APPENDIX 629 



Two grams of ordinary soil (or \ to i g. of peaty soil) are placed to- 

 .gether with 10 g. of sodium peroxid in the Parr explosion bomb, 0.7 

 to i g. (ordinarily about 0.8 g.) powdered magnesium added to start 

 combustion, the whole thoroughly mixed by shaking, and the charge 

 exploded by means of a hot iron plug or an electric current. (.7 g. 

 magnesium is used with soils high in organic matter.) The contents 

 of the bomb are then washed into a beaker by means of a fine stream of 

 hot water and brought to a boil to break up the coarse particles and expel 

 as much oxygen as possible. It is then run from a separatory funnel into 

 a flask containing dilute sulfuric acid (i H 2 SO 4 to 2 H 2 O) and the gas 

 collected in a measuring pipette. When all of the sample has been added 

 to the sulfuric acid and boiled until it is decomposed, the flask is filled 

 with water through the separatory funnel to force the last of the gas into 

 the measuring pipette. After noting the volume, the carbon dioxid is 

 absorbed in the potassium hydroxid pipette and the volume again read. 

 In taking the initial and the final readings, the same precaution should 

 be taken as for carbonate carbon. Duplicate samples should check 

 within i cc. for every 100 cc. gas obtained, and corrections must be made 

 for pressure and temperature. 



A blank determination must be run on the sodium peroxid, and this is 

 best done by using first a 2-gram, then a i-gram sample of the same soil, 

 calculating the amount of carbon in the reagents from the difference in 

 results, e.g. 



Let x = carbon in reagents ; 



then, if 2 g. soil + x = 45 mg. C, 



and i g. soil + x = 25 mg. C, 



we get by multiplying the last equation by 2 



2 g. soil + 2 x = 50 mg. C, 



and, subtracting the first equation from this, we get 



x = 5 mg. C 



Much better results can be obtained by determining the blank in this 

 way than where no soil is used. 



The total carbon thus found minus the carbonate carbon is reported as 

 organic carbon and is taken as a measure of the organic matter present 

 in the soil. 



Nitrogen. Nitrogen is determined by the regular Kjeldahl method. 

 Ten grams of soil (5 g. if high in nitrogen) are weighed into a Kjeldahl 

 flask, 20 cc. sulfuric acid (more if necessary) and approximately .65 g. 



