(137) 



plants are limited in some measure to man's labors and care 

 to secure their benefits. Thus, for illustration, suppose the 

 natural supplies of food for plants furnished by the atmos- 

 phere to be three- fourths of all received, and that one-fouith 

 only of the growth of any crop is derived from the soil and 

 its fertility, still, a strict proportion between the amount of 

 supplies from these two different sources does not the less 

 exist. If the cultivator's land at one time, from its natural 

 or acquired fertility, affords to the growing crop alimentary 

 principles of value to be designated as five, there will be 

 added thereto other alimentary parts, equal to fifteen in 

 value from the atmosphere. The crop will be made up of, 

 and will contain, the whole of twenty parts, of which five 

 only were derived from and served to reduce by so mucb, 

 the fertility of the soil. These proportions are stated merely 

 for illustration, and, of course, are inaccurate; but the the- 

 ory or principle is correct, and the law of fertilization and 

 exhaustion thence deduced is as certainly sound. Then, 

 upon these premises, there is taken from the land, for the 

 support of the crop, but one-fourth of the alitnent derived 

 from all sources for that purpose. And, if no other causes 

 of destruction of fertility were in operation, one green or 

 manuring crop (wholly given to the land, and wholly used 

 as manure), would supply to the field as much alimentary 

 or fertilizing matter as would be drawn thence by three 

 other crops removed for consumption or saK But in prac- 

 tice there are usually at work important agencies for de- 

 struction of fertility, besides the mere supply of aliment to 

 growing crops. Such agencies are the washing off of solu- 

 ble parts, and even the soil itself, by heavy rains; the has- 

 tening of the decomposition and waste of organic matter, by 

 frequent tillage processes and changes of exposure; and 

 ploughing or other' working of land when too wet, either 

 from rain or want of drainage. Also, a cover of weeds left 

 to rot on the surface, or any crop ploughed under, green or 

 dry as manure, is subject to more or less waste of its ali- 



