THE PROPERTIES OF SOIL 43 



frame or is spread over an acre of land. 

 Probably the heat liberated in the soil by 

 the decomposition of such a moderate dressing 

 of manure as 10 tons per acre would be 

 difficult to detect by means of an ordinary 

 thermometer, but if the dressing of manure 

 were 50 tons or more per acre as is not 

 uncommon in intensive horticultural and 

 agricultural practice the influence on the 

 temperature of the soil would be quite 

 appreciable. The decomposing remains of a 

 former crop, as represented by plant roots, 

 stubble, etc., also give out heat, although 

 not so rapidly as more easily decomposable 

 farmyard manure ; but still, as a source of 

 heat, they have to be taken into account in 

 considering the various factors affecting 

 temperature. The production of heat in this 

 way is a consequence of oxidation, that is to 

 say, it is due to the union of the oxygen of 

 the air with some substance which either 

 contains no oxygen, or contains less oxygen 

 than it is capable of combining with. 



Although vegetable matter oxidizes more 

 rapidly than other substances in the soil, 

 it may be mentioned that low oxides of iron 

 and other elements can take up more oxygen, 

 and, in the process of combining, heat is 

 liberated to affect bodies in its neighbourhood. 



