“(6) Evidence of the limiting factor in untreated soils is obtained by studying the effect of temperature on bacterial numbers. Untreated soils were maintained at 10°, 20°, 30° C. in a well-moistened aerated condition, and periodical counts were made of the numbers of bacteria per gram. Rise in temperature rarely caused any increase in bacterial numbers. But after the soil was partially sterilised the bacterial numbers showed the normal increase with increasing temperatures.
TABLE VI.
| Temperature of Storage. °C. | Untreated Soil. | Soil Treated with Toluene. | ||||||
|---|---|---|---|---|---|---|---|---|
| At Start. | After 13 Days. | After 25 Days. | After 70 Days. | At Start. | After 13 Days. | After 25 Days. | After 70 Days. | |
| 5°-12° | 65 | 63 | 41 | 32 | 8·5 | 73 | 101 | 137 |
| 20° | 65 | 41 | 22 | 23 | 8·5 | 187 | 128 | 182 |
| 30° | 65 | 27 | 50 | 16 | 8·5 | 197 | 145 | 51 |
| 40° | 65 | 14 | 9 | 33 | 8·5 | 148 | 52 | 100 |
“(7) It is evident, therefore, that the limiting factor in the untreated soils is not the lack of anything, but the presence of something active. The properties of the limiting factor are:—
“(a) It is active and not a lack of something.
“(b) It is not bacterial.
“(c) It is extinguished by heat or poisons.
“(d) It can be re-introduced into soils from which it has been extinguished by the addition of a little untreated soil.
“(e) It develops more slowly than bacteria.
“(f) It is favoured by conditions favourable to trophic life in the soil, and finally becomes so active that the bacteria become unduly depressed.