Number of Organisms Present and their Distribution in Depth.
In computing the number of invertebrates normally present in a given type of soil, the method adopted consists of making individual counts of all such organisms as occur in each sample of a series taken over a period of twelve months. This method considerably reduces errors due to season and to the possible deviation of one or more samples from the average. If the total number of these organisms is known for the samples taken, it becomes a simple procedure to arrive at their approximate numbers per acre.
TABLE XIV.
(Based on Morris, 1922 A.)
| Unmanured Plot. | Manured Plot. | |
|---|---|---|
| Insects | 2,474,700 | 7,727,300 |
| Larger Nematoda and Oligochæta Limicolæ | 794,600 | 3,600,400 |
| Myriapoda— | ||
| Diplopoda | 596,000 | 1,367,000 |
| Chilopoda | 215,400 | 208,700 |
| Symphyla | 64,000 | 215,500 |
| Total | 875,400 | 1,791,200 |
| Oligochæta (Terricolæ) | 457,900 | 1,010,100 |
| Arachnida— | ||
| Acarina | 215,400 | 531,900 |
| Areinida | 20,200 | 20,200 |
| Total | 235,600 | 552,100 |
| Crustacea (Isopoda) | 33,700 | 80,800 |
| Mollusca (Pulmonata) | 13,500 | 33,700 |
| Total Invertebrata | 4,885,400 | 14,795,600 |
Fig. 20.—Distribution in depth of the more important groups of soil invertebrates in the manured and unmanured (or control) plots at Rothamsted. (From Morris, “Annals of Applied Biology,” vol. ix., nos. 3 and 4, Cambridge University Press.)
[Table XIV.] represents a numerical estimate of the invertebrate fauna of two plots of arable land at Rothamsted. The soil is clay with flints overlying chalk, and the land in question has been devoted for eighty years to continuous cropping with wheat; one plot (No. 3) receives an annual dressing of farmyard manure at the rate of 14 tons per acre, and the other plot (No. 2) receives no natural or artificial fertilizer. The significant feature in a comparison of the fauna of the two plots is the great numerical increase in organisms due to the addition of manure. From the point of view of distribution in depth, [Fig. 20] clearly demonstrates that the bulk of the fauna is concentrated in the first three inches of the soil. With the exception of the Acarina it is evident that the limits of vertical distribution extend below the depth of nine inches investigated, although the numbers of organisms likely to be present are inconsiderable. The Oligochæta, or true earthworms, occur in Rothamsted soil in numbers very much in excess of the figures given by Darwin, who quoted observations by Hensen. The latter authority calculated that there were 53,767 earthworms in an acre of garden soil, and estimated that about half that number would be present in an acre of corn field. In the Rothamsted investigations their numbers exceeded Hensen’s estimate over 16 times in unmanured land, and over 36 times in manured land.
In an area of pasture-land in Cheshire few insects occurred below a depth of 2 inches, and they reached the limit of their vertical distribution at or near 6 inches. Their number (3,586,000 per acre) is considerably in excess of that present in unmanured arable land at Rothamsted.
1, Collembola; 2, Thysanura; 3, Orthoptera; 4, Thysanoptera; 5, Hemiptera; 6, Lepidoptera; 7, Coleoptera; 8, Diptera; 9, Hymenoptera.
Fig. 21.—Number of individuals in the different orders of insects in manured and unmanured arable land at Rothamsted. (From Morris, “Annals of Applied Biology,” vol. ix., nos. 3 and 4, Cambridge University Press.)