154 



SOIL PHYSICS AND MANAGEMENT 



(a) Loss on Ignition." The ignition method is sometimes 

 used, but at the best is only approximate for peats and sands 

 which contain very little water of hydration. Five grains of water- 

 free soil is heated to low redness in a crucible till all organic matter 

 is burned away. Cool and moisten with a few drops of a saturated 

 solution of ammonium carbonate. Dry and heat to 150 C. to 

 expel excess of ammonia. The loss in weight is the organic matter, 

 water of hydration, and volatile substances. 



Loss on Ignition Compared with Organic Matter 10 

 [Calculated from organic carbon] 



The per cent of loss on ignition is seen to be much higher than 

 that obtained from the actual amount of organic carbon determined, 

 taking the organic matter as containing 58 per cent of carbon or 

 multiplying the per cent of carbon by 1.724. 



(b) Combustion in Oxygen. 10 The combustion method has 

 been used to some extent. The soil is placed in a porcelain or plati- 

 num boat and ignited in a combustion tube partly filled with cupric 

 oxide. The tube is connected with a series of bulbs, those of sul- 

 phuric acid for absorbing nitrous fumes and water and a weighed 

 potash bulb for absorbing the carbon dioxide formed during com- 

 bustion. A current of air from which the carbon dioxide has been 

 removed by passing through a potash bulb is drawn through the 

 tube by means of an aspirator. The amount of carbon dioxide pro- 

 duced is then determined by weighing the bulb, and the organic 

 matter found by multiplying the weight of carbon dioxide by 0.471. 



(c) The Chromic Acid Method. 11 The apparatus consists 

 of a train of flasks and bulbs arranged as shown in figure 79. A 

 current of air is drawn through the apparatus by an aspirator at /. 

 The carbon dioxide is removed from the air by a solution of potas- 

 sium hydroxide in the flask G. The combustion takes place in flask 

 F, into which about ten grams of soil are placed, together with five to 



