DEPARTMENT OF AGRICULTURE. 



necessary to revise our conception of the soil, and it may be profitable 

 to rework some of the older lines of soil investigation in the light of 

 this new knowledge. 



It seems impossible to extract all the colloidal matter from a soil, 

 and it is a very time-consuming operation to extract any considerable 

 part of it. The bureau, therefore, has attempted to devise indirect 

 methods for determining the amounts of colloids in different soils. 

 The absorption methods tested thus far probably all give a very fair 

 approximation of the colloidal content of soils. The method depend- 

 ing upon the absorption of water vapor seems particularly likely 

 to be one of general usefulness. In fact, in cooperative work with 

 the Geological Survey this method was used for indicating the ap- 

 proximate colloidal content of some 50 samples of deep-sea mud. 

 The results checked up well witli other evidence concerning the quan- 

 tity of colloid present. A description of this method will shortly 

 be published in the Journal of Physical Chemistry. 



In order to perfect the methods of determining soil colloids it 

 has been necessary to investigate many of the properties of the col- 

 loids, such as the degree to which thej^ are affected by aging and b}' 

 heat, their similarity to synthetic inorganic gels, and their absorptive 

 capacities as compared with those of finely ground minerals. 



It is expected that during the coming year the work that has been 

 accomplished on methods of determining soil colloids can be pre- 

 sented for publication. A report on the investigation concerning the 

 chemical composition and formation of the soil colloids will also be 

 completed. Results regarding absorption by colloidal and non- 

 colloidal soil constituents are now in process of publication. 



As in past years, about half the time of the division has been de- 

 voted to making complete chemical analyses of soils for the Soil 

 Survey and analyzing samples for other branches of the Government. 

 A large number of miscellaneous samples submitted for identifica- 

 tion or determination of one or two constituents were also examined. 



The identification of certain soil constituents and the research work 

 in many instances has been greatly facilitated by the use of petro- 

 graphic methods of examination. In the course of this work it has 

 been necessary from time to time to gather together the optical con- 

 stants of practically all the common inorganic salts. By the arrange- 

 ment of these constants in appropriate tabular form it is now pos- 

 sible to identify any of these inorganic salts within a relatively few 

 minutes. These tables are being published with the idea of facili- 

 tating the work of other chemists and of encouraging the adoption 

 of petrographic methods where they possess marked advantages over 

 chemical analysis. 



FERTILIZER INVESTIGATIONS. 



During the last fiscal year the bureau has continued its general 

 fundamental work to develop our resources in fertilizer materials. 

 It has furnished to farmers and others interested in fertilizers tech- 

 nical information regarding fertilizers, lime, and manure, their 

 sources, supply, manufacture, purchase, and mixing, and has advised 

 on many problems bearing on fertilizers that have arisen in the vari- 

 ous departments of the Government. Cooperative work with the 

 other departments of the Government and with other bureaus of 



