PRESENT THEORY AND PRACTICE OF MINERAL MANURES. 431 
subservient to their nourishment.” Chalks, marls, and powdered lime- 
stones were, in his view, only amendments to the soil,‘and not any ma- 
terial which could supply needful food to plants. He writes: “They act 
merely by forming a useful earthy ingredient of the soil.” Thus Davy 
gave the stamp of his genius to the extension of the idea that manures 
are improvements or amendments to ground in the first instance, and 
that their value as food materials to the plant is only secondary. 
Jethro Tull was the first writer who brought forward the idea that 
minute earthy particles supply the whole nutriment of the vegetable 
world. He anticipated Liebig in the grand idea that the mineral mat- 
ters of the soil are the food of plants; but his notion of manures was 
that they act in no other way than in ameliorating the texture of the 
soil; that in fact their agency is mechanical. “AI sorts of dungs and 
compost,” writes Tull, “contain some matter which, when mixed with 
the soil, ferments therein, and by such ferment dissolves, crumbles, 
and divides the earth very much. This is the chief and almost the only 
use of dung, for, as to the earthy part of it, the quantity is so very small 
that, after a perfect putrefaction, it appears to bear a most inconsider- 
able proportion to the soil itis destined to manure, and therefore, in 
that respect, is next to nothing.” This view of Tuil, which was first 
published in 1753, was given out when he was unable to support it by 
any experimental proof; and the idea, therefore, fell lifeless, to be re- 
suscitated by Liebig, with the many proofs which he adduced. 
Nearly thirty years ago Liebig pointed out that farm-yard manure is 
not indispensable; that it may be successfully replaced (although not then 
economically) by the employment of various substances of mineral ori- 
gin, bodies containing nitrogen and those devoid of that element, se- 
lected from those which can yield to plants all the constituents which 
they need for development. Since that time the experiments of Kuhl- 
Imann, and subsequently these of Boussingault, Barclay, Hannan, Gil- 
bert and Lawes, Schattenmann, Turner, Wilson, and others, have con- 
firmed the views of that great chemist, and demonstrated the special 
fact that nitrates and ammonia salts are able to supply to the soil all 
the nitrogen necessary to endow it with fertility; and finally the more re- 
cent results of Georges Ville prove that these nitrates and ammonia 
salts, united with phosphate of lime, with potash salts and gypsum, are 
sufficient to replace farm manure where it cannot be supplied. 
It will be seen in this summary that no mention has yet been made ot 
humus, or the decomposed vegetable matter of varied composition known 
under that name. Boussingault, in his work, Rural Economy, treating 
of the rotation of crops, (Chapter 7,) gives his view as to what the food 
of plants consists of: ‘It is known that the atmosphere and the organic 
matters diffuséd through the earth concur simultaneously to maintain 
the life of plants, but how far each contributes is undetermined ;” but 
of the real practical value of humus he had no doubt. Some of Liebig’s 
strongest efforts were directed to show that it may be dispensed with asa 
food of plants, and consequently as a manure; and the experiments of 
Gilbert and Lawes and of Ville seem to show that humus is not abso- 
lutely necessary as an amendment to land; yet, after the lapse of twenty- 
nine years, the statement of J. W. Johnston, of England, as to the im- 
portance of humus in the soil, is as true now as then: 
It is consistent with almost universal cbservation that the same soil is more pro- 
ductive when organic vegetable matter is present than when it is wholly absent. In 
fact, humus acts in a way so different from the chemical substances which Liebig and 
others have shown may be substituted for it, that it eannot be fairly estimated when 
contrasted with them. Humus gives physical qualities to the soil which cannot be 
bestowed by any of the food materials of plants. It places a soil in relation with the 
