In the preceding chapters it is shown that the soil is normally inhabited by a very mixed population of organisms, varying in size from the smallest bacteria up to nematodes and others just visible to the unaided eye, on to larger animals, and finally earthworms, which can be readily seen and handled. These organisms all live in the soil, and therefore must find in it the conditions necessary for their growth. We have dealt in the first chapter with the supplies of water, air, and heat, without which life is clearly impossible. Equally necessary is the source of energy, for the organism requires energy material as surely as the motor engine requires petrol, and it ceases to function unless an adequate supply is forthcoming.
All the energy comes in the first instance from the sun, if we exclude the unknown but probably small fraction coming from radio-active elements. But this radiant energy is not utilisable by the soil population, excepting surface algæ; it has to be transformed into another kind. So far, chlorophyll is the only known transformer; it fixes the energy of sunlight and stores it up in bodies like hemicellulose, sugar, starch, protein, etc. The transformation is imperfect; even the heaviest yielding crops grown under glass, in conditions made as favourable as our knowledge permits, utilise only about 4 per cent. of the total energy available during their period of growth; in natural conditions not more than 0·4 per cent. is utilised. Such as it is, however, the energy fixed in the plant represents all, indeed more than all, that the soil organisms can obtain.
In the state of Nature, vegetation dies and is left on the soil. Two things may then happen. It may become drawn into the soil by earthworms and other agents; the energy supply is thus distributed in the soil to serve the needs of the varied soil population. This is the normal case, associated with the normal soil population and the normal flora. If, however, the mingling agents are absent, the dead vegetation lies like a mat on the surface of the soil, only partially decomposing, unsuitable for the growth of most seedlings, and effectually preventing most of the vegetation below from pushing a way through: thus there comes to be no vegetation at all, or only a very restricted and special flora. The soil population becomes also specialised. Peats and acid grassland afford examples.
On the neutral grass plots at Rothamsted, the dead vegetation does not accumulate on the surface but is rapidly decomposed or drawn into the soil, leaving the surface of the earth bare and free for the growth of seedlings. On the acid plots dead vegetation remains long on the surface, blotting out all new growth excepting two or three grasses which form underground runners capable of penetrating the mat, and sorrel, the seedling roots of which seem to have the power of boring through a fibrous layer of this sort. It is possible to remove the mat entirely by bacterial action alone, if sufficient lime be added periodically to make the reaction neutral, but failing these repeated additions the mat persists.
We shall confine ourselves to the normal case where earthworms bring the source of energy into the soil.
Directly the energy is available, it begins to be utilised. Two laws govern the change. The first is well-known to biologists: it states that the total energy of the system remains constant and can neither be increased nor diminished except from outside; in other words, that energy can be neither created nor destroyed. The second law is less familiar: it is that energy once transformed to heat by one organism cannot be used again by another. It is not destroyed; it remains intact, but is useless to the organism. One cannot have an indefinite chain of organisms living on each other’s excretory products; there was a certain quantity of energy in the food eaten by the first, and no more than this quantity can be got out whether one organism obtains the whole or whether others share it.
The outside value for the amount of energy fixed in the soil is obtainable by combustion of the soil in a calorimeter, but much of this is not available to the soil organisms. The normal sedimentary soils of England still contain decomposition products of the débris of plants and animals originally deposited with them, but in the long course of ages much of the extractable energy has been utilised. The soil population is thus dependent on recently grown vegetation, and it is therefore largely confined to the layer, usually in this country about 6 inches thick, through which the recently dead vegetation is distributed. Below this level there may be sufficient air, water, temperature, etc., but there is insufficient source of energy for any large population.
Unfortunately there is no ready means for distinguishing between the total and the actually available quantity of energy in the soil. But it is not difficult, by adopting the Rothamsted analytical method, to ascertain the approximate amount of energy that has been transformed in a given period. The Rothamsted plots are periodically analysed and a balance sheet is drawn up showing how much of each constituent has been added to and removed from the soil in the intervening period. For two of the Broadbalk plots the results are shown in [Tables XV.], [XVI.]
The dunged plot receives 14 tons farmyard manure per annum, a quantity in excess of what would usually be given; the unmanured plot, on the other hand, has received no manure for many years and is abnormally poor. Normal soils lie somewhere between these limits, but tending rather to the value for the dunged than for the unmanured plot. It will be seen that each acre of the dunged land loses on an average 41,000 calories per day, while each acre of the unmanured land loses on an average 2700 calories per day.
TABLE XV.—MATERIAL BALANCE SHEET: BROADBALK SOIL, ROTHAMSTED.
(Lb. per Acre per Annum.)