'292 ANNUAL REPOllTS OF DEPARTMENT OF AGRICULTURE. 



been determined. It is ;i well-known I'act that in u complex solution- 

 of tiiis kind the addition of another salt may more or less completely 

 ('lian<2:e the system. Temperature and other factors also enter in 

 chaniriiiii: the forms that are loft on evaporatiiifr such a solution to 

 dryness. It is at least probable that the addition ol" fertilizer ma- 

 terials to the soil may change the system, and that they may tend 

 to change the relation of the soil to the life activities of the plant. 



This inAcstij'ation shonld be pushed murli furtlier tlian is possible 

 with the limited funds now provided. 



In working np large lots of soil for the extraction of these mineral 

 bodies considerable trouble was encountered owing to the presence 

 of colloidal matter that had to be disposed of. This material has 

 itself been studied, and it has been found possible to separate it by 

 the use of the centrifuge from the insoluble mineral particles. Avhen 

 it appears as a true colloid, very sticky and plastic when wet and 

 having in the dry state the general appearance of a resin. It appears 

 to be a silicate of alumina, usuallj' with some iron and a trace of 

 potassium, sodium, magnesium, and calcium, Avhether combined or 

 merely absorbed having not yet been determined. We have desig- 

 nated this material " ultra-clay." 



When this material is separated completely from the mineral par- 

 ticles of the soil, the soil, even the most plastic soil, appears to lose 

 much of its plastic properties. When this material, up to 10 per cent 

 of the weight of the sand, is added to a loose and incoherent sand, 

 made into briquettes, and dried, it gives to the sand a crushing 

 strength greater than if an equal amount of Portland cement had 

 been, added. There is, however, this difference, that the dried 

 briquettes made up with Portland cement do not fall to pieces on 

 adding water, while the briquettes made up w^ith ultra-clay when 

 put in water fall to pieces. 



It appears evident from the results so far obtained that this is the 

 material that gives plasticity and binding power to ordinary soils. It 

 is, therefore, of vital importance that a full and comprehensive study 

 be made of the ultra-clay from different soil tj^pes and that if pos- 

 sible some way of controlling its adhesive powers be discovered. 

 This has an important bearing upon the physical properties of soils, 

 and is of importance to the engineer in connection with the foundation 

 of buildings and of roads. The bureau is cooperating wdth the Bu- 

 reau of Public Eoads along these lines. 



It has been found that ultra-clay is very absorbent of mineral mat- 

 ters and of certain organic matters, and that in the dry state it will 

 absorb under similar conditions as much ammonia gas as will char- 

 coal. 



The division of soil chemistry must prepare quantities of this rna- 

 teral from our principal soil types and study its chemical constitution 

 both as a basis for determining the differences in soils and to see if 

 its properties can be modified at will. 



SOIL PHYSICS. 



A part of the personnel of the Division of Physical Investigations 

 of Soils has continued to cooperate with the War Department in the 

 study of the Haber process for nitrogen fixation. Results of these in- 

 vestigations are noted under the report of "Fertilizer resources." 



