III.] COMPOSITION OF CROPS 57 



composition of their ash. These differences never 

 extend to the entire absence from one plant of an 

 element present in another : all plants contain the same 

 elements, except silica, which is abundant in some plants 

 and entirely absent from others, though, as far as is 

 known, it is unessential to either. 



The common idea that certain plants flourish only 

 in particular soils because they find there some par- 

 ticular constituent which is elsewhere lacking, is 

 therefore erroneous ; we might almost say that all soils 

 contain all the constituents that plants need, so that as 

 far as food goes, where one plant can grow any other should 

 be equally possible. It is true that the excess or 

 deficiency of certain soils in some particular constituent 

 renders them more or less appropriate to one crop or 

 another, but the circumstances which determine the 

 association of a given plant with a particular soil are 

 more often questions of texture, temperature, and water 

 supply, than of absolute nutrition, as represented by the 

 greater or less provision in a soil of a particular con- 

 stituent of the plant. 



Although the composition of different plants does 

 not vary within very wide limits, and though as we shall 

 see later the composition does not throw much light on 

 the suitability of particular soils for certain crops, it is 

 yet desirable to know what quantities of plant food are 

 removed from the soil by the ordinary farm crops. 



Only average figures can be given because of the 

 variations set up by season and soil, but in Table VI. 

 are set out the amounts of the chief nutrient constituents 

 contained in the usual farm crops, based chiefly upon 

 analyses at Rothamsted : — 



[Table YI. 



