ON PLANT COMPOSITION AND .MANI KIAL KKi^ IPKMENTS. I ll 
three principal existing mineral constituents of our arable lands, tlie 
sandy or siliceous, the clay (complex silicates of alumina and potash), 
and the calcareous (carbonate of lime). 
It is evident, therefore, that according to the nature of the primitive 
rocks the soils formed therefrom were more or less rich either in silica, 
in lime, or in clay. 
It is thus that the different types of soil were formed which we see 
around us to-day. This process of soil formation is also continuous. The 
mechanical action of water, friction, the influence of frost, and, above all, 
that of the carbonic acid in solution, produce every day fresh quantities 
of silica, lime, and clay. 
At an epoch, extremely remote, in the history of the earth, and which 
no branch of science can determine, there appeared upon it a cell of 
planetary or other derivation, which was, in any case, the origin of life. 
This organic cell, in order to adapt itself to the conditions of the environ- 
ments in which it found itself, must have had the power to seize upon 
the carbon of the carbonates and to decompose water, in order to find the 
energy necessary for its development : — the life of the nitric ferment is now 
the only example of life possible under these conditions. In any case this 
cell differentiated and adapted itself to external conditions, and became 
the source of the organic world. Yery soon the inferior plants developed 
themselves, and commenced to add their action to the other mechanical 
actions effecting the disintegration of the rocks. These plants perished 
on the soil which had nourished them. Their organs fell upon the 
soil and were decomposed. Thence w^as constituted in time an accumu- 
lation of substances, which, being rapidly oxidised, became principally 
composed of carbon, and gradually enriched also with nitrogen. This 
was the origin of humus, or leaf-mould. The association of this humus 
with the three other chief constituents gave birth to arable land, 
w^iich thenceforth w^as capable of nourishing vegetation of a more 
exacting nature, and of a more and more complex organisation. In 
brief, arable soil is an association of sand, clay, lime, and humus in 
such proportions that the qualities and defects proper to each of these 
constituents are balanced, from the point of view of physical qualities, by 
their reciprocal influences. From the chemical point of view also 
this association is one of the happiest. The sand contributes silica, 
often mixed with a little oxide of iron; the clay, an abundance of 
alumina, potash, a little soda, and often much oxide of iron ; and the 
calcareous portion, rarely pure, contains principally lime, but it also 
carries with it some magnesia, sulphuric acid, and phosphoric acid. 
The humus itself is a valuable composition, rich in carbon and associated 
nitrogen, w^hich, from a mineral point of view, may be said to be the reflex 
of the composition of the soil on which it has been produced. In any 
case, it is this which is the principal source of nitrogen, the most useful 
element for vegetation. 
Plant Requisites. 
Before studying in detail the manner in w4iich the soil affords to 
vegetation the requisite mineral food constituents, it will be as well to 
point out what are the alimentary needs of a certain number of plants 
used in horticulture. The first of the two accompanying tables, com- 
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