BUREAU OF SOILS. 377 



Progress was also made in developing methods for determining 

 the total quantities of colloids in soils. It was found that after all 

 colloid possible was removed from the soil the unextracted colloid 

 remaining in the residues could be estimated with considerable accu- 

 racy by microscopic count. The results of extraction and micro- 

 scopic count give a determination of the total colloid present in the 

 soil. A method of determining the colloidal content of a soil from 

 the relative adsorptive capacities of the soil and colloid for dye, 

 water, or ammonia was investigated in considerable detail. It was 

 found that the adsorptive capacity of the soil and colloid for water 

 vapor under certain conditions affords a fair estimation of the total 

 colloid in the soil. 



In the past it has been quite generally believed that soils as a rule 

 contain only 1 to 2 per cent of inorganic colloidal material. This 

 idea was based partly on early faulty methods of estimation and 



f)artly on peculiar interpretations of what constituted the soil col- 

 oids. By employment of the improved methods of estimation it 

 was found that soils may contain as high as 70 per cent of colloidal 

 material. 



While the properties of a soil are largely dependent on the total 

 quantity of colloids present, the character of the colloidal material 

 is also of great importance. Considerable progress has been made 

 in measuring quantitatively the properties of the colloids extracted 

 from different soils, and the investigations are being continued. 

 This fundamental knowledge of the soil colloids is very important. 

 In the past there has been considerable speculation regarding col- 

 loidal phenomena in soils mthout an exact knowledge of the prop- 

 erties of the substances supposed to be responsible for the phenomena. 

 A considerable volume of analytical work has been done on soils in 

 collaborative work with other bureaus and vnih other divisions of 

 this bureau. Many complete chemical analyses have been made 

 of the different strata present in virgin soils. This work is important 

 in the determination of soil characteristics and forms the basis in 

 part for the classification of many soil types. An investigation is 

 also being made of the variation in the mineralogical and coUoid 

 composition of different soil strata. In addition, chemical exami- 

 nations have been made of a wide variety of soils that are being used 

 by the Bm-eau of Standards in a test of the influence of the soil on 

 pipe corrosion. 



SOIL PHYSICS. 



The study of the influence of colloid on the physical condition 

 of the soil has been continued during this year, and a relation between 

 the coUoid content and the mineral content of the soil has been 

 estabhshed. This relation shows the great influence of the colloid 

 content upon the bearing strength of sofls, and may be of great 

 service in estimating the cohesion of the soil in problems related to 

 the use of soil material in drainage, irrigation, and other engineering 

 work and in tillage operations. A new, improved method of me- 

 chanical analysis has been worked out that will express the relative 

 amounts of coUoid and mineral grains in the soil. This method 

 gives better insight into the nature of the soil composition and is 

 applicable in the study of agricultural soils as well as soil material 

 to be used in engineering construction. 



