Dec. 2, 1918 Relation of Inorganic Soil Colloids to Plowsole 



509 



ammonia extract, and that practically all the iron was present in that 

 form. It would also appear that practically all the silica and aluminium 

 in the plowsole layer were colloidal. 



Fraps (4)^ used ammonium carbonate to precipitate the inorganic soil 

 colloids from ammoniacal solutions. It was found in this work that a 

 little more inorganic material was obtained by precipitating the extract 

 successively with nitric acid and ammonium carbonate. The acid car- 

 ried down practically all the organic matter as well, as judged by color. 



The following experiment shows the results obtained by precipitating 

 the colloids from ammonia extracts of a soil, with nitric acid and am- 

 monium carbonate as precipitants. Two 150-cc. portions of a composite 

 ammonia humus solution were treated separately with nitric acid and 

 ammonium carbonate. The filtrate from the acid-treated extract was 

 then treated with ammonium carbonate, and the filtrate from the car- 

 bonate-treated extract was treated with nitric acid. Each separate 

 precipitate was ignited and weighed. The results obtained are given in 

 Table IV. 



Table IV. — Comparison of efficiency of nitric acid and ammonium carbonate in pre- 

 cipitating inorganic colloids from, ammonia extracts of soil 



It appears that these two precipitants were about equally effective in 

 precipitating the inorganic colloids from humus solutions. When the 

 two were used successively, an appreciable increase in inorganic material 

 was obtained. The results of the first precipitations would indicate that 

 the flocculated organic matter caused by addition of nitric acid did 

 not carry down or occlude noncoUoidal mineral substances. 



Samples of soil mulch, plowsole, and subsoil from four other orange 

 groves not included in the discussion so far, were extracted with hydro- 

 chloric acid till calcium was absent, washed, and extracted with ammo- 

 nium hydrate. The inorganic colloids were precipitated as just described, 

 ignited, and weighed. The average percentage of total inorganic soil 

 colloids in the humus extracts of the separate soil tamples from the four 

 groves was as follows: Soil mulch, 0.0642; plowsole, 0.0912; subsoil, 

 0.073. 



These results agree with those given in Table III, in showing that an 

 accumulation of ammonia-soluble inorganic soil colloids occurs in the 

 plowsole layers. 



1 Reference is made by number (itaUc) to "Literature cited," p. 518-519. 



