PRACTICE XVI 



DETERMINATION OF LOSS ON IGNITION 



The loss that a soil suffers when it is ignited is often taken as 

 a measure of its organic matter, but it can be only a very rough 

 approximation at best for most soils. For some subsurface and 

 nearly all subsoils it gives little or no idea of the amount of or- 

 ganic matter. By igniting, the organic matter, volatile salts, car- 

 bon dioxide of carbonates present, and water of hydration will be 

 driven off. In heavy clay soils and all fine-grained ones water of 

 hydration forms a very large part of the loss. Subsoils with 

 little or no organic matter may lose as much as surface soils, 

 due to the larger amount of clay and consequently a larger 

 amount of water of hydration which is driven off by the heat. 

 The more organic matter present in a soil or the greater the sand 

 content, the nearer the loss on ignition will correspond to the 

 actual organic content, so that for sandy and peat soils ignition 

 may give a close approximation to the amount of organic matter 

 present. 



Use the same soils as used in Practice XIII. Bring four small 

 crucibles (25 cc.) to a constant weight by igniting in a blast flame. 

 Weigh out 5-g. duplicate samples of each soil and ignite in the 

 crucibles at a low red heat for one hour in a muffle furnace. Cool 

 in a desiccator and weigh. Calculate the weight of water-free 

 soil used. Express the loss due to ignition in per cent of the 

 water-free soil. 



Which soil loses more ? Why ? 



If a coarse- and a fine-grained soil have the same organic con- 

 tent, which will lose more on ignition ? 



REFERENCES. 



" Soils," Lyon and Fippin, pp. 124-126. 

 w The Soil," Hall, p. 43. 



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