26 THE FOOD OF PLANTS CHAP. 



make a shallow tray, and put it on the balance and 

 weigh it carefully. Note the weight. Put some 

 of the dried soil in it and weigh again. Subtract 

 the first weight from the second, and thus get the 

 weight of dried earth. 



Place the crucible or platinum dish on a pipeclay 

 triangle over a Bunsen flame or petroleum blowpipe, and 

 heat it strongly. 



The soil begins to smoke and burn. 



Keep up the heat until no more smoke is given 

 off, let it cool, and weigh it again. Again heat and 

 again weigh until the weight remains the same. It 

 is now much lighter than it was before, owing to the 

 loss of all the decaying remains of plants which 

 have been burnt away. 



We thus find that the soil consists of 

 two portions, the decaying remains of plants 

 or vegetable portion which burn away, and 

 the mineral portion, consisting of sand, stones, 

 clay, and other things which does not burn 

 away. 



The Subsoil contains little Vegetable Matter 



If we now dig a hole in the garden, we shall find 

 that the dark surface soil is only a few inches deep, and 

 that underneath we come to earth of a different colour. 



This is called the subsoil, and contains very little 

 decaying vegetable material. 



