232 REPORT OF COMMISSIONER OF FISH AND FISHERIES. 



because they contain large stores of tbe ingredients of plant-food, as 

 potash, lime, nitrogen, &c., and because these are furnished in available 

 forms, so that the plant can readily use them. As a rule, after cropping 

 for some time, the point is reached where the natural resupply of plant- 

 food is insufficient to produce large crops. In other words, in the 

 so-called "poor," " worn-out," or "exhausted" soils, the natural strength 

 is insufficient for profitable production. 



In order to know what fertilizers to use on such soils we must know 

 what ingredients of plant -food are deficient, and what manures will best 

 supply them. An idea of the essential ingredients of plant-food removed 

 from the soil in cropping may be obtained from the table below, which 

 is calculated from the extensive tables of analyses of plants by Wolff. 



Materials removed from the soil by various crops. 



Large quantities of silica, and small quantities of soda, chlorine, and 

 iron, are also removed from the soil by every crop. Iron is necessary to 

 the growth of all agricultural plants, but in very minute quantity. In 

 many cases s^qj.U amounts of chlorine seem to be requisite. Silica, if 

 needed at all, which is quite doubtful, is required only in extremely 



