92 CHEMICAL ANALYSIS OF SOIL 



mortar until it will all pass through a sieve with holes 

 I mm. in diameter. This is the soil used for analysis. 



70. Moisture. 



About 3 grams of the powdered soil are dried in 

 a porcelain basin in the steam oven until of constant 

 weight, or for a maximum period of twenty-four hours. 

 The loss in weight represents the hygroscopic water. 



Organic Matter, Nitrogen, etc. 



71. Loss on Ignition. 



The loss on ignition is intended primarily to measure 

 the amount of organic matter present, but unavoidably 

 includes also a quantity of the water of hydration, or 

 water combined with certain of the soil silicates. If 

 too high a temperature is used some of the carbonates 

 may also be decomposed. 



Procedure. — The residue from estimation of the 

 moisture (70) is heated over a very small flame from 

 a rose burner, or from an argand burner, preferably 

 in a platinum dish, for several hours, with occasional 

 stirring. The temperature should not be raised above 

 a dull redness, and after ignition and allowing to cool, 

 the mass is moistened with a few drops of ammonium 

 carbonate solution. The residue is again slowly heated 

 to about 150° C, to drive off the excess of ammonia, 

 allowed to cool in a desiccator, and weighed. 



72. Organic Matter. 



A more accurate determination of the organic matter 

 may be made by oxidising the soil with chromic acid and 

 measuring the amount of carbon dioxide given off. 



Procedure. — The apparatus described below (76) 

 for the estimation of carbonates may be used, with 

 slight alterations, for the estimation, which may follow 



