1846.] Theory of Agriculture. 15 



plain, Liebig has inculcated the doctrine that vegetable matter, in 

 the form of humus, does not, in the least, administer to the gro\\'th 

 of vegetables. He bases his assumption, as we have seen, on the 

 ground of its insolubility. Now, on this point, we take this po- 

 sition: that in all soils which have any claim to fertility, there is 

 a great abundance of soluble matter at any given time, amounting 

 to six or seven times the weight of the heaviest crops which are 

 ever removed from them. This view is sustained by the quantity 

 of soluble matter which can be obtained from a soil by the action 

 of cold water alone, and not by the action of acids or alkalies. 

 This fact shows, at any rate, that soluble matter exists in the soil, 

 and that it is derived from vegetable matter, which is the precise 

 fact required in order to show that plants can supply themselves 

 with carbon, and are not necessitated to absorb it by their leaves 

 from the atmosphere. We do not attempt to invalidate the position 

 of vegetable physiologists, that leaves receive from the air car- 

 bonic acid. What we do insist upon is, that there is a store- 

 house of carbon in the soil, in the combinations we have already 

 given; this is sufficient for our purpose. We do know, from Dr. 

 Jackson's experiments, that these combinations are highly active 

 in the promoting of the growth of plants. We do know too, that 

 perfect plants and perfect seeds can be produced only in soils 

 which contain vegetable matter — and we think that we are war- 

 ranted in saying that a soil is fertile in proportion to the quantity 

 of organic matter it contains, and that its capabilities for the 

 present and the future, rest mainly upon its quantity. If it is in 

 small quantity, the soil is comparatively unproductive, though the 

 crops growing upon it have all the advantages which they have 

 in other cases, to absorb carbonic acid from the atmosphere. 



One of the best facts of every day occurrence, which goes to 

 support the doctrine that plants derive a part of their matter from 

 the carbonic acid of the atmosphere, is, the successful cultivation 

 of clover by means of gypsum for the purpose of growing wheat, 

 by which plan the fertility of some soils is maintained a long 

 time. Yet even in this practice, exhaustion is to be expected in 

 the end, though we have soils in New-York which bear a constant 



