MANURES. 217 



proper cultivation, by a suitable rotation of crops, and by 

 the use of suitable manures. It is the latter with which 

 we are chiefly concerned in this chapter. 



The chief object then of manuring is to restore fertility 

 to soils which are becoming, or have become, more or less 

 exhausted, to enrich soils which are from their very na- 

 ture poor, or to prevent exhaustion taking place, by re- 

 turning to the soil in a useful form those valuable sub- 

 stances which are continually being removed by the crops 

 we grow. 



The word "fertility" is so often employed in connec- 

 tion with manuring that it is necessary to clearly under- 

 stand what is meant by it, and what the essential conditions 

 of fertility in a soil are. The reply which would generally 

 be hazarded to such a question is that a fertile soil is one 

 that contains a plentiful supply of plant food. This 

 answer would however be incomplete, for there are many 

 conditions that have to be fulfilled before a state of ferti- 

 lity is attained. We may say that fertility depends on 

 the properties of the soil as classified under these three 

 heads : 



I. Physical. 

 II. Chemical. 

 III. Bacteriological. 



A discussion in detail of the physical conditions which 

 influence the productiveness of a soil would involve tech- 

 nical details which it is desired to avoid, though, for a 

 complete understanding of the action of manures a brief 

 summary of them is necessary. One of the most import- 



