212 GEOLOGY AND CHEMISTRY OF SOILS. 



into the food of plants. It neutralizes acids, and, by uniting 

 with silicic acid, forms the outer covering or epidermis of 

 the grains and grasses. It is found in all plants in considera- 

 ble abundance, and is one of the greatest fertilizers of the 

 soil. Plants, as we have frequently remarked, eliminate it 

 from the rocks by galvanic action ; decomposing vegetable 

 matter also abstracts it ; the ordinary action of the air, wa- 

 ter, and many other agents: hence the use of clay, ashes, 

 of fallow crops, and ploughing in green crops, to induce the 

 soil to yield its potash. 



tSoda, as we have seen, is a constituent of many minerals, 

 such as albite containing 11.43 per cent., mica contain- 

 ing from 3 to 5 per cent., and basalt from 5 to 7 per cent, of 

 this alkali. But the proportion in the soil is much less, in 

 consequence of the action of growing plants, — many of which 

 take up and appropriate it as food. Common salt is a chloride 

 of sodium, and is found very widely diffused, so that this al- 

 kali exists probably in sufficient quantities in the soil to sup- 

 ply all the wants of plants. Its action is similar to potassa, 

 but it is not so essential to vegetation. Porphyritic soils con- 

 tain it in the greatest abundance. 



Oxide of iron exists in the soil as a protoxide, peroxide 

 and in combination with acids. It is found in all soils, in 

 one or all of these forms. The use of clay has been supposed 

 to result from its containing from 9 to 13 per cent, of this 

 substance. It is also found in green sand in great abundance, 

 constituting about |- part of the whole mass. In fact, it is 

 widely diffused through all the primitive and most of the 

 secondary rocks. 



The quantity of oxide of iron as determined by analysis, 

 varies considerably in different soils, from 1 to 5 per cent, in 

 the soils of Massachusetts. The soils of Maine contain from 

 2 to 12 per cent., those of Rhode Island from 2 to 8 per 

 cent., and generally soils contain at least 5 per cent, of this 

 oxide. 



