2i6 SOIL CONDITIONS AND PLANT GROWTH 



Soil rapidly decomposes hydrogen peroxide. Some decom- 

 position of protein and of phenol and hydrolysis of cane 

 sugar also occurs under conditions where living organisms 

 seem excluded. There is some evidence that soil can bring 

 about most of the chemical actions that a large number of 

 organisms can effect, such as ammonification, but not changes 

 peculiar to one or two organisms only like nitrification and 

 nitrogen fixation. Sestini/ indeed, supposed that ammonia is 

 oxidised catalytically by the ferric oxide always present to 

 nitrites and nitrates, while Loew (i8o^) states that nitrogen 

 can be catalytically "fixed" and converted into nitrates; 

 Russell and Smith,^ however, failed to reproduce these 

 changes catalytically. Indeed, the chief argument in favour 

 of the bacterial hypothesis is that all known soil processes 

 can be reproduced in the laboratory by soil bacteria acting 

 under conditions comparable with those known to obtain in 

 nature, whilst they have not been produced by catalysts. The 

 biological hypothesis, therefore, remains the simplest and most 

 satisfying, but there is room for more evidence before it can 

 be regarded as positively established.^ 



It is certain that living bacteria occur in the soil in addi- 

 tion to those present as spores. Some idea of the relative pro- 

 portions of these two forms was obtained by making gelatin 

 plate cultures of soil before and after treatment with toluene, 

 which destroys living forms but not spores, or at any rate not 

 all spores (Table LVL). Spores form only about 25 to 30 per 

 cent, of the total numbers, and for some unknown reason do 

 not accumulate. Conn (see p. 187) does not consider, however, 

 that they represent organisms of importance in the soil. The 

 bacterial numbers are seen to be very high, but even these 

 figures do not represent the true totals, and no medium has 

 yet been devised that allows of the growth of all the organisms 

 known to occur in the soil. 



^ Landw. Versuchs-Stat., 1904, 60, 103-112. 

 ^Journ. Ag. Sci., 1905, i, 444-453. 



3 For a recent discussion see The Proof of Microbial Agency in the Chemical 

 Transformation of Soil (H. J. Conn, Science, 1917, 46, 252). 



