( 113) 



fertility), and partly, and for much the larger portion, by matters 

 supplied, either directly or indirectly, from the atmosphere. More 

 than nine-tenths, usually, of the substance of every plant is com- 

 posed of the same four elements, three of which oxygen, nitro- 

 gen and carbon compose the whole atmosphere; the fourth 

 hydrogen is one of the constituent parts of water; and, also, as a 

 part of the dissolved water, hydrogen is always present in the 

 atmosphere, and in a great quantity. Thus, all these principal 

 elements of plants are superabundant, and always surrounding 

 every growing plant; and from the atmosphere (or through the 

 water in the soil), very much the larger portion of these joint sup- 

 plies is furnished to plants; and so it is of each particular element, 

 except nitrogen, much the smallest ingredient, and yet the richest 

 and most important of all organic manuring substances and of all 

 plants. This, for the greater part, if not for all of its small share 

 in plants, it seems, is not generally derived, even partially, froia 

 the air, though so abundant therein, but from the soil, or from 

 organic manures given to the soil. 



u But, though bountiful nature has offered these chief alimentary 

 principles and ingredients of vegetable growth in as inexhaustible 

 profusion as the atmosphere itself which they compose, still, their 

 availability and beneficial use for plants are limited in some measure 

 to man's labors and care to secure their benefits. Thus, for illus- 

 tration, suppose the material of food for plants furnished by the 

 atmosphere to be three- fourths of all received, and that one-fourth 

 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 much 

 the fertility of the soil. ' These proportions are stated merely for 

 illustration, and, of course, are inaccurate; but the theory or prin- 

 ciple 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-fodrth 

 8 



