166 FARM-YARD MANURE. 



ground into the plants grown upon it. If one of two 

 fields yields tmce as large a crop of wheat and straw as 

 the other, this necessarily presupposes that the wheat- 

 plants upon the one field have received from the ground 

 twice as much nutriment as those upon the other. 



If the same or different plants are cultivated in succes- 

 sion on a field, the crops will gradually decrease, and the 

 soil will be termed ' exliausted,' in an agricultm^al 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 ft-om the total nutritive substances, 

 just so the exhaustion of the field proceeds from a diminu- 

 tion 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 exliausted 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 former 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. 



