SAPROPHYTES 411 



the elements of fertility are gradually washed out of the soil 

 by the drainage water is known as LEACHING. 



It is evident that, if bacterial action is too rapid and sets 

 free soluble materials faster than they can be taken up by the 

 growing crop, these materials will be wasted by leaching. 

 Leaching is more rapid in cultivated soil than in soil covered 

 by a turf, for, in the first place, water soaks into the culti- 

 vated soil more rapidly, and, in the second place, more air 

 enters the cultivated soil and this leads to more rapid bacterial 

 action. For a similar reason, loose, sandy soils which permit 

 the air and water to enter them freely lose their humus more 

 rapidly than do heavy clay soils. 



468. Bacteria and Soil Acidity. We have seen that bacterial 

 action in the soil results in the liberation of considerable quan- 

 tities of carbon dioxide in the soil. Some of this carbon dioxide 

 escapes immediately to the air, while some of it goes into 

 solution in the soil water. A portion of the gas that goes into 

 solution in the soil water combines chemically with the water 

 to form CARBONIC ACID. The soil water which thus contains 

 carbonic acid readily dissolves limestone, or CALCIUM CAR- 

 BONATE, and other similar chemical compounds of the soil 

 (see Fig. 285). These soil compounds may now be taken up 

 by the roots of higher plants or they may escape from the soil 

 by leaching. Other acids formed in the soil by the action of 

 bacteria have similar powers of rendering the inert materials 

 of the soil soluble. Most natural soils contain originally large 

 quantities of these materials which can combine with the 

 acids formed by the microorganisms in the soil but by con- 

 tinuous cropping for many years, these materials are likely 

 to become more or less exhausted in the surface soil. When 

 these materials which are called BASES by chemists, become too 

 scarce to combine with the acids as fast as the latter are formed 

 by the microorganisms, the acids accumulate in the soil and 

 we have what are known as ACID SOILS. 



469. The Correction of Soil Acidity. The accumulation of 

 acids in soil hinders its productiveness in several ways. In 



