PHYSICAL AND CHEMICAL CONSTITUENTS OP SOILS. 29 



of moisture exists. Yet in a soil largely composed of clay, and as 

 much deficient in sand, a very large natural supply of humus will 

 prevent the tenacity and intractability which the clay otherwise 

 would have induced ; and cause the soil, when dry, to be friable, 

 loose, and permeable. In wet seasons, however, the same soil will 

 be again too close and adhesive. 



Further if we can conceive that other materials could be sub- 

 stituted, having entirely different chemical characters, they might 

 serve as well for physical constituents of soils, as the earths of 

 which they took the place. Thus the purest clay, or even pure 

 alumina, if calcined to the state of brick, and then reduced to fine 

 grains, would serve the same physical purposes in soil as silicious 

 sand. And if an artificial soil were thus composed, it might have 

 all the physical qualities of the most sandy soil, while its chemical 

 composition would be more aluminous than ever exists in nature. 



The physical or mechanical action of earths has been kept gene- 

 rally in view through the foregoing pages, inasmuch as the earths 

 have been considered as forming large ingredients of soils. But 

 besides this more obvious action of the agricultural earths, all of 

 them, as well as many other different bodies, act also by chemical 

 power. For the fullest exercise of this power by each, compara- 

 tively very small proportions of each ingredient are required. In 

 a soil composed of any proportion whatever of silicious, aluminous, 

 calcareous, magnesian, and vegetable earths, perhaps the quantity 

 of each acting chemically, might not exceed the hundredth, if the 

 thousandth, part of the whole mass of soil all the remainders of 

 each earth, whether great or small, having, for the time, no other 

 than mechanical action. 



But the magnitude and importance, and value to the farmer, of 

 the mechanical and chemical ingredients of soils are not at all in 

 proportion to the quantities required to exert the different powers. 

 The chemical action is much the more valuable in effect and benefit 

 produced ; and also because the producing agents, from the small 

 quantities required, are more or less under the control of man ; 

 while the great quantity alone of any material required for physical 

 effect, would generally place it entirely beyond control. 



All chemical ingredients of soils, whether of the agricultural 

 earths which also make the universal mechanical materials, or of 

 any other bodies so far as they operate in soils by chemical action, 

 are manures, which serve directly or indirectly, immediately or 

 remotely, to give food to and promote the growth and production 

 of plants. 



Tlms/according to my views, and in the sense in which I use 

 the terms, the physical or mechanical constituents of soils, and the 

 agricultural earths, when serving as earths, are the same; and 



so, that so much of these earths as act chemically, or as chemical 

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