PLANT FOODS 145 



gas of the air furnishes carbon ; water furnishes hydro- 

 gen and oxygen; and the remaining elements, nitrogen, 

 potassium, phosphorus, and calcium, are often insuffi- 

 cient in the soil, and must be supplied if a good crop is 

 to grow. So these, particularly the first three, are the 

 elements that the farmers buy in their fertilizers. 



Soil elements. The following table shows the com- 

 position of a typical soil, containing 100 pounds of mat- 

 ter. It will be noted that all these elements and com- 

 pounds enter into the plant as food: 



Water 12.67 Ibs. 



Carbon 



Silica 71.55 



Aluminum 6.94 



Iron 5.17 



Magnesium 1.08 



Soda 43 



Sulphuric acid 04 



85.21 Ibs. 



Nitrogen 12 



Phosphoric acid 43 



Potash '. 35 



Lime 1.22 



2.12 Ibs. 



The first and second groups given above are more 

 or less constant in the soil and in sufficient quantities 

 that the farmer need not concern himself about supply- 

 ing them. The third group, nitrogen, phosphoric acid, 

 potash and lime, although constituting so small a por- 

 tion of the soil, must be held up to a good standard 

 amount and proportion or the plant cannot grow. These 

 elements are often used up in the soil and must be ar- 

 tificially supplied by fertilizers of various kinds. 



Elements in a wheat plant. The following table 

 shows the composition of 100 pounds of wheat plants : 



