28 PHYSICAL AND CHEMICAL CONSTITUENTS OF SOILS. 



avoid the apparent contradiction which might be inferred, if, in a 

 general description of calcareous earth, I had omitted all allusion 

 to qualities that will afterwards be brought forward as all-important. 



Physical (or Mechanical} and Chemical Constituents of Soils. 



In the discussion of this general subject, we should always bear 

 in mind the different actions of the earths as the physical, or me- 

 chanical, and the chemical ingredients of soils. These different 

 actions have already been incidentally referred to ; but they require 

 more particular notice. 



Any of the earths which may serve as large materials in the 

 composition of a soil, must act, for much their greater proportion, 

 merely mechanically in the relation of the soil to the growth of 

 plants. Thus, the various mixtures of silicious and aluminous 

 earths existing in all ordinary soils and these more rarely with 

 large proportions of either calcareous or magnesian earth, or humus 

 serve, for much the larger proportions of each and all, to furnish 

 merely that mechanical position and support for growing plants 

 which is necessary for them to draw freely the available supplies 

 of water, air, and food. The conditions necessary for this purpose 

 are, that the soil shall have enough sand to be sufficiently permeable 

 by moisture, and for the extension of the rootlets ; that there shall 

 be enough clay to give firm support to the plant in its upright 

 position, and sufficiently to close the too great openness of thfi'sand. 

 These necessary physical conditions of the soil, in relation to its 

 texture and powers of receiving, retaining, and transmitting moist- 

 ure, are further improved, and opposite evils either modified or 

 prevented, by additional admixtures of calcareous (and perhaps 

 magnesian) earth, and humus. But so far the action of each and 

 all these materials, in large quantities, (and for much the larger 

 proportion being always understood), act only by their physical 

 qualities, and exert such powers in proportion to quantities. Any- 

 one of these materials, for much its greater part, might be substi- 

 tuted by some other, if offering like physical qualities, though 

 totally different in chemical character and constitution. Thus, 

 when chalk greatly predominates in soil disposed to dampness, 

 from position or climate, its physical qualities serve to increase the 

 evil, as would clay ; and the soil is both colder and wetter than if 

 there were no physical action of the calcareous earth. On the 

 other hand, in a soil disposed to suffer by dryness, the like chalky 

 constitution would increase that evil, as would sand, by its open 

 texture permitting the too rapid escape of moisture. Humus, in 

 large proportion, acting mechanically Ifke clay, serves to close the 

 too open pores of sandy soils ; and, by its remarkable absorbent 

 power, to make them more retentive of moisture wherever excess 



