22 THE CHEMISTEY OF THE FARM. 



tremely fertile. The surface soil (first nine inches) of a 

 pasture may contain when dry O25 of nitrogen per cent., 

 while soil of the same depth from an arable field may yield 

 0*10 0*15 per cent., and a clay subsoil 0'05 per cent. 

 A good surface soil may contain 0*20 per cent, of phosphoric 

 acid, or not unfrequently a smaller quantity. Potash 

 varies much, rising to 1*0 per cent, or more in some clay 

 soils, but being generally much smaller. 



The weight of soil on an acre of land is, however, so 

 enormous, that small proportions of plant food may amount 

 to very considerable quantities. Nine inches' depth of 

 arable soil (clay or loam) will weigh, when perfectly dry, 

 about 3,000,000 or 3,500,000 Ibs. A pasture soil will be 

 lighter, the first nine inches weighing when dried and the 

 roots removed about 2,250,000 Ibs. Supposing, therefore, 

 a dry soil to contain O'lO per cent, of nitrogen, phosphoric 

 acid, or potash, the quantity in nine inches of soil will be 

 from 2,250 Ibs. to 3,500 Ibs. per acre. 



A large part of the elements of plant food contained in 

 soils is present in such a condition that plants are unable 

 to make use of it. An acre of soil may contain many 

 thousand pounds of phosphoric acid or of nitrogen, and 

 vet be in a poor condition ; while a dressing supplying 

 50 Ibs. of readily available phosphoric acid, or nitrogen, 

 in the form of superphosphate or nitrate of sodium, may 

 greatly increase its productiveness. 



The fragments of silicates or limestone present in soil, 

 as stones, gravel, and sand, are as a rule of little value to 

 a plant, the elements of plant food which they contain 

 being mostly in too insoluble a condition to be attacked by 

 the roots. These fragments of rock may, however, be 

 slowly decomposed by the mechanical action of frost, and 



