172 FUNCTIONS OF THE NITRATES. 



combination more suited to the plant's growth. The nitrogen 

 they contain may also enter into new states of combination ; and 

 such may be the case likewise with their potash, soda, lime, and 

 magnesia, in the presence of the other mineral and vegetable 

 matters with which they come in contact in the soil. But upon 

 these points we have as yet very little light, and therefore I 

 refrain from advancing any mere speculations. 



Their function in the plant is probably threefold. First, to 

 supply nitrogen, which we know to be so necessary a part of 

 the substance of the plant. Second, to supply alkaline and 

 earthy matters, which are also necessary to their formation. 

 Third, to act chemically in the sap in producing or inducing 

 those chemical changes, on the rapid succession of which the 

 more speedy growth of plants depends. 



In reference to this last kind of action, I throw out two sugges- 

 tions which appear to me to have some weight. First, that the 

 large proportion of oxygen which the nitrates contain, and the ease 

 with which they yield it up, has something to do with their striking 

 action upon vegetation. Second, that the constant presence of 

 a large proportion of nitrogen at the extremities of the roots of 

 plants which nitrogen plays an important part in vegetable 

 growth, and which the soluble nitrates when applied to, or na- 

 turally produced in the soil, are in a condition readily to supply- 

 is connected with the immediate nature of the effect they are 

 seen to produce. No sooner do they enter into the roots than 

 their oxygen as well as their nitrogen may conspire to exalt or 

 hasten the chemical actions on which growth depends. 



It is not difficult to suggest experiments by which the former, 

 at least, of these two conjectural opinions may be tested. 



A constant production of nitric acid, and probably of ammonia, 

 is going on in the soil. It proceeds most rapidly in soils which 

 abound at once in alkaline and in organic matter. This I have 

 fully explained in my published Lectures.* I allude to it here 

 only for the purpose of observing that this is probably the reason 

 why the nitrates produce the most striking and profitable effect 

 on land which is not rich. On such land they are produced 

 naturally in less abundance. 



* Second edition, p. 287. 



