BULLETIN 1122, U. S. DBPAETMENT OF AGRICITLTUEE. 



PREPARATION OF SOIL FRACTIONS. 



Our aim was to secure only two fractions of the soil, colloid and 

 noncolloid; but we found the separation was facilitated in some cases 

 by dividing the noncolloidal fraction into two groups — a coarser 

 fraction containing the gravel and sands and a finer fraction consist- 

 ing of particles ranging approximately from 0.001 to 0.050 millimeter 

 in diameter.^ The cofloidal fraction, as previously noted, was made 

 up of particles less than 0.001 millimeter in diameter. 



Fractions were obtained from four soils. Details of the procedure 

 were as follows: 



Samples of 50 to 100 grams of soil were used, depending on the 

 probable amount of colloidal matter present. The samples were 

 first agitated with distilled water containing ammonia (about 1 

 part of ammonium hydroxide to 3,000 parts HjO). The suspension 

 was allowed to settle for several hours and the supernatant liquid 

 containing the finer soil particles was then decanted and whirled for 

 45 minutes in a bottle centrifuge having a diameter of 22 inches and 

 a speed of 850 revolutions per minute. The material remaining in 

 suspension was poured into the jar containing the colloid fraction. 

 The residues from the centrifuge were combined with the residues 

 left after decantation and were rubbed gently with a rubber pestle, 

 having just enough water present to make a paste. About a liter 

 of water was then added and the subsidence and centrifuging re- 

 peated. After 40 to 60 such treatments the residues yielded only 

 the merest trace of colloid and it was evident that no more colloid 

 was to be extracted by these methods. The colloidal matter was 

 concentrated to a thin paste by drawing off the water through Pas- 

 teur-Chamberland filters, using suction. The paste of colloidal 

 matter, as well as the coarse residues, was then air-dried. 



ABSORPTION BY THE FRACTIONS. 



The amounts of the different fractions separated in this manner 

 from four soils of the humid region, together with the absorption 

 per gram of these fractions for malachite green, water vapor, and 

 ammonia gas are shown in Table 1. The quantities of the different 

 fractions obtained are expressed as percentages of the whole soil 



Table 1. — Absorptive capacities of different soil fractions. 





Part of 

 whole 

 soil. 



Absoi-ption per gram 

 of material. 



Description of soil fractions. 



Mala- 

 chite 

 green. 



Water. 



Am- 

 monia. 



Huntington loam, soil: 

 Colloid. 



Perct. 

 10.3 

 21.9 

 64.0 



13.3 

 19.4 

 63.3 



14.4 

 20.6 

 61.9 



42.4 

 53.1 



Gravi. 



0. 0949 

 .0288 

 .0114 



.0918 

 .0143 



Gram. 



0. 2221 

 .0476 

 .0215 



.2996 

 .0665 



Gram. 

 0. 0270 



Fine fraction 



.0098 



Coarse fraction 



.0062 



Huntington loam, subsoil: 



Colloid 



.0226 



Fine fraction 



.0072 



Coarse fraction 



. 0053 . 0292 



.0031 



Sassafras silt loam, subsoil: 



CoUoid 



.1398 

 .0222 

 .0053 



.3720 

 .0657 



.2732 

 .0546 

 .0041 



.3072 

 .0772 



.0293 



Fine fraction 



.0079 



Coarse fraction 



.0004 



Sharkey clay, soU: 



Colloid 



.0516 



Fine fraction^ 



.0172 







o Very little material above 0. 050 millimeter present. 



^ This group corresponds approximately to the combined silt and clay groups of the Bureau of Soils 

 classification, except that in this classification no lower limit is given for the clay (10). 



