132 ACTION OF SOIL ON FOOD OF PLANTS IN MANURE. 



contains a nutritive substance in the nitric acid, and salts 

 of ammonia in the ammonia. Hence it is extremely diffi- 

 cult in individual cases to determine whether their action 

 is due to their nutritive constituents, or to the fact that 

 they have brought about the absorption of other nutritive 

 substances. 



In a fertile soil tillage and manuring have a definite 

 relation to one another. If, after a rich harvest, the field 

 is prepared by tillage alone to produce a similar rich crop 

 in the next year, that is, if the mechanical means are suf- 

 ficient to distribute the store of nutritive substances so 

 uniformly that the plants of the following season will find 

 as much nutriment in all parts of the soil as during the 

 last, any further supply of mineral constituents by manur- 

 ing would be mere waste ; but, where a field is not in that 

 condition, the deficiency must be suppHed by manure, in 

 order to restore the original power of production. Thus, 

 in a certain sense, the mechanical operations of tillage 

 and of manure are supplementary to one another. 



Of two similar fields, maniu-ed in exactly the same way, 

 if the one has been well tiUed, and the other badly tilled, 

 the former wiU yield a richer crop, i.e. the manure seems 

 to have a better effect upon this than upon the badly- 

 tiUed field. 



If one of two farmers knows his land better, and culti- 

 vates it more judiciously than the other, the former will, 

 in a given time, obtain as good crops with less manure, or 

 richer crops with the same quantity of manure. 



All these facts should be considered in estimating the 

 value of manuring agents ; but, as science has no 

 standard for measuring the results of the mechanical 

 operations of tillage, this cannot be taken into account 

 here, and we must confine ourselves to that which can be 

 scientifically measured and compared. 



