THEORY OF THE ACTION OF THE NITRATES. 343 



contains 26 per cent, of its weight of nitrogen—or one cwt. of pure dry 

 nitrate of soda contains about 19 lbs. of nitrogen. This nitrogen we 

 know to be a necessary constituent of plants— one which they obtain 

 almost wholly from the soil — ^but v hich nevertheless is generally pre- 

 sent in the soil in small quantity on.y. "We have already seen reason 

 (Lee. VIII., p. 159,) to believe that nitric acid exists naturally in the 

 soil, and is the form in which a large portion of their nitrogen is con- 

 veyed into the roots of plants ; — when we add it to our fields, therefore, 

 we only aid nature in supplying a compound by which vegetables are 

 usually sustained. And as the young plant will necessarily languish 

 in the absence of one essential kind of food, although every other kind 

 it may require be present in abundance, it is easy to see how the 

 growth of a crop — languidly proceeding upon a soil deficient in nitrogen 

 — may be suddenly re-animated by an application of nitrate of soda to 

 its roots. That this is the true way in which the nitrates generally act 

 is supported by the oUseivation that it is in the poorest soils that they 

 are most useful to the husbandman. 



We have already seen, also, that one function of the leaf in the pre- 

 sence of the sun is to decompose carbonic acid, and give off its oxygen 

 (Lee. v., sec. 5.) It exerts a similar action upon the nitric acid of the 

 nitrates, and upon the sulphuric acid of the sulphates, discharging their 

 oxygen into the air, and thus leaving the nitrogen and sulphur at liberty 

 to unite with the other elementary substances contained in the sap — for 

 the production of the several compounds of which the parts of the 

 growing plant consist. 



Nor, as shown in a previous lecture, (VIII. , sec. 8,) is the good effect 

 of these nitrates upon the crop limited to the supply of that quantity of 

 nitrogen only which they themselves contain. The excess of crop 

 raised by their aid often contains very much more nitrogen than they 

 have been the means of conveying to the roots, even supposing it all 

 to have been absorbed and appropriated by the plant. This arises from 

 the circumstance that the more the plant is made to thrive, the more 

 numerous and extended become its roots also, and these roots are thus 

 enabled to gather from the deeper and more distant soil those supplies 

 of nitrogenous and other necessary food, which would have remained 

 beyond their reach had the plant been allowed to remain in its pre- 

 viously feeble or more languid condition. This has been called the 

 stimulating effect of manures, and some substances have been said to 

 act only in this way upon vegetation. This, however, appears to me to 

 be a mistake. The supposed stimulating is always a secondary effect, 

 and necessarily follows from the use o^ every kind of manure, which by 

 feeding the plant gives it greater strength, and thus enables it to appro- 

 priate other supplies of food which were previously beyond its reach, or 

 which from the absence of one necessary constituent it could not render 

 available to its natural growtff. 



In this way the nitrates act as such — in contra-distinction to the sul- 

 phates and other salts of potash and soda. But there is every reason to 

 believe that the potash and soda themselves often aid the effect of the 

 nitric acid with which they are associated. In soils deficient in these 

 alkalies the nitrates would act beneficially, even though nitric acid 



