What has the plant food been made from? 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 grains of plant remains, and also put up two pots of sand, one containing 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:-- Green weight After drying Top soil and pieces of plants (Pot 6) 42.0 5.0 Top soil alone (Pot 3) 17.7 2.6 Difference in top soil 24.3 2.4 {50} Green weight After drying Subsoil and pieces of plants (Pot 7) 10.5 1.9 Subsoil alone (Pot 4) 5.1 1.1 Difference in subsoil 5.4 0.8 [Illustration: 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)]