190 THE CULTURE OF FARM CROPS. 



the larger will be these contributions of the richest kind of 

 plant food. It is equally evident that the more the soil is 

 worked and stirred, the more the changes from hot to 

 cool, and from moist to dry, will affect it; and thus in 

 consequence of all this, the soil that is cultivated during 

 the summer must gain the largest accessions of plant 

 food from the atmosphere. This is the first and greatest 

 benefit that thus accrues from the summer cultivation of a 

 growing crop. 



It has been shown too, that a large quantity of carbonic 

 acid is brought to the soil by the atmosphere which circu- 

 lates in it, and by the rain which descends upon it; and 

 that carbonic acid has most distinct and important relations 

 to plant growth. It furnishes the carbon, of which more 

 than one-half of the dry substance of plants consists. More- 

 over, water containing carbonic acid exerts a strong solvent 

 action upon the mineral compounds of the soil, decomposing 

 them and fitting them for use as food for plants. This is 

 another and most important benefit accruing from this me- 

 chanical operation upon the soil; for the larger amount of 

 water received and passed through the soil by evaporation, 

 the more effect is produced by the action of the carbonic 

 acid dissolved in it. 



The same may be said of the oxygen which is absorbed 

 by the rain water, and of the effect of the nitrifying influence 

 of the peculiar germ known to produce nitric acid in the 

 soil. These too, exert a more potent influence in porous 

 and moist soil than in compact and dry soil. Thus in 

 many ways we are able to perceive the useful results of the 

 frequent v/orking of the soil during the growth of a crop. 



But this is not all. The summer fallow was designed for 

 the destruction of weeds, as well as for the reduction of the 

 soil to a mellow and pulverulent condition. When a culti- 

 vated crop is worked as it should be, every weed is de- 

 stroyed most effectively. And just here a most important 

 point for consideration comes up. As a rule the summer 

 cultivation of the soil is not sufficiently thorough. Some 

 weeds are permitted to escape. This is an injury to the soil 



