vi EDITORS PREFACE. 



growth of a plant in all its parts, every clement of food 

 is required. Not one substance has any superior fertilis- 

 ing power over another. The average crop of an un- 

 manured field is always regulated 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 element. If the 

 minimum matter w^as 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 ascer- 

 tained, the application of farm-yard manure is certain 

 in producing a fertilising effect, simply because it is a 

 complex mixture containing all the food elements of 

 plants, and consequently whilst supplying other matters 

 which are not immediately wanted, it also fiu-nishes the 

 minimum substance. In chapter fourth, is discussed 

 the question of this altered composition of the ground 

 by cultivation. 



In chapter eleventh, the fact that not one of the 

 elements of food by itself possesses any superior nutri- 

 tive value over the others is further discussed, Nitro- 

 genous food, like aU the rest, must be present if a plant 

 is to grow properly, but no excess of this element of 

 food will of itself produce more abundant crops. The 

 analyses of soils show that they abomid in nitrogen. 

 Were all other sources of this element w^anting, 

 there would stiU be a continued supply provided for 

 in rain and dew, and in the many processes of oxida- 

 tion going on at the sm^face of the earth. Probably, 

 wherever w^e have a generation and circulation of car- 



