NOURISHMENT OF PLANTS. 37 



greater pecuniary advantage, by a judicious succes- 

 sion of crops, and likewise especially by the applica- 

 tion of very powerful manures (guano, bone-dust, 

 etc.), although these, abstractedly considered, can 

 furnish little humus. When by these a more vigor- 

 ous growth of plants is effected, the roots and leaves 

 become larger, and the soil acquires, therefore, by the 

 fall of the latter and the decay of the former, more 

 material for the formation of humus than by less 

 vigorous forcing and a poorer vegetable growth ; 

 perhaps more than it would have acquired by ma- 

 nuring with straw-manure. 



3. Plants receive nitrogen chiefly through the am- 

 monia which is generated during the putrefaction 

 and decay of vegetable, and, more particularly, of 

 animal substances. Plants are always surrounded 

 by air, and the air consists mainly of nitrogen. 

 Hence it might be concluded that they could never 

 lack the means of obtaining this element when it is 

 necessary to the structure of their tissue, inasmuch 

 as they have the opportunity of absorbing it in any 

 quantity from the atmosphere. And yet they are 

 without it in many, perhaps the greater number of 

 fields ; and it is for this reason that these do not 

 produce so many or such vigorous plants as they in 

 general might ; a fact which will be more precisely 

 shown in the following chapter. From the cir- 

 cumstance, therefore, that plants do not take up the 

 nitrogen of the air as nourishment, we must infer 

 their inability to do so, and come to the conclusion, 

 4 



