BUREAU OF SOILS. 207 



and the process is now being tried out by commercial interests at 

 Anniston. Ala. 



A bulletin describing the work of this bureau on phosphoric acid 

 and phosphates is now nearing completion. This report describes 

 and discusses the various phosphate fertilizers, both natural and man- 

 ufactured, which are available for agricultural purposes, compares 

 and describes methods of manufacture, and gives in detail the results 

 of the research work of this bureau on methods of producing phos- 

 phoric acid suitable for fertilizer use. 



POTASH. 



During the summer of 1919 the attention of this bureau was di- 

 rected to extensive injur^^ to crops by the use of fertilizers containing 

 borax. The evidence collected at that time pointed to borax in the 

 potash from Searles Lake as the primary cause of the injury, and as 

 a precautionary step a careful check has since been kept on the purity 

 of the potash salts from this source. Analyses made at intervals of the 

 })roducts produced since the adoption in 1919 of a new process for 

 the purification of the potash show a borax content ranging from 

 0.59 to 0.07 per cent, as compared with a maximum of about 20 per 

 cent in the salts produced by the original process. These results 

 show that the borax in the potassium chloride now produced at 

 Searles Lake compares favorably with that found in sodium nitrate, 

 a nitrogen carrier long used in fertilizing the crops of this country. 



Borax has been the only deleterious substance found in American 

 potash, and the quality of the materials produced since 1919 shows 

 that the producers have met the requirements of the Department of 

 Agriculture and that there no longer exists the former danger to 

 crops from the presence of borax in fertilizers. 



SOIL PHYSICS. 



A great advance has been made in the study of the physical com- 

 position of soils. Much of the material that previously has been 

 classed in the sand and silt groups has been found to be made up of 

 colloidal aggregates. Work looking to the development of a new 

 method that will take into account these facts has been started. 

 Results of analysis according to the new method should express the 

 percentage of colloid present in the soil. This will be exceedingly 

 valuable, because it has been long known that the colloidal material 

 in a soil was perhaps the most significant portion of its physical 

 composition, though no method was known by which this amount 

 could be determined and expressed in an ordinary analysis. A 

 method should be furnished that will express the colloidal content 

 as a part of the mechanical analysis. 



Work has been continued in studying the relation of colloidal con- 

 tent to the mechanical composition of the rest of the soil. Briquets 

 of selected material mixed with colloid are tested for their break- 

 ing strength. These tests have shown the great influence of colloid 

 upon the bearing strength of soils and have indicated that the opti- 

 mum condition for cultivation is closelv related to the colloidal con- 



