30 FIRST PRINCIPLES OF AGRICULTURE. 



titles as compared with other constituents. Its most 

 common combination is with lime, though it is frequently 

 found in combination with iron and aluminum. Bocks 

 which contain "fossils," or fossiliferous rocks, frequently 

 contain high percentages of phosphoric acid. 



Potash. Potash is also derived . from rocks, and 

 varies in the amount contained in different soils. Those 

 derived directly from granite or trap are the richest in 

 this element; it exists in the soil in combination with 

 silica, forming substances called " silicates," which are 

 of great importance. 



Lime. Lime is an ingredient of most soils, and is 

 derived from the decay of limestone, or from fossils. 



The Natural Fertility of Soils. The mineral con- 

 stituents, phosphoric acid and potash, though contained 

 in soils in relatively small amounts, ranging from less 

 than one-tenth per cent to over one per cent, give to 

 soils their chief claims to natural fertility; since most 

 agricultural plants require relatively large proportions 

 of these in proportion to other mineral constituents. 



The quantity of phosphoric acid and potash contained 

 in a soil is, however, comparatively great, since the sur- 

 face soil at a depth of nine inches will weigh, when per- 

 fectly dry, three to three and one-half million pounds 

 per acre ; hence, with even one-tenth per cent, it would 

 contain from three thousand to three thousand five hun- 

 dred pounds of each of these constituents. 



The other necessary mineral ingredients are found in 

 greater or less, and usually in sufficient amounts in all 

 soils. These, while all essential to the complete develop- 

 ment of the plant, are of course less liable to exhaustion. 



