OF AGRICULTURE. 17 



In subsequent lectures, evidence will be adduced 

 showing that Liebig was certainly right, in concluding 

 that humus is not an adequate source of the carbon 

 of our crops. It will be seen that, to some of them at 

 any rate, it is at most only a very limited source, if 

 indeed it is to them a source at all. It will, on the 

 other hand, be shown that the organic residue of pre- 

 vious vegetation accumulated in the soil is, to say the 

 least,a very material source of the nitrogen of our crops. 



Thus, though mistaken as to the explanation of 

 the fertility of soils rich in humus, Thaer and 

 others were, after all, not far from the truth w r hen 

 they maintained that the richness of a soil in such 

 matter in a condition readily susceptible to oxidation, 

 was, in a great degree, the measure of its fertility. 



With regard to the hydrogen of plants, at any 

 rate that portion of it contained in their non- 

 nitrogenous products, Liebig maintained that its 

 source must be water; and that the source of the 

 oxygen was either that contained in carbonic acid, or 

 that in water. 



With regard to the nitrogen of vegetation, both 

 from the known characters of free nitrogen, and as 

 he considered a legitimate deduction from direct 

 experiments, Liebig argued that plants did not 

 assimilate uncombined nitrogen, either from the 

 atmosphere, or dissolved in water and so absorbed by 

 the roots. The source of the nitrogen of vegetation 

 was, he maintained, ammonia ; the product of the 

 putrefaction of one generation of plants and animals 

 affording a supply for its successors. He pointed 

 out that, in the case of a farm receiving nothing 

 from external sources, and selling off certain products, 

 the amount of nitrogen in the manure produced by 

 the consumption of some of the vegetable produce on 



