252 SOILS. 



principally mechanical, tending to keep the soil 

 moist, from its attraction for water, and likewise 

 being highly useful in absorbing ammonia, both from 

 the air and from all decaying substances evolving it 

 in the vicinity, by virtue of that property which 

 many porous substances, and more especially char- 

 coal, possess, of absorbing or condensing that gas 

 (162) ; which, as we shall shortly show, is of great 

 importance in the growth of plants. 



665. Soils differ greatly in their mechanical as 

 well as in their chemical nature. The same sub- 

 stances constitute a soil possessing very different pro- 

 perties, according as they are in the form of little 

 grains like sand, or in very fine powder. This state 

 of mechanical division is of great importance for 

 several reasons, and most particularly in relation 

 to water. A soil containing a large quantity of 

 alumina is generally known by its stiff, tenacious 

 character, and is remarkable for its great retentive 

 power for water ; whilst those consisting principally 

 of silica, and more especially those in which it exists 

 in the form of sand, are generally light and porous 

 soils, and far less retentive of water. 



66Q. Again, a soil containing alkaline silicates in 

 the form of little grains, always contains free alkaline 

 matter in a soluble state, set free by the decomposi- 

 tion of those silicates, and this separation of alkaline 

 matter continues so long as there is any of the solid 

 silicates left ; this effect would cease in a short time 

 if all the silicates were very finely powdered ; they 



