538 
On the Food of Plants. 
of plants, more especially the saline or mineral substances indis- 
pensable to such plants, and the nitrogen, sparingly furnished by 
the atmosphere, but in nearly all cases of artificial culture largely 
required. It is to supply the deficiencies thus occasioned that we 
have recourse to manure. 
Now, it must be the object of every good farmer to follow such 
' a rotation system as shall economise to the utmost extent his farm- 
vard manure, the supply of which must of necessity be limited, 
l)y cultivating in the intervals of his corn crops those plants which 
f subsist to the greatest extent on atmospheric food, and which are 
; at the same time useful as fodder or otherwise, so that while very 
[ little saline matter is taken from the ground, large quantities of 
' carbon and nitrogen may be withdrawn from the air, and trans- 
; ferred to the soil through the agency of the plant, in the shape of 
\ roots, residue, &c., afterwards ploughed in, and furnishing by 
j their decomposition humus and ammonia. It is thus possible to 
\ imagine that by such means a sufficient supply of azotized matter 
even for grain might be got from the air alone without the use of 
animal manure at all ; but then it is impossible to select plants 
which do not, to a certain extent, rob the soil of salts, and thus at 
length destroy its fertility. Every one which has been examined 
contains more or less phosphate for example, and unless some 
artificial provision is made lor a suj)ply of such substances, the 
system cannot go on. 
It happens that in ordinary farmyard dung all the principles 
thus gradually lost by the soil are to be found in abundance : the 
decomposed straw furnishes silica in a minute state of division, 
still having with it a little potash and various saline substances; 
the solid animal excrements contain abundance of earthy phos- 
phates, as direct analysis has shown ; while the urine gives up by 
its putrefaction at once carbonate of ammonia and more phos- 
phate, besides smaller proportions of other principles : the only 
thing at all defective is potash, and that frequently exists plenti- 
fully in the soil, and is gradually liberated by disintegration. 
In fact, it can hardly l)e otherwise : the quantity of saline and 
azotized matter contained in the body of an animal at the end of 
its life is but small compared with that furnished by the food con- 
sumed, and since, except perhaps a little nitrogen, nothing is lost 
by respiration but carbon and hydrogen, the excess of azote and 
salts must be sought for in the excretions, and there accordingly 
they are to be found, and found also in that state most fitted for 
again passing into the vegetable form. 
The subjects have been so ably and so fully discussed by Liebig 
in his admirable treatise, now in the hands of all interested in 
agricultural pursuits, that a further detail becomes unnecessary : 
I could only give his matter in worse words. 1 subjoin the 
