10 BULLETIN 1122, U. S. DEPARTMENT OF AGRICULTURE. 



Ammonia gas was not absorbed at all by eight minerals and appre- 

 ciable amounts were absorbed by only two, muscovite and limonite. 

 Malachite green was absorbed in considerable amounts by muscovite, 

 biotite, and clilorite, and to a lesser degree by tourmaline and several 

 others. Four of the commonest soil minerals, on the other hand, did 

 not absorb a measurable amount of the dye. Water vapor was 

 absorbed by all minerals, although significant amounts were ab- 

 sorbed b}^ only five. The minerals which are usuallj'' present in soils 

 in the greatest amount — quartz and the feldspars — showed practi- 

 cally no absorption for the dye, water, or ammonia. 



There appeared to be no correspondence between the amounts 

 of malachite green, water, or ammonia absorbed by the different 

 minerals. Rutile, for instance, was highly absorptive of water but 

 not of malachite green or ammonia. On the other hand, tourmaline 

 had a considerable absorption for malachite green, only a very slight 

 absorption for water, and no absorption for ammonia. Limonite, 

 which shows especially high absorption for water and ammonia, is 

 usually considered as being a colloidal mineral. These results agree 

 with Greenland's {11) conclusion that crystalline minerals as weU as 

 the gel minerals may show some absorption. 



While there is of course some proportionahty between the outer 

 surface exposed by any one mineral and its absorptive capacity, that 

 is the finer any particular mineral is the more it will absorb, it is- 

 apparent from the above table that the absorptive capacities of the 

 different minerals were not proportional to the outer surfaces ex- 

 posed. Each mineral seems to have a characteristic absorptive 

 capacity for the different substances. 



ABSORPTION BY THE MINERAL PARTICLES IN THE SOIL. 



With determinations available of the absorptive values of practi- 

 cally all the important unaltered minerals found in normal soils, it is 

 possible to make an approximate evaluation of the part such minerals 

 have in the total absorption of a soil, provided the mineral com- 

 position is known. ^ Thus, it is possible to ca,lculate what is probably 

 the truly noncoUoidal absorption of the fractions described under 

 ^'Estimation of noncolloidal absorption from soil fractions." With 

 this in view, a quantitative mineralogical examination was made of 

 the "fine fraction" in each of the soils. The results were as foUows: 



Huntington loam, soil, fine fraction, 50 per cent quartz, 2 per cent 

 other crystaUine minerals, and 48 per cent indeterminate coUoid-hke 

 material. 



Huntington loam, subsoil, fine fraction, 75 per cent quartz, trace of 

 other crystalline minerals, 25 per cent indeterminate colloid-like ma- 

 terial. 



Sassafras silt loam, subsoil, fine fraction, 60 per cent quartz, 1 per 

 cent muscovite, 1 per cent other minerals, 38 per cent indeterminate 

 colloid-like material. 



Sharkey clay loam, soil, fine fraction, 50 per cent quartz, 5 per cent 

 muscovite, 3 per cent other crystaUine minerals, 42 per cent indeter- 

 minate colloid-hke material. 



8 While optical methods are not applicable for the identification of minerals below 0.005 millimeter in 

 jfliameter, crystalline outline can usually be distinguished in material down to about 0.001 millimeter. 



