110 FOOD OF PLANTS, AND HOW THEY TAKli IT. 
cultural chemistry, and by its aid, if properly conducted, we 
learn not only the elements contained by any given soil, but 
those substances that appear to be the necessary consti¬ 
tuents of any particular crop; and thus we may be guided 
in the application of manures to any soil, and for any crop, 
with some degree of certainty, instead of blindly and em¬ 
pirically applying this or that kind of manure, which we 
may have heard or read was a successful application to the 
crop in question. What proved successful on one soil might 
be quite useless on another, where a different element of plant- 
food is deficient. This will explain the very contradictory 
accounts which we receive from sources equally reliable in our 
agricultural publications, and in the experience of farmers. 
We sometimes find that the richest soils, as we call them, 
rich in humus or vegetable mould, will produce enormous 
crops of one kind of plant, but fail to produce an adequate 
return, if sown with other crops. This has been satisfac¬ 
torily explained by Liebig, to whom we are indebted for 
much of our knowledge on true agricultural chemistry at 
the present day, for the clearest views and the safest, be¬ 
cause the soundest, postulates. He tells us that wheat does 
not flourish on soils that are rich in pure vegetable mould, 
because this plant needs silex as an element necessary to its 
healthy constitution, and without which indeed it cannot 
exist. This substance it does not find in vegetable mould. 
Liebig discovered this by burning plants; in this process 
the organic elements were dissipated by forming gaseous 
compounds that escaped, while a certain portion remained 
fixed, as ashes—composed of lime, silex, soda, and potash, 
salt, bone-earth, gypsum, &c.—substances which could not 
be volatilized. His investigations further proved, that any 
given plant generally yields nearly the same proportion of 
ash, composed of almost exactly the same substances, and 
also that different plants furnished ashes for very different 
composition. 
Theodore de Saussure, in his * Researches on Vegeta¬ 
tion/ was the first to point out the constant occurrence of 
determinate mineral constituents in determinate plants. 
(Schleiden.) 
Upon which Liebig takes this position:—“Since the or¬ 
ganic nutriment stands everywhere in equal abundance at 
the service of all plants, the cause of the great difference of 
vegetation cannot be sought therein, consequently it must 
lie in the inorganic constituents, and it is essentially indif¬ 
ferent whether we convey manure to the field, or burn it 
first and strew the ashes on the soil, since its efficacy is de¬ 
pendent solely on the constitution of the ashes a position 
