“(4) The improvement in the soil brought about by partial sterilisation is permanent, the high bacterial numbers being kept up even for 200 days or more. It is evident from (3) and (4) that the factor limiting bacterial numbers in ordinary soil is not bacterial, nor is it any product of bacterial activity, nor does it arise spontaneously in soils. “(5) But if some of the untreated soil is introduced into partially sterilised soil, the bacterial numbers, after the initial rise, begin to fall. Thus the limiting factor can be reintroduced from untreated soils. “(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. +--------+------------------------+------------------------+ | | | Soil Treated | | | Untreated Soil. | with Toluene. | |Tempera-+------+-----+-----+-----+------+-----+-----+-----+ | ture of| |After|After|After| |After|After|After| |Storage.| At | 13 | 25 | 70 | At | 13 | 25 | 70 | | °C. |Start.|Days.|Days.|Days.|Start.|Days.|Days.|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.