62 The plant food in the soil 



soil. That is to say, there has been more change in 

 Pot 6 containing surface soil than in Pot 7 containing 

 subsoil. The "something," whatever it may be, that 

 changes plant remains like leaves, stems, pieces of grass, 

 roots, etc. into plant food therefore acts better in the 

 surface soil than in the subsoil. Here then we have 

 another difference between surface and subsoils. 



Summary. The experimental results obtained in 

 this chapter may now be summed up as follows: — 



(1) Plant food is present in the top soil only and 



not to any extent in the subsoil 



(2) There is not much present, so little indeed 



that we could not detect it by weighing. 



(3) It is, however, always being made in the top 



soil, if water is present. Only little is made 

 fi^om the subsoil. 



(4) The remains of leaves, stems, roots, etc. furnish 



an important source of plant food. 

 (6) But they have first to undergo some change, 



and the agent producing this change is 



more active in the top soil than in the 



subsoil. 

 (6) The top soil is much the most useful part of 



the soil and should never be buried during 



digging or trenching, but always carefully 



kept on top. 



