36 The part that burns away 



brown or black combustible part is really mould formed 

 by the decay of plant roots, etc., then we should expect 

 that as the percentage of mould in the soil increased, so 

 its blackness would increase and its loss on burning 

 would become greater. This actually happens. 



This, then, is our idea. We suppose that the plants 

 that have lived in past years have decayed to form a 

 black material like leaf mould which stops in the soil, 

 giving it a darkish colour. The more mould there is, 

 the darker the colour of the soil. We know that along 

 with this decay there is a great deal of shrinkage. As 

 the black material is formed from the plant, it only 

 extends as far into the soil as the plant roots go, so 

 that there is a sharp change in colour about 6 inches 

 below the surface (see also p. 2). Like the plant the 

 black material all burns away when the soil is heated 

 sufficiently. 



Thus we can explain all the facts we have observed, 

 and in what seems a very likely way. This does not 

 show that our supposition is correct, but only that it is 

 useful When you come to study science subjects you 

 will find such suppositions, or hypotheses as they are 

 called, are frequently used so long as they are found 

 to be helpful. In our present case we could only get 

 absolute proof that the black combustible part of the 

 soil really arose from the decay of plants by watching 

 the process of soil formation. We shall turn later to 

 this subject. 



The black material is known as humus. Farmers 

 and gardeners like a black soil containing a good deal of 

 humus because they find it very rich, and we shall see 

 later on why this is so. Vast areas of such soils occurring 

 in Manitoba, in Russia, and in Hungary are used for 



