408 NATURE AND PROPERTIES OF SOILS 



function to a certain extent in this direction, some are es- 

 pecially active. It is known that B. mycoides, B. mesenteric 

 cus and B. megatherium are capable of assimilating phos- 

 phorus in considerable quantities, while such organisms as 

 Beggiotoa and OpMdomonas store up sulfur in large amounts. 

 In the same way iron, potassium, calcium, and like elements 

 may be utilized. While such biological action is at the time 

 a direct competition with higher plants, more mineral ma- 

 terial is ultimately available in the soil through such activ- 

 ities. 



While the direct effects of organisms on soil minerals is 

 no doubt very important, the direct influences seem to be 

 more vital in a practical way. While this indirect influence 

 may be in part enzymic, it is probably largely due to the 

 production of carbon dioxide, which accompanies all types of 

 life processes. The sulfurous acid and nitrous acid of the 

 sulfur and the nitrogen cycles, respectively, are also active 

 to a certain extent. The preceding discussion of the sulfur 

 compost indicates how vigorous the biological oxidation with- 

 in the sulfur cycle may become under certain conditions. In 

 the soil, however, carbon dioxide is probably by far the most 

 important. 1 Since the significance of carbon dioxide has 

 already been adequately discussed (pars. 17, 58 and 132), it is 

 sufficient at this point to state that this gas, because of its 

 large amounts and its intimate relationship to the mineral 

 material, is probably the most effective solvent agent in the soil. 



1 Typical reactions involving tri-calcium phosphate, orthoclase and cal- 

 cium carbonate are as follows: 



Ca,(P0 4 ) a + 2C0 2 + 2H a O = Ca 2 H 2 (P0 4 ) 2 + Ca(HC0 3 ) 2 . 

 2KAlSi 3 8 + C0 2 + 2H 2 = H 4 Al,Si a O, + K a CO. + 4SiO a . 

 CaCG 3 + H 2 + CO a = Ca(HCO,) a . 



