152 Bacteria in Relation to Country Life 



moors and heaths is due not so much to absence of air 

 as to absence of moisture. The place of the bacteria is 

 taken, in this case, by various molds and fungi which, 

 in their development, give rise to the formation of acid 

 compounds. 



When the soil-moisture is neither excessive nor de- 

 ficient, fine-grained soils, like those of coarser texture, 

 allow the change of their carbon to carbon dioxid, but 

 at a less rapid rate. For this reason, the disappearance 

 of humus from such soils is comparatively slow, even 

 when the land is frequently tilled. The effects of barn- 

 yard manure, or of green -manure, may be observed 

 years after the influence of similar treatment is no 

 longer apparent on sandy soils. Taking it altogether, 

 then, clay soils, or clay loams do not part with their 

 humus as rapidly as do sandy soils or sandy loams. 

 When they are very compact, the normal rate of decom- 

 position is too slow for profitable crop-production, and 

 resort is then had to drainage, liming, or manuring, 

 operations which serve, among other things, to admit 

 more air into the soil. The soil bacteria are then per- 

 mitted to develop more rapidly, and to cause the de- 

 sired changes. 



In most soils under cultivation, the exhaustion of 

 humus is rapid. To offset this loss, the soil is seeded 

 down to grass from time to time, and receives, also, 

 applications of barnyard manure. The undisturbed sod 

 diminishes the aeration of the soil and reduces the rapid 

 oxidation of the humus. The latter is thus allowed to 

 increase in quantity. Barnyard manure furnishes 

 humus-forming material and serves to replenish the 



