184 



HIGHER PLANTS AND SOIL MICROORGANISMS 



intensity is low, little or no increase in plant growth follows an 

 increase in the carbon dioxide content; the assimilation processes 

 of plants under such conditions are limited by the low light inten- 

 sity. Any acceleration of decomposition processes by soil organ- 

 isms tends to offset deficiencies of carbon dioxide in the atmos- 

 phere. Generally, however, the content of carbon dioxide is 

 below the level for maximum plant development in brilliant sun- 

 light. The movements of the gas between soil, atmosphere, and 

 plant are shown in Fig. 70. 



Evolution of Carbon Dioxide. — The carbon dioxide evolved 

 from soils is almost entirely of biological origin, although some may 

 arise from soil carbonates. The origin of this gas is divided 

 between the activities of plant roots and soil microorganisms. 

 Whether roots or microorganisms produce most of the gas is deter- 

 mined by the type of plant and soil conditions. Generally the 

 roots of higher plants produce much less carbon dioxide than the 

 microbes. In very sandy soils deficient in organic matter, the 

 roots may give rise to much the greater amount of carbon dioxide, 

 as shown from certain results obtained by Lundegardh: 



Respiration of sandy soil and plant roots . . 45 mgm. of carbon dioxide per hour 



Respiration of sandy soil 0.14 mgm. of carbon dioxide per hour 



Respiration of plant roots 0.31 mgm. of carbon dioxide per hour 



Per cent of the carbon dioxide liberated 



from i)lant roots 68 . 9 



Under most soil conditions, however, plants produce a smaller 

 portion of the total carbon dioxide which comes from the soil. 



TABLE 44 

 Carbon Dioxide Produced per Square Meter per Hour in Soil Sup- 

 porting Plant Growth and in Bare Soil (prom Lundegardh) 



Date 



Bare soil, 

 gm. 



Soil growing 

 oats, gm. 



Increase due 



to plant 



development, 



gm. 



Per cent of 

 the gas due to 

 plant devel- 

 opment 



June 29 . . . 

 July 24... 

 July 27 . . . 

 August 10 . 

 August 14 , 



0.220 



. 273 

 0.250 



0.216 

 0.268 



. 399 



0.053 

 0.034 

 0.131 



19.4 

 13.6 

 32.8 



