Chapter 6
THE PLANT FOOD IN THE SOIL
Apparatus required.
_The pot experiments (p. xiii)._
It is a rare sight in England to see land in a natural uncultivated state devoid of vegetation. The hills are covered with grasses and bushes, the moors with ling and heather, commons with grass, bracken and gorse, a garden tends to become smothered in weeds, and even a gravel path will not long remain free from grass. It is clear that soil is well suited for the growth of plants. We will make a few experiments to see what we can find out about this property of soil.
We have seen that a good deal of the soil is sand or grit, and we shall want to know whether this, like soil, can support plant life. We have also found that the subsoil is unlike the top soil in several ways, and so we shall want to see how it behaves towards plants. Fill a pot with soil taken from the top nine inches of an arable field or untrenched part of the garden; another with subsoil taken from the lower depth, 9 to 18 inches, and a third with clean builder's sand or washed sea-sand. Sow with rye or mustard, and thin out when the seeds are up. Keep the pots together and equally well supplied with water; the plants then have as good a chance of growth in one pot as in any other.
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Figs. 20 and 21 are photographs of sets of plants grown in this way; the weights in grains were:--
Green weight After drying
Rye Mustard Rye Mustard
Plants grown in top soil (Pot 3) 14.5 17.7 5.6 2.6
" " " subsoil (Pot 4) 2.9 5.1 1.6 1.1
" " " sand (Pot 5) 2.0 4.6 0.8 1.0
The plants in the soil remained green and made steady growth. Those in the sand never showed any signs of getting on, their leaves turned yellow and {44} fell off; in spite of the care they received, and the water, warmth and air given them, they looked starved, and that, in fact, is what they really were. Nor did those in the subsoil fare much better. The experiment shows that the top soil gives the plant something that it wants for growth and that it cannot get either from sand or from the subsoil; this something we will call "plant food."
Further proof is easily obtained. At a clay or gravel pit little or no vegetation is to be seen on the sloping sides or on the level at the bottom, although the surface soil is carrying plants that shed innumerable seeds. A heap of subsoil thrown up from a newly made well, or the excavations of a house, lies bare for a long time. The practical man has long since discovered these facts. A gardener is most particular to keep the top soil on the top, and not to bury it, when he is trenching. In levelling a piece of ground for a cricket pitch or tennis court, it is not enough to lift the turf and make a level surface; the work has to be done so that at every point there is sufficient depth of top soil in which the grass roots may grow.
How much plant food is there in the top soil? To answer this question we must compare soil that has been cropped with soil that has been kept fallow, i.e. moist but uncropped. Tip out some of the soil that has been cropped with rye, and examine it. Remove the rye roots, then replace the soil in the pot and sow with mustard; sow also a fallow pot with mustard. Keep both pots properly watered. The soil that has carried a crop is soon seen to be much the poorer of the two. Fig. 22 shows the plants, while their weights in grams were:--
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Green weight After drying
Mustard growing in soil previously cropped with rye, Pot 1 17.8 62.3
Mustard growing in soil previously uncropped, Pot 2 3.3 8.6
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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."
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:--
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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
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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
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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
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 {51} could push the experiment still further and find out more about plant food, but this involves chemical problems and must be left alone for the present. We can, however, say that plant remains are an important source of plant food, and since we suppose the black material of the soil to be made of plant remains (see p. 36), it will be quite fair to say also that this black material, the humus, is a source of plant food. We have therefore answered the question we set, and we can explain some at any rate of the differences between the surface soil and the subsoil. The surface soil contains a great deal of the black material, which forms plant food, while the subsoil does not. Thus plants grow well on the surface soil and starve on the subsoil. We can also explain why gardeners and farmers speak of black soils as rich soils; they contain more than other soils of this black material that makes plant food. Still further, we can explain why the farmer often sows plants like mustard, tares or clover, and then ploughs them into the ground. They are not wasted, but they make food for the next crop that goes in.
Now let us turn to the results of the subsoil experiments. The leaves and stems have increased the crop, but only by 5.4 grams: they have not been nearly so effective as in the surface soil. It is evident that the mustard did not feed directly on the leaves and stems put in; if it had there should have been an equal gain in both cases. The leaves and stems clearly have to undergo some change before they are made into plant food and the soil has something to do with this change. After the crops are cut the soils should be tipped out and examined. More of the original pieces of leaf and stem are found in the subsoil than in the surface {52} soil. That is to say, there has been more change in Pot 6 containing surface soil than in Pot 7 containing subsoil. The "something," whatever it may be, that changes plant remains like leaves, stems, pieces of grass, roots, etc. into plant food therefore acts better in the surface soil than in the subsoil. Here then we have another difference between surface and subsoils.
SUMMARY. The experimental results obtained in this chapter may now be summed up as follows:--
(1) Plant food is present in the top soil only and not to any extent in the subsoil.
(2) There is not much present, so little indeed that we could not detect it by weighing.
(3) It is, however, always being made in the top soil, if water is present. Only little is made from the subsoil.
(4) The remains of leaves, stems, roots, etc. furnish an important source of plant food.
(5) But they have first to undergo some change, and the agent producing this change is more active in the top soil than in the subsoil.
(6) The top soil is much the most useful part of the soil and should never be buried during digging or trenching, but always carefully kept on top.
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