232 



PRINCIPLES OF AGRICULTURAL CHEMISTRY 



AVERAGE OF 19 SUMMER WEEKS. 



Volume of 



carbon dioxide 



in 1,000 volumes air 



Under grass 2.1 



Under straw mulch 7.2 



Bare soil 9.4 



The thicker the plants, the greater their effect in reducing 

 oxidation. The following experiment of Wollny show this fact : 



Volume of 

 carbon dioxide 

 Number of plants in 1,000 volumes air 



3 oats to o.i meter 5.0 



6 oats to o. i meter 3.4 



12 oats to o.i meter 2.3 



24 oats to o.i meter. ' 1.9 



The difference appears to be largely due to the drying out of 

 the soil by the plants. 



A covering of vegetation thus conserves the organic matter of 

 the soil ; cultivation and bare fallow decrease it. 



Value of Humus. The chief chemical action of humus results 

 from its solvent action, and the solvent action of the carbon 

 dioxide which it produces in its decomposition. Water contain- 

 ing carbon dioxide has a much greater solvent power on minerals 

 than pure water. This is illustrated by the following experi- 

 ments of Dietrich. 



MINERAL MATERIAL DISSOLVED BY SOLVENTS. 



The addition of humus (turf) to the soil increases its water 

 capacity. 



