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Some Effects of Organic Growth-Promoting Substances (Auxi- 

 mones) on the Soil Organisms concerned in the Nitrogen 

 Cycle. 



By Florence A. Mockekidge, B.Sc, King's College, London. 

 (Communicated by F. W. Oliver, F.E.S. Received January 2, 1917.) 



It is a "well-established fact that the presence of a certain amount of 

 humus in soil is essential to complete fertility, and it is equally well known 

 that this organic matter, whether supplied in the form of stable or green 

 manure, is far more effective wben decomposed, or " rotted," than when 

 fresh. This has been attributed to the soluble humus formed during the 

 rotting, but wherein lies the peculiar merit of this soluble humus has long 

 been a debatable point, some considering that it serves primarily as a plant 

 nutrient, while others claim that its most important effect is upon the 

 bacterial flora of the soil. 



It is well known that raw peat, although rich in humus, is practically 

 useless as a manure, on account of its acid and insoluble nature. This 

 insoluble humus can be neutralised and rendered largely soluble by 

 extraction with alkalies, and almost all experiments on the effect of 

 soluble humus on bacteria have hitherto been carried out with such 

 extracts. Too much reliance must not be placed on the results obtained 

 with these chemically prepared substances, for they are not strictly com- 

 parable with the soluble humus produced in the soil by natural processes. 

 However, Bottomley* has shown that it is possible, by inoculating peat 

 with certain aerobic soil bacteria, and keeping it under suitable conditions, 

 to convert it into a partially soluble humus in a comparatively short time. 

 A similar bacterial action is taking place more slowly in every rotting 

 manure heap and in all soils, for the longer stable and farm manures are 

 kept, the more water-soluble brown humus can be extracted from them. 

 Bottomley 's bacterial treatment of the peat simply reproduces and hastens 

 these natural processes, and the product of the treatment, which is known 

 as " bacterised peat," is practically the counterpart, in a more concentrated 

 form, of rotting stable manure or of green manures which are undergoing 

 decomposition in the soil. The soluble humus which can be extracted 

 from it may be justly considered to approximate more closely to the 

 natural product than any extracts obtained by chemical processes, and the 



* Bottomley, W. B., ' Journ. Roy. Soc. Arts,' vol. 62, No. 3199 (1914). 



