5o8 



Journal of Agricultural Research 



Vol. XV, No. 9 



Table II. — Analysis of humus extracts of soil mulch, plowsole, and subsoilfrom orange 

 groves. Average percentage of eight groves 



There is no evident accumulation of these elements in the plowsole 

 that would account for its formation. 



Another set of ammonium extracts was made from samples of the 

 same soils. The colloidal material was precipitated with a little nitric 

 acid, and filtered out. The filtrate was next treated with a little am- 

 monium carbonate, and again filtered. The two precipitates were 

 taken up with a little ammonium hydrate, evaporated to dryness, and 

 ignited just enough to bum off the organic matter. The ignited mate- 

 rial was digested as before with nitrohydrochloric acid on the water 

 bath until no perceptible residue remained, and the solution was analyzed. 

 Table III gives the results of the partial analyses made, each figure 

 being the average percentage found in soils from eight different groves. 



Table III. — Analysis of inorganic colloids in humus extracts of soil mulch, plowsole, 

 and subsoilfrom orange groves. Average percentage in soils from eight groves 



Soil layer. 



Mulch . . . 

 Plowsole 

 Subsoil. . 



Percentage on basis of dry soil. 



Silicon 

 dioxid. 



O. 016 



.030 

 .015 



Ferric oxid. 



o. 005 



.008 

 . 006 



Aluminiiun 

 oxid. 



O. 016 



• 025 

 .018 



Phosphorus 

 pentoxid. 



O. 004 

 . 004 

 .003 



Silica, iron, 



and altunin- 



ium. 



0.037 

 • 063 

 •039 



There is thus an accumulation of colloidal iron, aluminium, and silica 

 in the plowsole layer. The detailed analyses showed this to be con- 

 sistently the case for each grove. 



The analyses given in Tables II and III were made on ammonia ex- 

 tracts of the same composite samples, but each table represents a separate 

 extraction on fresh samples. Hence, the ratio of the colloidal elements 

 to the total amounts of these same elements determined in the ammonia 

 extracts can not be definitely stated from the data obtained. A com- 

 parison of Tables II and III, however, would indicate that only about 

 one-third of phosphoric acid was present in the colloidal form in the 



