96 THE CULTURE OF FARM CROPS. 



some unnatural and excessive product is yielded, is a wholly 

 wrong and mistaken one. \Yrong terms and ideas are in- 

 jurious, notwithstanding that a name has no effect in chang- 

 ing the nature of anything; for they lead to wrong prac- 

 tices and grave errors in the management of the crops, and 

 these cannot fail to result in loss. 



The inorganic substances upon which plants feed and 

 which they extract by their roots from the soil, have been 

 mentioned in a previous chapter, but they may be conven- 

 iently repeated. They are lime; potash; soda; magnesia; 

 sulphur and sulphuric acid; phosphoric acid; silica and 

 chlorine. These, with the exception of sulphur and chlor- 

 ine, which are elements, are the oxides of metals which are 

 elementary substances. The first four are usually found in 

 the ashes of plants combined with carbonic acid as carbon- 

 ates; lime however is found ae a sulphate being combined 

 with sulphuric acid in the ashes of clover and a few other 

 plants. There are a few other substances of inorganic ori- 

 gin which are occasionally found in plants, such as iron, 

 manganese, iodine, &c. but these are evidently accidentally 

 absorbed with the water in which they happen to be in sol- 

 ution, and being innoxious do not interfere with the devel- 

 opment of the plants, but are not strictly plant food. 



The proportion of the various mineral elements of plant 

 growth varies greatly in the different species of vegetables; 

 so much so as to become a leading characteristic with them. 

 Thus there are what may be called potash plants; lime 

 plants; soda plants, &c.; and these dominant elements will 

 be found to have a considerable bearing upon the question 

 of fertilizing crops, to be hereafter treated. Thus on refer- 

 ence to the tables given in the next chapter, it will be seen 

 that the ash of the stems and leaves of potatoes contain from 

 39 to 46 per cent, of lime and 16 to 22 per cent, of mag- 

 nesia; pea straw cantains 38 per cent, of lime; but wheat 

 straw only 6 per cent.; and the tubers of potatoes only 2? 

 per cent.; while the ash of the last mentioned contains 60 

 per cent, of potash; that of turnips 50 per cent., clover 35 

 to 50 per cent.; of young grass 56 per cent.; and of tobacco- 



