Material Changes.
These transformations of energy involve transformations of matter. The original plant residues may be divided roughly into substances forming the structure of the plant, such as the hemicelluloses, the pentosans, gums, and the contents of the cell—the protoplasm and the storage products, protein; in addition, there are smaller quantities of fats and waxes and other constituents. Some of the easily-decomposable carbohydrates never reach the soil at all, being broken down by intracellular respiration or attack of micro-organisms. But much of the structure material—hemicelluloses, pentosans, etc.—remains.
Once the plant residues pass through the earthworm bodies they become completely disintegrated and lose all signs of structure.
The only visible product so far known is humus, the black sticky substance characteristic of soil and of manure. Two modes of formation have been suggested. Carbohydrates, sugars, pentosans, etc., are known to yield furfuraldehyde or hydroxymethylfurfuraldehyde on decomposition, and it has been shown at Rothamsted that this readily condenses to form a humus-like body, if not humus itself. In the laboratory the reaction is effected in presence of acid, but even amino-acids suffice. All the necessary conditions occur in the soil, and humus formation may proceed in this way.
Some of the structure material—the lignin—contains aromatic ring groupings. Fischer and Schrader have shown that in alkaline conditions these ring substances absorb oxygen and form something very like humus. It is quite possible that humus formation also proceeds in the soil in this way. Whether the two products are chemically identical is not known.
The scheme can be represented thus:—
| Cell structure material | ||
| Aliphatic (Hemicelluloses, Pentosans, etc.) | Aromatic (Lignin, etc., in presence of oxygen and under aerobic conditions) | |
| Fatty acids | Furfuraldehyde or Hydroxymethylfurfuraldehyde (in presence of acid) | |
| Calcium carbonate. | Humus. | |
The disintegration of the cell and the first stages in the decomposition of the structure material are almost certainly brought about by micro-organisms. Whether they complete the process is not known: purely chemical agencies could easily account for part.
The decomposition of protein in the soil has not been studied in any detail. From what is known of the acid hydrolysis and the putrefactive decompositions, however, it is not difficult to draw up a scheme which, at any rate, accords with the facts at present known. It is probable that the protein gives rise to amino-acids, which then break down by one of the known general reactions.
Two types of non-nitrogenous products may be expected: The aliphatic amino-acids give rise to ammonia and fatty acids; these form calcium salts which break down to calcium carbonate. The aromatic amino-acids—tyrosin, phenylalanine, etc.—which would account for about 6 per cent. of the nitrogen of vegetable proteins, would be expected to give ammonia and phenolic substances. Now phenols are poisonous to plants and if no method existed for their removal the accumulation would ultimately render the soil sterile. Matters would be even worse on cultivated soils, since cows’ urine, which enters into the composition of farmyard manure and is the chief constituent of liquid manure, contains, according to Mooser, no less than 0·25 to 0·77 grams of p-cresol per litre,[I] a quantity three to ten times that present in human urine. Fortunately this contingency never arises, for the soil contains a remarkable set of organisms capable of decomposing the phenols and leaving the soil entirely suitable for plant growth. This affords an interesting case of an organism—in this case the plant—growing well in a medium in spite of some adverse condition, not because it is specially adapted to meet this condition, but because some wholly different agent removes it.
[I] Mooser, Zeitschrift physiol. Chem., 1909, lxiii., 176. No phenol was found. It is possible that the p-cresol is not entirely derived from the protein, but that some comes from the glucosides in the animals’ food.
Other ring compounds, e.g. pyrrol, arise in smaller quantity in the decomposition of protein, but their fate in the soil is not known.
We may summarise the probable changes of the protein as follows:—
| Protein. | ||||||
| Aliphatic amino-acids | Aromatic amino-acids | Other compounds (Pyrrol, etc.) | ||||
| Fatty acids and hydroxy acids | Ammonia | Phenolic compounds | ||||
| Nitrite | ||||||
| Calcium carbonate | Nitrate | CO2 | ||||
It must be admitted that the evidence is indirect. The rate of oxidation of ammonia by bacteria in the soil is more rapid than the rate of formation, so that ammonia is practically never found in the soil in more than minimal amounts (1 or 2 parts per 1,000,000); indeed, the only evidence of its formation was for a long time the fact that no compound other than ammonia could be oxidised by the nitrifying organism. It has, however, since been shown at Rothamsted that ammonia accumulates in soils in which the nitrifying organism has been killed.
Nothing is known of the mechanism of the oxidation of ammonia beyond the fact that it is biological; the reaction is not easily effected chemically at ordinary temperatures. Possibly the organism assimilates ammonia at one end of a chain of metabolic processes and excretes nitrates at the other. Or, the reaction may be simply a straight oxidation for energy purposes, the ammonia changing to hydroxylamine and then to nitrous and nitric acids.
The nitrate does not remain long in the soil. Some is taken up by the plant and some is washed out from the soil. Part, however, either of the nitrate itself or of one of its precursors is converted into an insoluble form: probably it is changed into protein by the action of micro-organisms; it then goes through the whole process once more.
These are the general outlines; they present no particular chemical difficulties. When we come to details, however, there is much that cannot be understood.
First of all, there is the slow rate at which complex nitrogen compounds disappear from the soil in comparison with the rate of oxidation of the carbon. Thus, in the original plant residues, there is some forty times as much carbon as nitrogen: before they have been long in the soil there is only ten times as much carbon as nitrogen; this seems to be the stable position. What is the reason for this preferential oxidation of the carbon? No explanation can yet be given.
Fig. 24.
X-axis: 1887-8 1890-1 1900-1 1910-11
Y-axis: ℔ per acre
An equally difficult problem arises in connection with the length of time the process will continue. Decomposition of the nitrogen compounds never seems to be complete in the soil; it dribbles on interminably. In the year 1870 Lawes and Gilbert cut off a block of soil from its surroundings and undermined it so that the drainage water could be collected and analysed. The soil has been kept free from vegetation or addition of nitrogen compounds from that time till now; yet it has never failed to yield nitrates, and the annual yield falls off only very slowly ([Fig. 24]). This same peculiarity is seen in the yield of crops on unmanured land: it decreases, but very gradually; even after eighty years the process is far from complete, and there is no sign that it will ever come to an end.
TABLE XVII.—APPROXIMATE LOSS OF NITROGEN FROM CULTIVATED SOILS: BROADBALK WHEAT FIELD, ROTHAMSTED, FORTY-NINE YEARS (1865-1914.)
| Rich Soil: Plot 2. Lb. per Acre. | Poor Soil: Plot 3. Lb. per Acre. | |||
|---|---|---|---|---|
| Nitrogen in soil in 1865 | ·175 per cent. = 4340 | ·105 per cent. = 2720 | ||
| Nitrogen added in manure, rain (5 lb. per annum), and seed (2 lb. per annum) | 10,140 | 340 | ||
| Nitrogen expected in 1914 | 14,480 | 3060 | ||
| Nitrogen found in 1914 | ·259 per cent. = 5950 | ·095 per cent. = 2590 | ||
| Loss from soil | 8530 | 470 | ||
| Nitrogen accounted for in crops | 2500 | 750 | ||
| Balance, being dead loss | 6030 | -280 | [J] | |
| Annual dead loss | 123 | - 6 | [J] | |
| [J] Gains.Possibly the result of bacterial action. | ||||
A further remarkable fact connected with the decomposition of the nitrogen compounds is that it seems invariably to be accompanied by an evolution of gaseous nitrogen. Apparently there are two cases. Under anaerobic conditions many of the soil organisms have the power of obtaining their necessary oxygen from nitrates, thereby causing a change in the molecule which leads in some cases to liberation of gaseous nitrogen; but the same result seems to be attained in aerobic conditions, especially when carbon is being rapidly oxidised.
It is possible that the reaction is the same, and that in spite of the general aerobic conditions there is locally an anaerobic atmosphere. But it is also possible that some direct oxidation of protein or amino-acids may yield gaseous nitrogen. However it is brought about it affects a considerable proportion of the entire stock of nitrogen, and it becomes more serious as cultivation is intensified. Thus, on the Broadbalk plot receiving farmyard manure the loss is particularly heavy; on the unmanured plot it cannot be detected. The nitrogen balance-sheet is shown in [Table XVII.]
The oxidation of carbonaceous matter, however, is not invariably accompanied by a net loss of nitrogen; in other circumstances there is a net gain. In natural conditions there seems always to have been some leguminous vegetation growing; the gain may, therefore, be ascribed to the activity of the nodule organism. In pot experiments, however, it has been found possible, by adding sugar to the soil, to obtain gains of nitrogen where there is no leguminous vegetation, and this is attributed to the activity of Azotobacter.
The nitrogen cycle as observed in the soil is as follows:—
| Protein | |||
| By certain organisms and by growing plants | Ammonia | Mechanism uncertain | By Azotobacter, Clostridium, nodule organisms, etc. |
| Nitrite | |||
| Nitrate | Gaseous Nitrogen | ||
| By denitrifying organisms | |||
There has been but little study of the process of decomposition of the other compounds in plants. Part, if not all, of the sulphur is known to appear as sulphate, and some of the phosphorus as phosphate. It is certain that the plant constituents decompose, for there is no sign of their accumulation in the soil. They may exert transitory effects, but there is nothing to show permanent continuance. The toxic conditions which cause trouble in working with pure cultures of organisms in specific cultures media do not, so far as is known, arise in the soil. All attempts to find bacterio-toxins or plant toxins in normal soils have failed. The product toxic to one organism seems to be a useful nutrient to another, and so the mixed population keeps the soil healthy for all its members.
There is little precise knowledge as to the part played by the different members of the soil population in bringing about these changes.
We know in a general way that earthworms effect the distribution of the plant residues in the soil, and serve to disintegrate them; there is no evidence, however, that they play any indispensable part in the decomposition. Many root and other fragments do not go through this process; observation shows that fungi can force a way in, and they may be followed by nematodes which continue the disintegration. Possibly some of the flagellates help, and certainly the bacteria do. After that nothing is certain. We cannot, with certainty, assign any particular reaction in the decomposition to any specific organism, with the exception of the oxidation of the phenolic substances, the conversion of ammonia to nitrite and nitrate, and the fixation of nitrogen. With these exceptions many organisms seem capable of bringing about the reactions, and indeed some of the reactions may be purely chemical and independent of biological agencies.
The relationships between the soil population and soil fertility are readily stated in general outline, but they are by no means clear cut when one comes to details; fertility is a complex property, and some of its factors are independent of soil micro-organisms.
The general relationship between plants and soil organisms is one of complete mutual interdependence. The growing plant fixes the sun’s energy and converts it into a form utilisable by the soil organisms; without the plant they could not exist. The plant is equally dependent on the soil organisms in at least two directions: their scavenging action removes the dead vegetation which would, if accumulated on the surface of the soil, effectively prevent most plants from growing. Further, the plant is dependent on the soil population for supplies of nitrates. Nothing is known about the relative efficiencies of the various soil organisms as scavengers. Numerous fungi and bacteria are effective producers of ammonia, the precursor of nitrates; it is not known, however, whether flagellates and such higher forms as nematodes act in this way.
This widespread power of producing ammonia makes it impossible in our present knowledge to regard any particular group of organisms as par excellence promoters of fertility. Indeed, it is safest not to attempt to do so. The primary purpose of the activities of a soil organism is to obtain energy and cell material for itself; any benefit to the plant is purely incidental. For cell material it must have nitrogen and phosphorus; here it competes with the plant. If it produces more ammonia than it utilises—in other words, if it is driven to nitrogen compounds for its energy, then the plant benefits. If, on the other hand, it absorbs more ammonia than it produces, as happens when it derives its energy from non-nitrogenous substances, the plant suffers. Thus, addition of peptone to the soil or an increase in bacterial numbers effected without addition of external energy (e.g. by partial sterilisation) leads to increased ammonia supply, and, therefore, to increased fertility. But addition of sugar to the soil causes so great an increase of numbers of bacteria and other organisms that considerable absorption of ammonia and nitrate occurs, and fertility is for a time depressed.
Both actions proceed in soils partially sterilised by organic substances, such as phenol, which are utilised by some of the soil organisms; there is first a great rise in numbers of these particular organisms with a depression of ammonia and nitrate, then a drop to the new level, higher than the old one, and an increased production of ammonia and nitrate resulting from the partial sterilisation effects.
We must then regard the soil population as concerned entirely to maintain itself, and only incidently benefiting the plant, sometimes, indeed, injuring it; always essential, yet always taking its toll, and sometimes a heavy toll, of the plant nutrients it produces.
This effect makes it difficult to deduce simple quantitative relationships between bacterial activity and soil fertility, and the difficulty is increased by the fact that bacteria and plants may both be injured or benefited by the same causes, so that high bacterial numbers in a fertile soil would not necessarily be the cause, but might be simply the result of fertility.
The circumstance that certain soil organisms—bacteria, algæ, and fungi—themselves assimilate ammonia and nitrate may account for the remarkable slowness of nitrate accumulation, to which reference has already been made. The protein formed from the assimilated nitrogen remains in the bodies of the organisms, living or dead, till decomposition sets in. It is not difficult to picture a cycle of events in which much of the nitrate formed is at once reabsorbed by other organisms, and only little is actually thrown off into the soil. Such a process might continue almost interminably so long as any carbonaceous material remained.
Finally, we come to the very interesting problem—is it possible to control the population of the soil?
The problem may seem superfluous in view of the difficulties just mentioned. Some aspects of it, however, are fairly clearly defined.
In the first instance, some organisms appear to be wholly harmful to the plant; among them are parasitic eelworms and fungi, and bacteria causing disease.
Control of these organisms can be brought about by partial sterilisation, and of all methods heat is the most effective, but it is costly, and attempts are now being made to replace it by chemical treatment. The results are promising, but the investigation is laborious; the organisms show specific relationships, and in finding a sufficiently potent and convenient poison it is necessary in each case to make an investigation into the relationship between chemical constitution and toxicity to the particular organism concerned. Formaldehyde is usually potent against fungi, and the cresols, and particularly their chlor- and chloronitro-derivatives, are potent against animals (eelworms, etc.).
One group of organisms is wholly beneficial, those associated with leguminous plants. Attempts have been made to increase their activities by inoculating the soil with more vigorous strains. The practical difficulties still remain very considerable, but there is hope that they may be overcome.
It is also possible to shift the balance of the soil population in certain directions. Special groups of soil organisms can be caused to multiply temporarily, if not permanently, by satisfying their particular requirements. Thus, when a soil has been heated above 100° C. it becomes specially suited to the growth of fungi, and quite unsuited to certain bacteria such as the nitrifying organisms and others; if this heated soil is infected with a normal soil population the fungi develop to a remarkable extent. The nodule organisms appear to be stimulated by addition of farmyard manure and of phosphates, and the phenol-destroying organisms by successive small additions of phenol.
Finally, quite apart from the control of disease organisms, it is possible to alter the soil population considerably by partial sterilisation, using a temperature of only about 60° C., or a poison like toluene that favours few of the soil organisms. This problem has already been discussed in [Chapter I.]
The control of the soil population is still only in its infancy, but it already promises useful developments. It cannot, however, be too strongly insisted that the only sure basis of control is knowledge, and we cannot hope to push control further till we have learned much more about the soil population than we know at present.