298 GENERAL FUNCTIONS OF THE SOIL. 



must be firmly fixed, therefore the soil must have a certain consistency, 

 — -their roots must find a ready passage in every direction ; therefore the 

 soils must be somewhat loose and open. Except for these purposes, we 

 see linle immediate use for the sand and alumina whicli form so much 

 oi -ne substance of soils — till we come to study their physical properties. 

 The siliceous sand is insoluble, and the alumina exists in plants In very 

 minute quantity only, while during the progress of natural vegetation, 

 the proportion of vegetable matter in the soil actually increases. The 

 immediate agency, therefore, of these substances is not chemical but 

 physical. 



The alumina of the clays is of immediate use in absorbing and retain- 

 ing both water and air for the use of the roots — while the vegetable mat- 

 ter is advantageous in reference to the same ends, as well as to the power 

 of absorbing quickly and largely the warmth of the sun's rays. The 

 soil, in short, in reference to vegetation, performs the four following dis- 

 tinct and separate, but each of them important and necessary, func- 

 tions : — 



1°. It upholds and sustains the plant, affording it a sure and safe an- 

 chorage. 



2°. It absorbs water, air, and heat, to promote its growth 



These are its mechanical and physical functions. 



3°. It contains and supplies to the plant botli organic and inorganic 

 food as its wants require ; and 



4°. It is a workshop in which, by the aid of air and moisture, cliemi 

 cal changes are continually going on ; by which changes these several 

 kinds of food are prepared for admission into the living roots. 



These are its chemical functions. 



All the operations of the husbandman are intended to aid the soil in the 

 performance of one or other of these functions. To the most important 

 of these operations — tlie methods adopted by the practical farmer for 

 improving the soil — it is my intention, in the following division of these 

 Lectures, briefly to direct your attention. 



