444 LEGUMINOUS PLANTS. 



But, though bountiful nature has offered these chief alimentary princi- 

 ples 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 illustration, suppose the natural 

 supplies 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 de- 

 rived 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 aequired 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 twenty parts,, of which five only were derived from 

 and served to reduce by so much, the fertility of the soil. These propor- 

 tions are stated merely for illustration, and, of course, are inaccurate; but 

 the theory or principle is correct, and the law of fertilization and exhaus- 

 tion ihence 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 aliment 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 aB manure) 

 would supply to the field as much of alimentary or fertilizing mattei as 

 would be drawn thence by three other crops removed for consumption or 

 sale. But in practice there are usually at work important agencies for 

 destruction of fertility, besides the mere supply of aliment to growing 

 crops. Such agencies are the washing off of soluble parts, and even the 

 soil itself, by heavy rains ; the hastening 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 alimentary principles in the course of the ensuing decom- 

 position. Therefore, it is nearer the facts that two years' crops or cul- 

 ture, for market or removal, would require one year's growth of some 

 manuring crop to replace, and to maintain undiminished or increasing 

 the productive power of the field. The poorest, and also the cheapest, of 

 such manuring crops will be the natural or "volunteer'' growth of weeds on 

 lands left cultivated, and not grazed; and the best of all will be furnished 

 in the whole product of a broadcast sown and entire crop of your own 

 most fertilizing and valuable field peas. 



Thus, of each manuring crop, (as of all others,) or of the fertilizing 

 matter thus given to the land, the cultivator has contributed but five parts 

 from the land, or its previous manuiing, and the atmosphere has supplied 

 fifteen parts. If, then, the cultivator, by still more increasing his own 



