ADDENDUM. 



THE third Edition of Professor Liebig's Agricultural Chemistry, 

 recently published, contains much additional evidence in support 

 of the view entertained by him, and embodied in Chap. VI. of 

 the present Treatise, that the relative fertility of different soils 

 depends, in great degree, upon the supply they yield of the 

 mineral ingredients, which are required by the crops of vegetables 

 raised upon them. 



It has been already shown (. 186) that the carbon of plants is 

 derived rather from the atmosphere than from the soil ; the latter 

 having for its chief purpose, to afford a supply to the young plant, 

 which is repaid with interest, when its full growth has been attained, 

 by the fixation of carbon from the air through the leaves ; so that 

 the quantity of carbon in a soil, which supports a flourishing vege- 

 tation, is continually increasing rather than diminishing. By 

 similar observation upon an extended scale, it may be shown that 

 their nitrogen also is chiefly derived from the atmosphere. For 

 centuries past, a very large, quantity of cheese has been annually 

 exported from Holland, the produce of the cows fed upon its 

 pastures. Now the fertility of these pastures has not diminished, 

 but has rather increased ; notwithstanding the withdrawal of so 

 large a quantity of a highly-azotised substance, the casein or cheesy 

 matter. The cows are permitted to remain on the pastures day 

 and night ; and they thus return to the soil, in their solid and 

 fluid excrements, nearly all the mineral matter which has been 

 withdrawn from it, with a considerable proportion of the elements 

 of its organic compounds. The nitrogen as well as the carbon 

 removed from it, whether in the form of milk, animal flesh, or the 

 products of respiration imparted to the atmosphere, must be again 

 obtained by the soil from the atmosphere ; since there is no dimi- 

 nution in the amount of these elements through a long period of 

 time. But if the mineral matters, which cannot be obtained from 

 the atmosphere, were not restored in the manure, a rapid diminution 

 in fertility would certainly ensue ; this having occurred in many 

 similar cases. 



It appears from recent chemical analyses of various kinds of 



