of the soil, a smaller quantity penetrates to the lower lay- 

 ers, and scarcely any at all to the deep layers and subsoil. 

 Hence when a subsoil is exhausted, manure cannot restore 

 its fertility. From this peculiar property of soils of ar- 

 resting the circulation of solutions of the food of plants, 

 arises the necessity of employing means for the distribu- 

 tion of food, and for the uniform mixture of the different 

 layers of the soil. The manner in which this is effected by 

 mechanical operations, by organic matter, by manures, by 

 certain chemical salts, &o., is pointed out in chapters sec- 

 ond, third, and twelfth. 



The quantity of food in a state of physical combination 

 in any fertile soil is only limited. Continuous cultivation 

 without replacement of all the mineral matters removed 

 hi the crops destroys fertility, either by causing the abso- 

 lute loss of the assimilable food, or by altering the proper 

 relative proportions between the different elements of food, 

 to such an extent that the due growth of all parts of the 

 plant is altered. For the successful growth of a plant in 

 all its parts, every element of food is required. Not one 

 substance has any superior fertilising power over another. 

 The average crop of an unmanured field is always regu^ 

 lated by that element of food which is present in minimum 

 quantity. The effect of a manure when beneficial is merely 

 to increase the relative proportion of this minimum ele- 

 ment. If the minimum matter was known in each case, 

 its direct application would be sufficient to increase the 

 fertility of the soil. But as in general this point is not 

 ascertained, the application of farm-yard manure is certain 

 in producing a fertilising effect, simply because it is a com- 

 plex mixture containing all the food elements of plants, 

 and consequently whilst supplying other matters which 

 are not immediately wanted, it also furnishes the minimum 

 substance. In chapter fourth, is discussed the question of 

 this altered composition of the ground by cultivation. 



