The plant food in the soil 



49 



These should be compared with the figures on p. 45. 

 Although the subsoil lay fallow for a long time it pro- 

 duced no plant food but is just as poor as the subsoil 

 that has been previously cropped. These observations 

 give us a clue that must be followed up in answering 

 our next question. 



What has the plant food been made Arom? 

 Clearly it is not made from the sand, the clay or the 

 chalk since all these occur in the subsoil. We have 

 seen (Chap. I.) that the top soil differs from the subsoil 

 in containing a quantity of material that will burn 

 away and is in part at any rate made up of plant 

 remains. It will be easy to find out whether these 

 remains furnish any appreciable quantity of plant food. 



Fill one pot with surface soil and another with the 

 same weight of surface soil well mixed up with 30 grams 

 of plant remains — pieces of grass, or stems and leaves 

 of other plants cut up into fragments about half an inch 

 long. At the same time put up two pots of subsoil, 

 one of which, as before, is mixed with 30 grams of plant 

 remains, and also put up two pots of sand, one contain- 

 ing 30 grams of plant remains and the other none. 

 Sow all six pots with mustard and keep watered and well 

 tended. The result of one experiment is shown in 

 Fig. 23 and the weights of the crop in grams were : — 



