ESSAY 



CALCAREOUS MANURES. 



PART FIRST— THEORY 



CHAPTER I. 



GENERAL DESCRIPTION OF EARTHS AND SOILS. 



It is very necessary that we should correctly distinguish earths and soils 

 and their many varieties ; yet these terms are continually misapplied — and, 

 even among authors of high authority, no two agree in their definitions, or 

 modes of classification. Where such differences exist, and no one known 

 method is so free from material imperfections as to be referred to as a com- 

 mon standard, it becomes necessary for every one who treats of soils to 

 define for himself — though perhaps he is thereby adding to the general 

 mass of confusion already existing. This necessity must be my apology 

 for whatever is new or unauthorized in the following definitions. 



The earths important to agriculture, and which form nearly the whole of 

 the known globe, are only three — silidous, aluminous and calcareous. 



Silicious earth, in its state of absolute purity, forms rock crystal. The 

 whitest and purest sand may be considered as silicious earth in agriculture, 

 though none is presented by nature entirely free from other ingredients. It 

 is composed of very hard particles, not soluble in any common acid, and 

 which cannot be made coherent by mixing with water. Any degree of 

 coherence, or any shade of color that sand may exhibit, is owing to the 

 presence of other substances. Tlie solidity of the particles of sand renders 

 them impenetrable to water, which passes between them as through a sieve. 

 The hardness of its particles, and their loose arrangement, make sand inca- 

 pable of absorbing moisture from the atmosphere, or of retaining any valu- 

 able vapor or fluid, with which it may have been in any manner supplied. 

 Silicious earth is also quickly heated by the sun, which adds to the rapidity 

 with which it loses moisture. 



Aluminous or argillaceous earth, when dry, adheres to the tongue, ab- 

 sorbs water rapidly and abundantly, and when wet forms a tough paste, 

 smooth and soapy to the touch. By burning, it becomes as hard as stone. 

 Clays derive their adhesiveness from their proportion of aluminous earth. 

 This also is white when pure, but is generally colored deeply and variously — 

 red, yellow, or blue — by metallic substances. When drying, aluminous 

 earth shrinks greatly, and becomes a mass of very hard lumps, of various 

 sizes, separated by cracks and fissures, which become so many little reser- 

 voirs of standing water when filled by rains, and remain so, until the 



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