164 Decomposition of Soil Organic Matter 



Influence of COo on Soil Minerals 



An increase in the evolution of COo, as a result of the decomposi- 

 tion of plant and animal residues added to the soil or of the soil 

 humus, leads to an increase in the COo content of the soil atmosphere. 

 This results in an increase in the hydrogen-ion concentration of the 

 soil. 



CO2 + HoO = H2CO3 



H2CO3 = H+ + HCO3- 



The hydrogen ion will interact with the various soil minerals, espe- 

 cially the phosphate and silicates, and bring about their greater solu- 

 bilization and availability for plant growth. This is shown in the 

 following reactions : 



Ca3(P04)2 + 2H2CO3 = 2CaHP04 + Ca(HC03)2 



NagSiOs + 2H2CO3 = 2NaHC03 + HaSiOa 



H2Si03 = H2O + SiOa 



The COo content of the soil atmosphere thus hastens the whole 

 process of weathering of rock constituents of the soil. It exerts an 

 important solvent effect upon the soil minerals, bringing them into 

 solution and making them more readily available for plant growth. 

 An increase in CO2 concentration in the soil will thus influence the 

 availability of various minerals essential for plant growth. This 

 affects first of all the solubility of the phosphates and silicates and 

 also that of other anions (borates). It also results in bringing into 

 solution greater concentrations of various cations, such as potassium, 

 calcium, and magnesium. 



Influence of COo on Plant Growth 



Carbon dioxide affects plant growth both directly and indirectly. 

 It has been definitely established by Lundegardh and others that 

 plants depend a great deal upon the COo liberated from the soil 

 humus for their nutrition, to supplement the COo present in the 

 atmosphere above the plants. It has even been said that the major 

 function of the addition of manure to soil is to increase the amount 

 of COo liberated, thus rendering larger concentrations of this im- 

 portant element available for plants. 



