Fig. 22. Mustard growing in surface soil previously cropped with rye (Pot 1) and in surface soil previously uncropped (Pot 2)
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 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."
But to return to our experiment with Pots 1 and 2. They had been kept moist before the mustard was sown. Did this moisture have any effect on the soil? Take two of the pots that have been kept dry and uncropped, and two that have been kept moist and uncropped, also one of dry uncropped subsoil and one of moist uncropped subsoil. Sow rye or mustard in each pot and keep them all equally supplied with water.
It is soon evident that the top soil is richer in plant food than the subsoil, and the soil stored moist is rather richer than that stored dry, although the difference here is less marked. In an experiment in which the soils were put up early in July and sown at the end of September the weights of crops in grams obtained were:—
Green weight After drying
Plants grown in top soil stored in 16.9 2.6
moist condition (Pots 10 & 11) 18.9 2.8
Plants grown in top soil stored in 12.1 1.8
dry condition (Pots 8 & 9) 14.4 1.9
Plants grown in subsoil stored in
moist condition (Pot 13) 5.5 0.9
Plants grown in subsoil stored in
dry condition (Pot 12) 5.6 0.8
The crops on Pots 10 and 11 ought of course to weigh the same, and so should the crops on Pots 8 and 9. The differences arise from the error of the experiment. In all experimental work, however carefully carried out or however skilful the operator, there is some error.
There is clearly an increase in crop as a result of storing the surface soil in a moist condition, showing that additional plant food has been made, since these pots were put up. On the other hand it does not appear that much plant food has been made in the subsoil during this time. Further evidence on this point is given by an experiment similar to that in Fig. 22, but where mustard is grown in subsoil kept moist, but uncropped for some time, and in subsoil previously cropped with rye. The results in grams were:—
Green weight After drying
Mustard growing in subsoil
previously cropped with rye 12.6 2.27
Mustard growing in subsoil
previously uncropped 12.9 2.26
These should be compared with the figures on p. 45. Although the subsoil lay fallow for a long time it produced 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 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
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
