PLANT-FOOD MATERIALS IN SOILS 107 



It is apparent that the effect of the growth of plants has 

 been to increase the amount of carbon dioxide in the soil 

 air. The figures represent the period of the greatest pro- 

 duction of carbon dioxide by the corn plant. 



127. Production of carbon dioxide by microorganisms. 

 —  In addition to the carbon dioxide excreted from roots, 

 there are large quantities produced by microorganisms that 

 exist in soils. These organisms are concerned in the decom- 

 position of organic matter, and one final product of such 

 action is carbon dioxide. It has been estimated that in 

 one acre of soil to a depth of sixteen inches, there are sixty- 

 eight pounds of carbon dioxide produced by bacteria and 

 fifty-four pounds excreted by roots during the growing 

 season. 



128. Solvent action of roots in other ways. — Many in- 

 vestigators think that the large quantities of mineral matter 

 that plants remove from soils could not be obtained from 

 the water solution even with the aid of carbon dioxide. 

 Several different ways have been suggested by which plants 

 may assist in rendering soluble the nutrients contained 

 in soils. It will not be necessary to discuss these as there 

 has been no definite and conclusive outcome to the investi- 

 gation of the subject. The indications are, however, very 

 strong that the plant aids in obtaining its food material in 

 some way or ways other than by the excretion of carbon 

 dioxide. 



129. Difference in absorptive power of crops. — Crops 

 differ greatly in their ability to draw nourishment from the 

 soil. The difference between the quantities of nitrogen, 

 phosphoric acid and potash taken up by a corn crop of 

 average size and a wheat crop of average size is very 

 striking. In Table 27 it may be seen that two tons of 

 red clover contain three times as much potash, nearly ten 

 times as much lime, and somewhat more phosphoric acid 



