116 NATURE AND PROPERTIES OF SOILS 



that of Hilgard 1 and that of Houston and McBride 2 seem 

 most important. 



In the procedure an attempt is made to keep the concen- 

 tration of the ammonia in contact with the soil constant dur- 

 ing the extraction. Consequently the sample, after treatment 

 with the acid, is washed into a 500 cubic centimeter flask, 

 which is filled to the mark with 4 per cent, ammonia. Diges- 

 tion is allowed to proceed for twenty-four hours, with fre- 

 quent shakings. The solution is then filtered and evaporated 

 to dryness. The residue is weighed, after drying thoroughly 

 at 100° C, and then ignited, the loss being considered as 

 humus. 



This method is open to serious criticism in that it is wholly 

 arbitrary and subject to considerable inaccuracy through 

 manipulation and the ignition of the humic residue. There 

 is also some doubt whether the figures obtained have any 

 direct relation to the fertility of the soil. 3 ^ 



62. The organic matter and nitrogen of representative 

 soils. — The amount of organic matter in soils varies so widely 

 according to the nature of the soil and climate conditions that 

 it is difficult to present representative figures. Excluding 

 peat and muck, which are 20 to 80 per cent, organic, the aver- 

 age mineral surface soil is found to contain from .50 per cent, 

 to 18 or 20 per cent, of organic matter. Some surface soils 

 of West Virginia, 4 averaging 2.88 per cent, organic matter, 



F. J., and others, The Determination of Humus; Neb. Agr. Exp. Sta., 

 Bui. 115, June, 1910. 



1 Hilgard, E. W., Humus Determination in Soils; U. S. Dept. Agr., 

 Div. Chem., Bui. 38 (edited by H. W. Wiley), p. 80, 1893. 



"Houston, H. A., and McBride, F. W., A Modification of Grandeau's 

 Method for the Determination of Humus; U. S. Dept. Agr., Div. Chem., 

 Bui. 38 (edited by H. W. Wiley), pp. 84-92, 1893. See also, Smith, O. C, 

 A Proposed Modification of the Official Method of Determining Humus; 

 Jour. Ind. and Eng. Chem., Vol. 5, No. 1, pp. 35-37, Jan., 1913. 



•Gortner, R. A., The Organic Matter of the Soil; III. On the Pro- 

 duction of Humus from Manures; Soil Sci., Vol. Ill, No. 1, pp. 1-8, Jan., 

 1917. Carr, R. H., Is the Humus Content of the Soil a Guide to Fer- 

 tility ; Soil Sci., Vol. Ill, No. 6, pp. 515-524, June, 1917. 



4 Salter, R. M., and Wells, C. F., Analyses of West Virginia Soils; 

 W. Va. Agr. Exp. Sta., Bui. 168, Dec, 1918. 





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