320 EXHAUSTION OF SOILS. 



gravel or earth, before digging, make 27 cubic feet when 

 dug. 



As a rough estimate, it may be stated that an acre of or- 

 dinary soil weighs 100 tons for every inch of its depth. 



EXHAUSTION OF SOILS. 



Each crop taken from a field exhausts the soil to the ex- 

 tent of the inorganic or earthy substances that are found in 

 the totality of the crop removed. Unless, therefore, these 

 elements are returned to the soil in some shape it gradually 

 loses its fertility, and finally refuses to produce altogether. 

 Hence the necessity for manuring, irrigating, or resting the 

 soil, that it may again, by accumulating these elements, re- 

 cover its fertility. By returning a crop in toto to the soil, 

 by ploughing it in or leaving it to decay and mingle again 

 with it, it accumulates in mass and grows in fertility, not 

 by the substances thus returned to it, but by fertilizing ele- 

 ments gathered in or combined from the atmosphere, by rains 

 and dews descending on it, and by capillary attraction from 

 beneath. 



By knowing the composition of the subtracted crops and 

 the added manures, the farmer can keep a debit and credit 

 account with his fields, which will be sufficiently accurate 

 to enable him always to keep his land improving. To enable 

 him to ascertain approximately what his various crops remove 

 from the soil, we introduce the following tables, &c. To 



