VI. MANURING AND GENERAL MANURES 105 



By artificial manures are indicated products either derived from 

 mineral deposits or manufactured in the arts, though the term is often 

 extended to substances of animal or vegetable origin, which are not 

 produced on the farm. In this sense, guano, sea-weed, oil-cakes and 

 other substances are sometimes classed as artificial manures. 



Another, perhaps more satisfactory classification, is into 



1. General Manures. 



2. Special Manures. 



A general manure is one which contains all the necessary con- 

 stituents of plant food and thus imparts, to the soil to which it is 

 applied, a complete store of the nutriment required for fertility. Ex- 

 amples of such manures are afforded by farm-yard manure, guano and 

 most plant and animal remains. 



A special manure contains only one or two constituents of plant 

 food, and cannot therefore supply all the requirements of plants. 

 Nitrate of soda, sulphate of ammonia, potash manures and phosphates 

 are good examples of this class. 



General manures are the safest to employ in practice, especially 

 when the manurial requirements of the soil are not well known, for 

 though, by their use, the soil may be receiving additions of certain con- 

 stituents which it does not require, such additions do no harm and a 

 better crop results from the increased supply of the other constituents 

 in which the soil may be lacking. 



Special manures, however, if intelligently employed, possess great 

 advantages and are often more economical. By their aid, the farmer, 

 if sufficiently well informed, is enabled to supply the soil with just 

 those constituents which it most needs, without the waste of labour 

 and expense entailed by using materials which are not necessary. 



To regard manures with reference to their chemical composition 

 only and to value them exclusively by the amounts of plant food which 

 they contain, is, however, distinctly erroneous. All manures have some 

 influence upon the texture and physical properties of the soil, while 

 some also exert a powerful effect upon the activity and development of 

 the micro-organisms of the soil, and in these ways produce effects upon 

 its fertility, apart altogether from their power of supplying one or more 

 items of plant food. 



But to ascribe the beneficial effect of the application of manures to 

 soil, solely to their physical action in altering its mechanical properties, 

 as seems to have been done recently by certain American writers, is 

 contrary to the experience of many years and to the results of thou- 

 sands of field trials, and cannot be seriously considered. It is true that 

 the mechanical effects of bulky organic manures, e.g., farm-yard 

 manure, upon light, sandy soils is often very great and sometimes, 

 perhaps, as important as their power of supplying plant food. Even 

 w T ith concentrated " artificial " manures, marked effects upon the text- 

 ure of the soil are often produced and doubtless greatly affect its fer- 

 tility. Examples are seen in the beneficial action of manures containing 

 free lime upon heavy, clay soils, while repeated applications of nitrate 

 of soda to such soils, render them more plastic and tenacious and 



