PRACTICAL AGRICULTURE. 99 



ashes ; the same quantity of barley straw, 8 1-2 : of oat 

 straw, only 4 ; the ashes of the three are, chemically, of 

 the same composition. Upon the same field which will 

 yield only one harvest of wheat, two successive crops of 

 barley may be raised, and three of oats. . We have, ia 

 these facts, a clear proof of what is abstracted from the 

 soil, and, consequently, what plants require for their 

 growth.— a key to the rational mode of supplying the 

 deficiency. 



Potash is not the only substance requisite for the ex- 

 istence of most plants ; indeed, it may be replaced, in 

 some cases, by soda, magnesia, or lime ; but other sub- 

 stances are required also. 



Plants obtain phosphoric acid (found in combination 

 with lime or magnesia) from the soil, and they, in their 

 turn, yield it to animals, to assist in the formation of 

 their bones. Creatures that feed upon flesh, bread, fruit, 

 and husks of grain, take in much more phosphorus than 

 is required for the building up of the animal fabric ; and 

 this excess is again usefully thrown out by them, chief- 

 ly in their liquid excrements. Some plants, however, 

 extract other matters from the soil besides silica, potash, 

 and phosphoric acid, which are essential constituents of 

 the plants ordinarily cultivated. 



American farming presents us with varied instances of 

 plants sown, and growing together in the same 

 field. Two such vegetables will mutually injure each 

 other, if they withdraw the same food horn the soil. — 

 Plants will thrive beside each other, either when the 

 substances necessary for their growth, extracted from 

 the soil, are of different kinds, or when they themselves 

 are not both in the same stage of growth at the same 

 time. On a soil containing potash, wheat and tobacco 

 may be reared in succession, because the latter plant 

 does not require the phosphates which the wheat has ap- 

 propriated to itself. Now, tobacco requires only alkalies, 

 and food containing nitrogen. When we growdifl^erent 

 plants in the same soil, for several years in succession, 

 the first of which leaves behind that which the second, 

 and the second that which the third may require, the 



