CHEMICAL AND AGKICULTUEAL EXHAUSTION. 165 



If the same or different plants are cultivated in suc- 

 cession on a field, the crops will gradually decrease, 

 and the soil will be termed ' exhausted,' in an agricul- 

 tural sense, when the crops cease to be remunerative, i e. 

 do not cover the expense of labour, interest of money, 

 &c. As the high crops were caused by the soil giving to 

 the plant a certain number of parts from the total nutri- 

 tive substances, just so the exhaustion of the field pro- 

 ceeds from a diminution in the sum of those nutritive 

 substances. 



The same number of plants cannot thrive upon the 

 same field as formerly, if the same quantity of nutritive 

 substances enjoyed by the previous crop is no longer to 

 be found. The exhaustion of a cultivated field in a 

 chemical sense differs from the agricultural use of the 

 term in this, that the former refers to the total amount 

 of nutritive substances in the soil, the latter to that 

 portion only of the total amount which the ground can 

 furnish to plants. A field is termed exhausted in a 

 chemical sense when it altogether fails to produce any 

 more crops. 



Of two fields, one of which contains, to the same 

 depth, a hundred times, the other only thirty times, the 

 amount of food required by a full wheat crop, the for- 

 mer furnishes to the roots of the plants more nutriment 

 than the latter in the proportion of 10 : 3, supposing 

 the condition and mixture of the soil to be the same in 

 both cases. If the roots of a plant receive from certain 

 spots of the one field 10 parts by weight of nutriment, 

 the roots of the same plant will find upon the other 

 field only 3 such parts available for absorption. 



An average wheat crop of 2000 kilogrammes ( = 39 

 cwts.) of grain, and 5000 kilogrammes ( 98 cwts.) 

 of straw, receives from a hectare ( 2-J- acres) of ground 

 250 kilogrammes ( 5 cwts.) of ash-constituents on, 

 an average. Now, upon the supposition that a field, 

 to give an average crop, must contain 100 times that 

 quantity (or 25,000 kilogrammes) of ash-constituents in 

 a perfectly available state, it follows that such a field 

 gives 1 per cent, of its total store to the first crop. 



