112 ELEMENTS OE AGRICULTURE 



soil. Now, ordinary soil weighs about 90 pounds to the 

 cubic foot. In other words, an acre of soil twelve inches 

 deep weighs 3.920,400 pounds. If such a soil contains .1 

 per cent of phosphoric acid it would yield 3,920 pounds 

 to the acre. Poor, sandy soil contains sometimes only 

 0.05 per cent phosphoric acid, or 1,960 pounds to the 

 acre. Now, a crop of 1,000 pounds of tobacco removes 

 3.4 pounds of phosphoric acid. So an acre of good 

 soil contains enough phosphoric acid to supply about 

 1,150 crops, of 1,000 pounds each, of tobacco, and poor 

 sandy soil has enough for about 575 crops of the same 

 size. These figures do not mean that the soil con- 

 taining .1 per cent of phosphoric acid could be cropped 

 in tobacco, continuously, for 1,150 years without be- 

 coming -exhausted, for tobacco requires other elements 

 besides phosphoric acid ; but they do show what a great 

 store of phosphates the soil contains which, if properly 

 used, will furnish many crops with food. But we have 

 considered only the first twelve inches of soil; the roots 

 of most crops penetrate to greater depths than twelve 

 inches; the roots of the cotton plant have been known 

 to reach into the subsoil between seven and eight feet. 

 The roots of wheat plants often reach a depth of four 

 or even five feet. As the subsoil often contains nearly 

 as much of phosphates as the soil, such crops have 

 much larger supplies of this food to draw on than are 

 contained in the first twelve inches. There is, of course, 

 some phosphoric acid lost from the soil through leach- 

 ing, but as most of the phosphates are very insoluble 

 this loss in well cultivated soil is but small. An ordi- 

 nary soil, if carefully cultivated, may be steadily 



