Another section of the population, the fungi, was simply found, and at present we have only limited views as to their function. The older workers considered that they predominated in acid soils, while bacteria predominated in neutral soils. Present-day workers have shown that fungi, including actinomycetes, are normal inhabitants of all soils. The attempts at quantitative estimations are seriously complicated by the fact that during the manipulations a single piece of mycelium may break into fragments, each of which would count as one, while a single cluster of spores might be counted as thousands. Little progress has therefore been made on the quantitative lines which have been so fruitful with protozoa. Dr. Brierley gives, in [Chapters VII.] and [VIII.], a critical account of the work done on fungi.
In addition to the organisms already considered there are others of larger size. The nematodes are almost visible to the unaided eye, most of them are free living and probably help in the disintegration of plant residues, though a few are parasitic on living plants and do much injury to clover, oats, and less frequently to onions, bulbs, and potatoes. Further, there are insects, myriapods and others, the effects of which in the soil are not fully known. Special importance attaches to the earthworms, not only because they are the largest in size and in aggregate weight of the soil population, but because of the great part they play in aerating the soil, gradually turning it over and bringing about an intimate admixture with dead plant residues, as first demonstrated by Darwin. Earthworms are the great distributors of energy material to the microscopic population. Systematic quantitative work on these larger forms is only of recent date, and Dr. Imms, in [Chapter IX.], discusses our present knowledge.
TABLE I.
Soil Population, Rothamsted, 1922.
(The figures for algæ and fungi are first approximations only, and have considerably less value than those for bacteria and protozoa.)
| Numbers per Gram of Soil. | Approximate Weight per Acre of— | |||||||
|---|---|---|---|---|---|---|---|---|
| Living Organisms. | Dry Matter in Organisms. | Nitrogen in Organisms. | ||||||
| Bacteria— | lb. | lb. | lb. | |||||
| High level | 45,000,000 | 50 | } | 2 | 0·2 | |||
| Low level | 22,500,000 | 25 | ||||||
| Protozoa— | ||||||||
| Ciliates— | ||||||||
| High level | 1,000 | — | — | — | ||||
| Low level | 100 | — | — | — | ||||
| Amœbæ— | ||||||||
| High level | 280,000 | 320 | } | 12 | 1·2 | |||
| Low level | 150,000 | 170 | ||||||
| Flagellates— | ||||||||
| High level | 770,000 | 190 | } | 7 | 0·7 | |||
| Low level | 350,000 | 85 | ||||||
| Algæ (not blue-green) | [100,000 | ] | 125 | 6 | 0·6 | |||
| Blue-green | Not known. | — | Say 6 | Say 0·6 | ||||
| Fungi— | ||||||||
| High level | [1,500,000 | ] | 1700 | } | 60 | 6·0 | ||
| Low level | [700,000 | ] | 800 | |||||
| 93 | 9·3 | |||||||
| = 4 parts nitrogen per 1,000,000 of soil. | ||||||||
| Larger Organisms. | ||||||||
|---|---|---|---|---|---|---|---|---|
| Numbers per Acre.[D] | Approximate Weight per Acre of— | |||||||
| Living Organisms. | Dry Matter in Organisms. | Nitrogen in Organisms. | ||||||
| Ma- nured. | Un- ma- nured. | Ma- nured. | Un- ma- nured. | Ma- nured. | Un- ma- nured. | Ma- nured. | Un- ma- nured. | |
| Oligochaeta (Limicolae)— | lb. | lb. | lb. | lb. | lb. | lb. | ||
| Nematoda, etc. | 3,609,000 | 794,000 | 9 | 2 | 3 | 1 | — | — |
| Myriapoda | 1,781,000 | 879,000 | 203 | 99 | 85 | 42 | 4 | 2 |
| Insects | 7,727,000 | 2,475,000 | 34 | 16 | 14 | 6 | 1 | 1 |
| Earthworms | 1,010,000 | 458,000 | 472 | 217 | 108 | 50 | 10 | 5 |
| Total | 210 | 99 | 15 | 9 | ||||
| Total organic matter (dry weight) in this soil = 126,000 lb. per acre. | ||||||||
| Total nitrogen = 5700 lb. per acre. (1 lb. nitrogen per acre = 0·4 parts per1,000,000 of soil.) | ||||||||
| [D] To a depth of 9 inches. The weight ofsoil is approximately 1,000,000 kilos. | ||||||||
Are there any other members of the soil population that are of importance? As already shown, the method of investigating the soil population in use at Rothamsted is to find by chemical methods the changes going on in the soil; to find by biological methods what organisms are capable of bringing about these changes; and then to complete the chain of evidence by tracing the relationships between the numbers or activities of these organisms and the amount of change produced. The list as we know it to-day is given in [Table I.]
The method, however, does not indicate whether the account is fairly complete, or whether there are other organisms to be found. We might, of course, trust to empirical hunting for organisms, or to chance discoveries such as led Goodey to find the mysterious Proteomyxan Rhizopods, which cannot yet be cultured with certainty, so that they are rarely found by soil workers. It is possible that there are many such organisms, and it is even conceivable that these unknown forms far outnumber the known. The defect of the present method is that it always leaves us in doubt as to the completeness of the list, and so we may have to devise another.
Reverting to [Table I.], it obviously serves no purpose to add the numbers of all the organisms together. We can add up the weights of living organisms, of their dry matter or nitrogen, so as to form some idea of the proportion of living to non-living organic matter, and this helps us to visualise the different groups and place them according to their respective masses. But a much better basis for comparing the activities of the different groups would be afforded by the respective amounts of energy they transform, if these could be determined. It is proposed to attempt such measurements at Rothamsted. The results when added would give the sum of the energy changes effected by the soil population as we know it: the figure could be compared with the total energy change in the soil itself as determined in a calorimeter. If the two figures are of the same order of magnitude, we shall know that our list is fairly well complete; if they are widely different, search must be made for the missing energy transformers. There are, of course, serious experimental difficulties to be overcome, but we believe the energy relationships will afford the best basis for further work on the soil population.
Finally, it is necessary to refer to the physical conditions obtaining in the soil. These make it a much better habitat for organisms than one might expect. At first sight one thinks of the soil as a purely mineral mass. This view is entirely incorrect. Soil contains a considerable amount of plant residues, rich in energy, and of air and water. The usual method of stating the composition of the soil is by weight, but this is misleading to the biologist because the mineral matter has a density some two and a half times that of water and three times that of the organic matter. For biological purposes composition by volume is much more useful, and when stated in this way the figures are very different from those ordinarily given. [Table II.] gives the results for two Broadbalk arable plots, one unmanured and the other dunged; it includes also a pasture soil.