20 



INTRODUCTORY 



[CHAP. 



ionised solution, and each particular kind of ion 

 establishes its own conditions of equilibrium within 

 and without the cell. 



The soil, however, is not to be regarded merely as 

 an inert medium to anchor the plant and convey the 

 manure to it when convenient, but is itself an enormous 

 potential reserve of plant food. 



We may take, by way of an example, the 

 Rothamsted soil. On the one hand, it is neither 

 richer nor poorer than the majority of British soils 

 and has no abnormal characteristics, so that it repre- 

 sents a very fair average type ; on the other hand, 

 there is no other soil about which so much knowledge 

 has been accumulated. 



Table II.— Analysis of the Soil of Broadbalk Field, 

 Rothamsted, Unmanured for 50 Years. 



The accompanying analysis shows, as usual, that 

 the greater part of the soil consists of insoluble 

 siliceous matter, of which no account need be taken ; 

 there is, further, a certain amount of organic material, 

 important as containing a store of nitrogen which may 



