704 
The Nitrifying Ferments of the Soil. 
eventually to supply the nitrogenous ingredients of tlie matured 
plant or grain. About this there can be no doubt. A 
nitrate in solution is readily carried through the roots into the 
circulation of a plant ; its presence in the leaves and stem can 
easily be proved by testing their juice, and it is found to be 
thus taken in and stored up until b}' the increasing demands 
of the growing cells it is used up in forming new com- 
pounds. Pari passu with this, the decrease in the nitrate 
of the soil can be followed by the same tests, and (if the 
experiment is conducted on a plant gi'owing in a flower- 
pot) at the end of the summer's gro^vth the soil will in many 
cases be found to be pumped absolutely dry of this particular 
ingredient. 
To say that nitrate of lime is the sole natural source (rf 
nitrogen supply for any plant or class of plants would be 
perhaps to put the case too strongly, but this much at any 
rate may be said : — The illimitable supply of pure nitrogen 
gas in the atmosphere is only available as food in a curious 
indirect way to a limited class of plants having a special 
manner of gi'owth (e.g. the Leguminosw) ; the great stock of 
nitrogen locked up in the soil in the form of organic or 
decayed vegetable matter (some hundred or a thousand times 
greater in quantity than the nitrate) is not known to be 
immediately available as food to any plant ; and the ammonia 
compounds of liquid manure, dung-heaps, guano and ammo- 
niacal manures are certainly to a great extent converted into 
nitraAes by the time the plant makes use of them as nitrogenous 
food. As to the lime present in the nitrate of lime, that must 
be looked upon as an accidental rather than an essential or even 
important concomitant of the nitrogen. Were potash the most 
abundant base in the soil, nitrate of potash (saltpetre or nitre) 
would be the nitrate pi-esent, and plants would obtain a more 
valuable food than even nitrate of lime. Were soda the most 
abundant base, nitrate of soda would prevail, and would be the 
salt absorbed by the plant to the same purpose as the lime salt. 
How well nitrate of soda supplies the natural deficiencies or the 
annual loss of our tilled soils in nitrate of lime is shown by the 
present annual consumption of 1 50,000 tons for this sole purpose. 
Boussingault, the pioneer of the experimental method 
in agricultural science, was well aware of the importance of 
nitrates and of the reason of it, one of his earliest essays 
bearing the title " On the influence of Saltpetre on the develop- 
ment of Plants." As early as 1856 he had succeeded in 
devising a method for estimating the nitrate present in soils, 
and he gives us the result of testing over thirty samples. He 
