5o6 Journal of A gricultural Research voi. xii. no. s 



tracted from peat. In the great number of humus determinations made 

 by the Office of Biophysical Investigations in the Citrus areas studied in 

 California not a single humus extract obtained has been off-color, when 

 compared with the standard solutions prepared for comparison. 



Gortner (4) obtained a black soil pigment from the soils he studied by 

 extracting them successively with 4 per cent sodium hydroxid, i per cent 

 hydrochloric acid, 4 per cent sodium hydroxid followed by water. The 

 soil residue obtained from the last sodium-hydroxid extraction, when 

 shaken with water in quantity, yielded the black pigment. 



Soils from various Citrus areas studied in this work did not produce 

 black pigment in appreciable amounts when they had been extracted 

 with hydrochloric acid to the absence of calcium, boiled for two minutes 

 in a 7.5 per cent sodium-hydroxid solution, and washed with hot dilute 

 sodium-hydroxid solution on the filter till the "humus color" had been 

 removed. It might be noted that a boiling 5 per cent solution of sodium 

 hydroxid did not remove all the color from the soils, and that a 10 per 

 cent solution removed no more than a 7.5 per cent solution. 



Following Gortner's procedure, black pigments were obtained from 

 some of the soils investigated in the Citrus areas of southern California. 



It should be noted that the colorimetric reading should be made as 

 soon as the humus extract is obtained, as the color partially fades out 

 on standing. 



As no evidence was obtained to indicate that any coloring matter was 

 left in the soil after the extraction with boiling sodium hydroxid of the 

 strength noted, and as the color tint was always the same as that of the 

 standard used, it seems that this rapid method for humus determination 

 is reliable for comparative studies, especially when used with soils in 

 the same general area. 



Another objection to the colorimetric method is that undecomposed 

 organic matter, like dry alfalfa, sweet clover, etc., produces a color when 

 treated in the manner just described for mating humus determinations. 

 This objection is also brought out by Gortner (4). This matter was 

 looked into when the method was worked out. When dry hay susbtances 

 were extracted \\dth hydrochloric acid to the absence of calcium, the 

 chlorin washed out, and the residue boiled for several minutes with a 

 7.5 per cent solution of sodium hydroxid, the organic materials yielded 

 a yellow-colored solution entirely different in color from the brown to 

 black humus color. 



A number of readings of these organic extracts gave a density of color 

 corresponding to about 0.0012 per cent humus in soil, if it is assumed 

 that I per cent of the substances was to be added to the soil. This is 

 about the working error in making humus determinations by the method 

 used, as will be seen from Table II, and such errors could have no influ- 

 ence on comparative results in the kind of study here reported. Indeed 

 there could never have been much of this off-color in the extracts, as a 



