The micro-organisms of the soil

by Edward J. Russell; Rothamsted Experimental Station

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.