312 



NATUEE AND PROPERTIES OF SOILS 



The methods of determining the amount of organic matter in 

 any soil have already been discussed (par. 60), the conclusion 

 being that the figure for organic carbon, or this figure multi- 

 plied by 1.724, was the most reliable indication of the organic 

 content of a soil. The bomb method is cited as one of the 

 more suitable procedures for obtaining 

 the organic soil carbon. 



The method for estimating the soil 

 humus, although it is not a bulk method, 

 should be considered at this point because 

 of its close relationship to the determina- 

 tion of organic carbon. The modified 

 Grandeau procedure (par. 61) is used for 

 humus estimation and is supposed to dis- 

 tinguish between the more active and less 

 active organic matter. Of the two 

 methods the determination of organic car- 

 bon is by far the more accurate. As 

 there is also some doubt about the com- 

 parative activity of the material ex- 

 tracted by the Grandeau procedure the 

 figure for organic carbon seems in general 

 the more significant and it is the deter- 

 mination usually made. The estimation 

 of soil humus may, therefore, be con- 

 sidered as a chemical method of sec- 

 ondary importance except in special 



Pig. 52.— Auger used 

 in the field exam- 

 ination of soil and 

 in taking samples. 

 Note modified cut- 

 ting edge. 



cases. 



The total nitrogen of the soil is determined by either the 

 Kjeldahl, the modified Kjeldahl, or by the Gunning method. 

 The determination of nitrogen is such a common laboratory 



is obtained. Where large areas are involved, as in the case of a soil 

 survey, only one sample, representative of the soil type being studied, is 

 usually taken. 



