32 



Morris J. Blish 



show a comparison of these gravimetric results with colorimetric 

 determinations made on the same soils. In the majority of cases, 

 the results obtained by the different methods agree fairly well, 

 while in many instances they are strikingly close together. 



The photometric method, which has been previously described, 

 was tried but very briefly on transition soils, with only indifferent 

 success. It was tried only on the first foot samples of the com- 

 posites. The photometric method is convenient only for dark 

 solutions, and is not practical for soils containing less than i.oo 

 per cent humus. It is not only slower in operation than the 

 colorimetric method, but its results did not agree nearly so well 

 with those of the gravimetric method, as the accompanying table 

 shows. In soils of comparatively high humus content, however, 

 it will give a fair estimation of the amount of humus per cent, as 

 Alway and Pinckney demonstrated when they examined a large 

 number of soils by this method several years ago. In this method 

 it seems especially desirable that the standard be from the same 

 locality as the soil under examination. The following table 

 shows the agreement of the photometric with the gravimetric 

 results obtained from determinations on the first foot of the 

 composites from the transition series. ' 



Table XVIII. Comparison of Photometric and Gr.wimetric Humus 

 Determin.vtions 



Samples of first foot composites from Lincoln, Weeping 

 Water, Hastings and Holdrege were extracted with 4 per cent 

 ammonia by the Hilgard method and it took fourteen days for 

 a complete extraction. Humus determinations were made both 

 colorimetrically and gravimetrically and the results compared with 

 the results using " Rather jMethod " extractions from the same 

 soils. Gravimetrically, the results were fairly concordant, but 



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