PRACTICAL AGRICULTURE. 41 



manure can long answer on the same land ; it can only 

 sup[)lv the materials; it contains. When all the sihcate 

 of potasii in grain-fields is exhausted, urine will not, can- 

 not, supply the deficiency, because it contains no silicate 

 of potash. So long as the land remained rich in this 

 material, urine or blood would supply the requisite ni- 

 trogen. Hence, in all ages and countries, the habit of 

 employing mixed manures and artificial composts has 

 been universally diffused. What is wanting is a more 

 accurate knowledge of the precise deficiency at any 

 given moment, and a consequent saving of capital from 

 unnecessary waste, together with an immense increase 

 in fertility, as the reward of so accurate an adaptation 

 of means and ends. The knowledge of a disease is es- 

 sential to the correct application of a remedy. 



It is by no means difficult to prevent the destructive 

 fermentation and heating of farm-yard compost. The 

 surface should be defended from the oxygen of the at- 

 mosphere. A compact marl, or a tenacious clay, offers 

 the best protection against the air; and before the dung 

 is covered over, or, as it were, sealed up, it should be 

 dried as much as possible. If the dung be found at 

 any time to heat strongly, it should be turned over, 

 and cooled by exposure to air. Watering dung-hills is 

 sometimes recommended for checking the process of 

 putrefaction, and the consequent escape of ammonia ; 

 but this practice is not consistent with correct chemistry. 

 It may cool the dung for a short time ; but moisture is 

 a principal agent in all processes of decomposition. Wa- 

 ter, or moisture, is as necessary to the change as air; 

 and to supply it to reeking dung, is to supply an agent 

 which will hasten its decay. 



If a thermometer, plunged into the dung, does not rise 

 much above blood-heat, there is little danger of the 

 escape of ammonia. When a piece of paper, moistened 

 with spirit of salt, or muriatic acid, held over the steams 

 arising from a dung-hill, gives dense fumes, it is a cer- 

 tain test that decomposition is going too far; for this 

 indicates that ammonia is not only formed, but is es- 



