AGRICULTURE. 141 



its substance, the mineral elements which constitute the 

 food of plants ; with the power of absorbing, in greater 

 or less degree, the gaseous elements needful for their 

 growth ; of absorbing and retaining moisture, of absorb- 

 ing and radiating heat, in their capacity of subdivision 

 and cohesion ; and in their furnishing the wherewithal 

 for the rooting of plants. 



The chief substances which, as a rule, comprise what 

 may be called the mechanically acting portion of the soil, 

 are clay (insoluble silica), sand, and chalk or lime. Soils 

 are classified according to the relative proportions which 

 they contain of these substances, with other subdivisions 

 determined by the proportion of organic matters (vege- 

 table substances) which are mixed through them, and 

 are denominated respectively clayey, sandy, calcareous, 

 medium ; also clayey loam, sandy or calcareous loam, 

 &c. &c. The physical or mechanical conditions of these 

 different soils can be and are modified to an almost un- 

 limited extent by tillage, by which the most compact and 

 impervious of soils can be rendered friable. 



The process of disintegration, under the action of the 

 atmosphere, the winds, the waters, and the light and heat 

 of the sun, by which soils were formed, is repeated on the 

 soil, and prepares its elements for the use of plants ; 

 chemical changes are effected, and new and soluble com- 

 binations are formed, and the minerals, alkalies, and earths 

 yield to this action their substance, which assumes forms 

 available for the use of plants ; these become diffused 

 through the mass of the soil, and are retained in it by its 

 inherent power of attraction. 



Plants are composed of two principal parts the under- 

 ground, or roots, and the overground, or leaves and stems. 

 The organic construction of these two parts fits them for 

 the different offices they have to perform. The roots 



