COMPOSITION OF SOILS.

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modified by the presence of a greater or less quantityof organic substances.

303. Soils differ greatly in their mechanical as wellas in their chemical nature. The same substances con-stitute a soil possessing very different properties accord-ing as they are in the form of little grains like sand, orin very fine powder. This state of mechanical divisionis of great importance for several reasons, and most par-ticularly in relation to water. A soil containing a largequantity of alumine is generally known by its stiff, tena-cious character, and is remarkable for its great retentivepower for water; whilst those consisting principally ofsilica, and more especially those in which it exists inthe form of sand, are generally light and porous soils,and far less retentive of water.

304. The best soils are those in which the earthyconstituents are so proportioned that the light, porousqualities of the one are balanced by the close, retentiveproperties of the others; for they are then most uni-formly suitable to vegetation.

305. The silica and alumine in soils are of coursealmost wholly free and uncombined with any acid, asthe former is not a base, and the latter has hardly anyaffinity for the weaker acids, such as the carbonic. Smallquantities of silica are almost always found in soils, com-bined with either soda or potash, forming those curiouscompounds before alluded to, in which the silica seemsto play the part of an acid (185) ; soils never containmore than a very small quantity of these substances ;

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