258 



On the Applicaiion of Manures: 



fallow-crop ploughed into the ground with others, as leguminous 

 plants^ according to the experiments of Boussingault, have the 

 greatest power of absorbing nitrogen from the air. 



On the same principle this chemist suggests the introduction 

 of the Jerusalem artichoke into light soils, which, owing to the 

 entire absence of mould, appear irreclaimablj barren ; this vege- 

 table, the tubers of which afford nourishment to cattle almost 

 equal to potatoes, having great power of absorbing both carbon 

 and nitrogen from the air, and thus by degrees generating a cer- 

 tain amount of soil.* 



I have seen this vegetable very commonly cultivated for the use 

 of cattle in the light lands of the Grand Duchy of Baden, and in 

 certain parts of Alsace. 



But if it be true, as Liebig has endeavoured to establish, that 

 plants obtain everything except their alkaline and earthy con- 

 stituents from the atmosphere, what, it may be asked, becomes of 

 the theory of which mention was ma<]e in the preceding lecture, 

 that attributes the unfitness of a soil for yielding several successive 

 crops of the same plant to the excretions given out by its roots ? 



For if plants receive the whole of their volatilisable ingredients 

 from the atmosphere, these excrementitious matters, being com- 

 posed chiefly of carbon, hydrogen, and oxygen, will not be ab- 

 sorbed, and therefore cannot affect the succeeding vegetation. 



The above inference would seem, unavoidable, if it were con- 

 sidered absolutely proved that nothing but the fixed ingredients 

 of a plant were derived from the earth ; but this is not fully es- 

 tablished even with respect to the humus, much less with respect 

 to the more soluble matters which the soil contains. 



These latter, there seems no reason for doubting, may be taken 

 up by the spongioles of the roots dissolved in water, together with 

 the alkaline and earthy ingredients which are derived from the 

 S;oil, nor am I aware of any proof that they may not likewise be 

 assimilated when so introduced. 



The theory of M. Decandolle, therefore, is not affected by the 

 above experiments, but must rest on its own merits, and continue 

 to afford a subject for inquiry to the scientific agriculturist. 



* It is to be observed, that Boussingault attributes to plants the power 

 of absorbing nitrogen from the air, but he alleges no proof that they have 

 that power, and his results may be just as well explained by supposing them 

 to have difierent powers of absorbing ammonia. It is to be remarked, that 

 the helianthus tuberosus belongs to a tribe of plants remarkable for their 

 power of absorbing and exhaling water, and hence it is evident that they 

 will be brought into contact within a given time with a larger amount of 

 ammonia than other plants which possess a less degree of energy in that 

 respect. 



