The rye has taken most of the plant food that was in Pot 1 leaving very little for the second crop. Our soil therefore contained only a little plant food, not more, in fact, than will properly feed one crop. But yet it did not seem to have altered in any way, even in weight, in consequence of the plant food being taken out. In our experiment the soil was dried and weighed before and after the mustard was grown; the results were:-- Pot 2 Pot 2_a_ lbs. oz. lbs. oz. Weight of dried soil before the experiment 6 6 6 7 " " " after " " 6 6 6 6 ------ ------ Difference 0 0 0 1 The experiment is not good enough to tell us exactly how much plant food was present at the beginning. But we can say that the amount of plant food in the soil is too small to be detected by such weighing as we can do. Here is an account of a similar experiment made 300 years ago by van Helmont in Brussels, and it is interesting because it is one of the first scientific experiments on plant growth:-- "I took an earthen vessel in which I put 200 pounds of soil dried in an oven, then I moistened with rain water and pressed hard into it a shoot of willow weighing 5 pounds. After exactly five years the tree that had grown up weighed 169 pounds and about 3 ounces. But the vessel had never received anything but rain water or distilled water to moisten the soil (when this was necessary), and it remained full of soil which was still tightly packed, and lest any dust from outside should have got into the soil it was covered with a sheet {47} of iron coated with tin but perforated with many holes. I did not take the weight of the leaves that fell in the autumn. In the end I dried the soil once more, and got the same 200 pounds that I started with, less about two ounces. Therefore the 164 pounds of wood, bark and root arose from the water alone." The experiment is wonderfully good and shows how very little plant food there is in the soil. The conclusion is not quite right, however, although it was for many years accepted as proof of an ancient belief, which you will find mentioned in Kingsley's _Westward Ho!_, that all things arose from water. It is now known that the last sentence should read, "Therefore the 164 pounds of wood, bark and root arose chiefly from the water _and air_, but a small part came from the soil also."