358 MICROBIOLOGY OF SOIL 



only 138,500 bacteria per g. in newly broken and unlimed peat soils; 

 whereas in similar soils that had been limed and cultivated for several 

 years the numbers averaged about 7,000,000 per g. and reached a 

 maximum of 22,132,000 per g. 



FOOD SUPPLY 



ORGANIC MATTER. It may be said truly that a soil devoid of 

 organic matter is practically devoid of bacteria. To the fresh and the 

 partially decomposed organic matter (humus) the soil organisms must 

 look for most of their food and energy. Being largely of plant origin 

 this organic matter contains starches, fats, organic acids, higher al- 

 cohols, proteins and amino-compounds. Because of the different 

 relations that these vegetable substances bear to the several species of 

 soil bacteria, a high or low proportion of starch, of cellulose, or protein 

 must necessarily modify both numbers and species relationships. For 

 instance, observations have been made by Coleman and others that 

 small amounts of dextrose favor nitrification, whereas larger quantities 

 retard it; similarly, it has been noted that in the spontaneous de- 

 composition of protein bodies bacteria are prominent and molds absent 

 or relatively few in numbers. But where dextrose is added to the 

 decomposing proteins molds soon appear in large numbers. There 

 may also be cited, in this connection, the observation of Hilgard that 

 humus should contain at least 4 per cent of nitrogen if it is to furnish 

 a sufficient quantity of available nitrogen compounds; otherwise, the 

 soil bacteria seem to be unable to decompose it, so as to meet the 

 needs of the growing plants. Many similar facts could be cited to 

 show that as a culture medium the soil is influenced by green manures, 

 barnyard manure, commercial fertilizers, lime, tillage and any other 

 treatment that will modify the quantity as well as the quality of its 

 organic matter. 



THE MINERAL PORTION or THE SOIL. The moisture films sur- 

 rounding the soil grains contain in solution substances derived from 

 these soil grains. A particle of calcium carbonate will be surrounded 

 by a moisture film containing some calcium bicarbonate. In the 

 same way particles of feldspar may give rise to a solution of potassium 

 bicarbonate; particles of apatite to a solution of calcium phosphate; 

 particles of selenite to a solution of calcium sulphate; particles of 

 protein to a solution of ammonia, etc. In view of the fact that these 



