ROTATION OF CROPS. 817 
Although the amount of inorganic matter in plants is very 
much smaller than that of the organic, still this portion, however 
small, is necessary to the life and vigorous development of most 
plants, and probably of all; although in certain Moulds no 
inorganic constituents have been detected. 
The inorganic constituents of plants are of great impcrtance 
in an agricultural point of view, and in growing plants for use 
in medicine, &c., as it is to their presence or absence, their 
relative quantities, and the solubility or insolubility of their 
compounds, in a particular soil, that it owes its fertility or 
otherwise, and its adaptability of growing with success one or 
another kind of plant. 
Rotation of Crops.—-The principle of the rotation of crops 
in agriculture is founded upon the fact of different plants re- 
quiring different inorganic compounds for their successful culti- 
vation ; and hence a particular soil which is rich in materials 
necessary for some plants, may be wanting or deficient in 
those required by others. Thus, Wheat or any cereal crop re- 
quires more especially for its proper growth a full supply of 
silica and phosphates ; hence it will only flourish in a soil con- 
taining the necessary amount of such substances. As growth 
proceeds, these constituents are absorbed in a state of solu- 
tion by the roots, and are applied to the requirements of the 
plants. When the grain is ripe, it is removed as well as the 
straw, and the silica and phosphates obtained from the soil will 
thus be also removed with them ; the result of this is necessarily, 
except in fertile virgin soil, that these ingredients will not be 
then contained in the soil in sufficient quantities to support 
immediately the growth of the same species of plants. But by 
growing in a soil thus exhausted by Wheat, another crop of a 
different kind, such as Clover, Peas or Beans, which requires 
either altogether different substances, or a different amount, or 
other combinations of the same substances, or whose roots 
penetrate to a greater or less depth, we may obtain a profitable 
crop ; while at the same time certain chemical changes will go 
on in the soil, and other ingredients be taken up from the atmo- 
sphere, and in other ways, by which the land will be again 
adapted for the growth of Wheat. 
The consideration of the above facts shows how important it 
is for the agriculturist to have some acquaintance with vegetable 
physiology and chemistry. He should know the composition of 
the various soils, and the plants which he cultivates, as well as 
the nature of the compounds required by them, and the modes 
in.which they are taken up, and thus be able to adapt particu- 
lar plants to the soils proper for them. If such soils do not 
contain the substances necessary for their life and vigour, he 
must supply them in the form of manures. The applications of 
chemistry and vegetable physiology to agriculture are thus seen 
to be most important, and the great practical improvements 
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