vil] DEC A V OF ORGANIC MATTER 1 73 



particularly sodium chloride; an acid reaction also 

 diminishes considerably the rate of decay. Speaking 

 generally, bacteria do not thrive as soon as the 

 medium passes the neutral point, and all the decay 

 processes must be carried out by the development of 

 fungi when the medium is acid. The presence of 

 chalk, or any form of carbonate of lime, by neutral- 

 ising any acids as fast as they are formed, promotes 

 the destruction of organic matter. Wollny has also 

 shown that calcium humate will oxidise much more 

 rapidly than uncombined humic acid placed under 

 similar conditions. To the absence of carbonate of 

 lime and mineral salts generally, may be ascribed the 

 tendency of -humus to accumulate and persist on the 

 very light, sandy heaths, where the soil is dry and 

 hot in summer, and also well aerated. It has already 

 been indicated, in treating of humus, that the various 

 organic compounds of nitrogen show very different 

 susceptibility to the breaking-down process which even- 

 tually renders the nitrogen available for the crop — 

 amongst the most resistent substances being the nucleo- 

 proteins in the undigested portions of food which form 

 dung, and the humus residues from poor, cropped-out 

 land. As in all cases much of the nitrogen of both soil 

 and manure seems to pass into obstinately persistent 

 compounds yielding slowly, if at all, to oxidation, and 

 hence wasted to the farmer, an attempt has been made 

 to increase the preliminary breaking down of nitrogen 

 compounds in the soil by the introduction of certain 

 very active bacteria. Stoklasa has shown that various 

 organisms — B. megatherium> B. fluorescens, etc. — when 

 seeded into soil manured with bone-meal or similar 

 materials, increase both the nitrogen and the phosphoric 

 acid obtained by the plant. A pure cultivation of some 

 such organism, B. Ellenbachensis> was for a time sold 



