VIII.] ELEMENTS FOUND IN SOIL 155 



less dependent upon mineral manures than are the 

 shallow-rooted crops like turnips and barley. Soil 

 analysis, even by the most refined methods, is chiefly of 

 value when the analysis of the particular soil under 

 consideration can be compared with a number of 

 analyses of similar soils on the same type of land ; so 

 as to ascertain its relative deficiencies or excesses, and 

 interpret them in the light of what has been ascertained 

 beforehand about this type of soil by experience or by 

 field trials. If, for example, the average percentage of 

 phosphoric acid in soils of a given type, defined perhaps 

 by a particular geological formation, is O-IS, and it is 

 also known that phosphatic manures usually meet with 

 a fair response on such land, then if the soil of a 

 particular field on that formation only shows o-i2 per 

 cent, of phosphoric acid, it will be safe to predict the 

 necessity for phosphatic manures on that field, though 

 on other soils 0-12 of phosphoric acid might indicate 

 such richness in that constituent as to remove the need 

 for phosphatic manures. 



One other point is brought out in a series of soil 

 analyses: besides a considerable uniformity in the 

 proportions of nitrogen, phosphoric acid, and potash, all 

 soils contain very much the same substances. It is 

 extremely rare to find that any one of the regular 

 constituents is missing, i.e., soda, potash, lime, magnesia, 

 iron, manganese, alumina, chlorine, silica, phosphoric 

 and sulphuric acids ; and it is almost as rare to find an 

 appreciable quantity of any other element, though one 

 or two occur occasionally in small quantities. Now this 

 fact puts an end to the very common idea that differ- 

 ences in character or quality in the crops grown on a 

 given soil are due to specific deficiencies in the soil 

 which may be made up artificially. Of course, there 

 are cases where a crop shows certain special features 



