1912.] PUBLIC DOCUMENT — No. 31. 123 



been demonstrated by Prof. A. Vincent Osmun ^ and others, and 

 the interpretation of these results we believe can be found in 

 chemical stimulation. However, it is not at all unlikely that 

 even in this case the aeration of the soil resulting from steaming 

 may play a small role in the increased number of bacteria, since 

 it is known that cultivation gives rise to an increase in the num- 

 ber of bacteria in soils. 



There are great differences in soils as regards the stimulating 

 effects of sterilization, and judgment must be exercised in draw- 

 ing deductions from this fact alone. Many commercial florists 

 and market gardeners in various parts of the United States have 

 had some experience in gi'owing different crops in sterilized soil, 

 and the results of their experience in this work are not always 

 the same. The best results which we have observed as arising 

 from sterilization have invariably been given by lettuce. 



The soils used in growing lettuce are rich in organic matter 

 from the repeated application of horse manure year after year, 

 and it is such soils as these, rich in humus, that sterilization 

 affects most advantageously for plant growth. Some experi- 

 ments, however, which we have made, with decomposed leaves 

 (leaf mold) and decayed vegetable matter obtained from florists, 

 gave results somewhat different from those obtained from soils 

 rich in organic matter largely derived from horse manure. 

 When seeds were soaked in decoctions of either sterilized or un- 

 sterilizcd leaf mold they showed little or no stimulation, and 

 when the decoction was strong we obtained positive injury to 

 seed. Neither did we obtain any stimulus to crops in sterilized 

 forest humus except when the humus was first washed out and 

 then sterilized. 



The idea recently advanced by Russell and Hutchinson, that 

 the increased bacterial flora characteristic of sterilized soil is 

 biological rather than chemical, does not in the least appeal to 

 us, at least for our conditions. The theory is to a certain extent 

 an adaptation or application of the Metchinikoff phagocyte 

 theory to the soil. Russell and Hutchinson report finding pro- 

 tozoa devouring bacteria in the soils, and they account for the 

 increase of bacteria in sterilized soils by the absence of protozoa 



■ A Comparison of the Numbers of Bacteria in Sterilized and Unsterilized Soils, by A. Vincent 

 Osmun, Hatch Exp. Sta. Rept., 1905. 



