300 , MICROBIOLOGY OF SOIL 



satisfactory decomposition of organic matter in the soil, and the abund- 

 ant supply of available nitrogen compounds, as well as of other con- 

 stituents of plant food to growing crops. The influence of lime on the 

 multiplication of soil bacteria is well illustrated, for instance, by the 

 experiments of Fabricius and yon Feilitzen. These investigatorslound 

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

 whereas in similar soils that had been limied and, cultivated, for several 

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

 maximum of 22,132,000 per g. 



rooD 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 relationshipSi For 

 instance, observations haye 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, linie, 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 sur- 

 rounding th? soil grains contain in solutipn substance? derived frgni 



