THE PEINCIPLES OF MANURING 203 



other substances which are generally regarded as arti- 

 ficial, have often been treated as natural manures. As 

 a popular expression it is useful and often convenient, 

 but it is to be regarded as only vaguely descriptive and 

 not as an exact definition. 



Light and Heavy Manures. The terms light and heavy 

 were at one time introduced and used practically as 

 substitutes for artificial and natural. Farmyard manure, 

 seaweed, plant refuse, and all kinds of bulky organic 

 substances which affect the physical properties of the 

 soil, may be described as heavy manures in contra- 

 distinction to the more concentrated light manures which 

 affect the chemical properties only. These terms imply 

 a distinction of some importance, and are therefore pre- 

 ferable, but they never Jbecame popular and are now 

 practically obsolete. 



General and Special Manures. The first of the above 

 methods is a clumsy attempt to classify manures accord- 

 ing to their origin, and the second is an attempt to 

 classify them according to their uses. No classification 

 can be altogether satisfactory that does not include both 

 ideas. The latter is the more important, and should be 

 made the primary basis of division. 



It has been shown (p. 181) that the chemical functions 

 of manures are twofold, viz., restitution and addition of 

 plant foods. Eestorative manures must be of a complete 

 or general character, i.e., must contain all the fertilising 

 ingredients. Additive manures must be adapted to the 

 special requirements of the soils and crops to which they 

 are applied. They may be, but are not necessarily or 

 usually complete. Manures may therefore be conveniently 

 divided first into 



1. General manures. 



2. Special manures. 



The essential characteristic of general manures is that 



