MICROORGANISMS AS A FACTOR IN SOIL FERTILITY. 237 



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 (humus) organic matter 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 alcohols, proteins and 

 amino-compounds. Because of the different relations that these vege- 

 table 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 decomposition 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 observations 

 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 OF THE SOIL. The moisture films surround- 

 ing 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 reactions are more or less localized 

 and diffusion slow, there are, undoubtedly, in the soil minute zones where 

 individual species are more prominent than they are in others. For 

 example, Heinze has found it convenient to isolate Azotobacter by inocu- 



