THE FOOD OF PLANTS. 281 



dium crop of wheat, so that in two years with 

 wheat the land is deprived of no more of the agent 

 than it loses in one year with potatoes. 



The aim has been in this essay to point out the 

 nature of the materials which plants require, and 

 to impress upon the mind of the reader the great 

 truth'that when the farmer has gained this knowl- 

 edge, and also learned the quantity necessary for 

 a given crop, the accumulation and use of these 

 materials are as simple as supplying raw materials 

 for making cloth, boots and shoes, or any other man- 

 ufacture. A field in proper condition for culture 

 should contain in ample abundance all the inor- 

 ganic materials which the intended crop requires, 

 and these materials should be in an assimilable con- 

 dition, or in other words, they should be in a solu- 

 ble condition, so that by the aid of water they can 

 be taken up and carried through the plant organ- 

 ism. The proper manures for wheat and corn are 

 the nitrogenized varieties, or those which hold 

 nitrogen, either in the form of ammonia or as nitric 

 acid. These should be conjoined with phosphates 

 and potash in considerable amounts. For potatoes, 

 potash, phosphates, and lime are required; the 

 latter element, lime, enters largely into the leaves, 

 and is an important article of food for the vegetable. 

 Gypsum or plaster, which holds lime and sulphurous 

 acid, is a valuable manurial agent for potatoes, es- 



