96 Dr. Hugo Mohl, on Liebig's Organic Chemistry. 



again render the soil suitable for the same crop, which is only the 

 case to a slight extent. We must, therefore, assume that the first 

 crops do communicate to the soil substances detrimental to the sub- 

 sequent crops. These substances must be of an organic nature. It 

 has been shown that these cannot be excrementitious, and therefore 

 it follows that such deleterious substances must consist of organic 

 compounds, derived from the roots which have accumulated and re- 

 mained behind in the land. If, then, in a soil filled with the remains 

 of roots, the same crop will only succeed after a lapse of years, 

 whilst other crops will thrive luxuriantly, we may conclude, that 

 the organic compounds of such roots will be absorbed by plants 

 previous to their being decomposed into inorganic substances ; and 

 that, consequently, plants of a different kind will use them for 

 food, although those of the same kind will be injured by them. 



After having assigned the utility of rotation to the formation of 

 humus, Dr. Liebig states his views of vegetable nutrition at the dif- 

 ferent periods of growth. He says, that a plant returns just 

 so much carbon to the soil as it has absorbed from it in the form of 

 carbonic acid produced by decomposing humus. This supply of car- 

 bon is sufficient for many plants at the first period of their growth, 

 but it is not sufficient to supply some of their organs with the neces- 

 sary maximum of food. But the object of agriculture is to gain the 

 maximum of produce, and this, says Liebig p. 154, " stands in 

 a direct ratio to the amount of food which has been given to a plant 

 during the first period of its development," therefore all pains are to 

 be taken to increase the amount of humus. 



The short and the long of these rather vague assertions (says Dr. 

 Mohl), is, apparently, that a crop will be the greater the more food 

 a plant has received from the soil before its period of flowering. 

 But this axiom, although true in the main, is somewhat contradicted 

 by another at p. Ill, where it is stated that humus is use- 

 ful to young plants, by contributing to the increase of their organs of 

 atmospheric nutrition; but it is not indispensable, and its ex- 

 cess may even be detrimental in the first stages of development. 

 The food, namely, which a young plant receives from the air in the 

 form of carbonic acid and ammonia, ib restricted within certain li- 

 mits, — it can assimilate no more that the air contains. If, therefore, 



