Mineral Nutrients 



153 



soil the value of using, about once in twelve or fifteen years, 

 a lime with high magnesia content. Finally, of all the 

 important elements furnished by the soil, nitrogen, phos- 

 phorus, and potassium are less abundant, relatively more 

 in demand by the growing crop, and accordingly to be 

 conserved and consistently restored. 



83. Nutrients removed by farm crops. - - In order to 

 appreciate properly the relation of cropping to the fertility 

 of the land it is necessary first to note the amounts of the 

 more important soil nutrients - - nitrogen, phosphorus, 

 and potassium - - which may be removed by various 

 crops annually. The table below is adapted from data 

 given by Hopkins : l - 



1 Hopkins, C. G., "The Fertility in Illinois Soils.' 

 123: p. 189. 



111. Agl. Exp., Bui. 



