184 § 104. ANALYSIS OF SOILS AND EOCKS. 



set free by the oxidation of tlie humus by the chromic 

 acid. 



Since humus contains on an average 58"! ^ of carbon, 

 multiply the quantity of carbonic acid found by 0.4702, 

 for the amount of humus. 



The difference between the sum of the humus and the 

 nitrogen and the total loss suffered on ignition (§ 98) 

 gives the amount of water, chemically combined or other- 

 wise retained at 100° C. 



Some information in regard to the nature of the or- 

 ganic matter, and the extent to which decay has pro- 

 gressed, may be obtained by comparing the amount of 

 humus, or of the carbon in it, with that of the nitrogen, 

 by a microscopic examination of the various products 

 of the silt analysis and by the loss suffered by these c:a 

 ignition, and also by the following tests. 



1. The reaction of the soil or of the humus contained 

 in it, which is tested by allowing moistened lumps of the 

 soil to remain in contact with carefully prepared blue and 

 red litmus-paper ; a change from blue to red may be 

 caused by carbonic acid, but, if the red color remains 

 after the paper is thoroughly dry, the change was due to 

 acids of the humus, unless the soil gives the same reaction 

 after gentle ignition, in which case it may have been 

 caused by acid sulphates. 



2. Mix 100 grms. of the soil with 200 c.c. of a stand- 

 ard ammoniac al solution of calcic nitrate of such a 

 strength that 200 c.c. contain 1 grm. of lime, and an 

 amount of ammonia chemically equivalent to this amount 

 of lime. After frequent shaking of the mixture in the 

 course of 24 hours, filter, and determine lime in a measur- 

 ed quantity of the filtrate ; the lime that is missing was, 

 according to Knop, absorbed by the humus, and may be 

 taken as an approximate measure of the amount of the 

 same. 



3. To determine the amount of organic matter, mainly 



