112 SCIENCE OF AGRICULTURE. 



Most of the rocks in New England contain, on an 

 average, 66 per cent, of silica; 16 per cent, of alum- 

 ina; 6 or 7 per cent, of potash, and 5 per cent, of 

 iron; lime and magnesia in much less quantity. And 

 the composition of our soils will be found to corres- 

 pond very nearly with these numbers ; excepting 

 potash and lime in their free state, which are soon so 

 far exhausted by vegetation as not to be detected by 

 the ordinary methods of analysing soils. By potash 

 in a free state, 1 mean to distinguish it from its state 

 or condition when combined with silica, alumina, &c. 

 in rocks, pebbles, and grains of sand, consisting of 

 broken pulverized but not decomposed compound 

 minerals. The terms decompose, and decomposition 

 must be understood to mean to divorce, or to separate, 

 or the separation of the elements of a compound 

 substance. Felspar, a large component part of gra- 

 nite, greenstone, and other of our most common 

 rocks is, for example, composed of silica 63, alumi- 

 na 17, potash 13, lime 3, oxyd of iron 1 in 100 parts; 

 broken or pulverized, each particle is composed of 

 these elements in their natural state chemically 

 combined. Decompose felspar and we have the sili- 

 ca, alumina, potash, &c. still mixed perhaps, but 

 divorced or separated so far as these terms express, 

 the breaking of the bonds of adhesion which exist 

 in their combined state. 



In their combined, confederated, compact state, 

 they aid each other in resisting the action of the 

 hungry roots of vegetables upon them; decomposed, 

 they readily become the prey of their vegetable de- 

 vourers. Hence the potash, for example, of the de- 

 composed felspar does not long remain in the soil, 

 but will be found in the plants and trees growing 

 thereon. 



Among the elements which compose our cultivated 

 vegetables, are found silica, lime, magnesia, oxyd of 

 iron, potash, soda, sulphuric, and phosphoric acids. 

 Hence these will be found constituents of all soils 

 capable of producing them. 



