50 



The plant food in the soil 



Fig. 23. 

 Pieces of grass, leaves, etc. change into plant food in the surface but not 

 to any great extent in the subsoil. Mustard is growing in surface 

 soil (Pot 3), in surface soil and pieces of grass (Pot 6), in subsoil 

 (Pot 4), and in subsoil and grass (Pot 7) 



Now let us look at these results carefully. The 

 experiment with surface soil shows that the pieces of 

 stem and leaf have furnished a good deal of food to the 

 mustard and have caused a gain of 24*3 grams in the 

 crop. If we knew what the pieces were made of we 



