PLANT FOOD 



63 



Nitrogen sometimes makes as high as 4 per cent of the 

 dry matter. 



No other element occurs in nearly so large an amount, 

 and the amount of most of them is very small indeed. 

 It may not require over a pound of iron to grow an acre 

 of hay, but this iron is absolutely necessary. How small 

 a part of the plant's substance is obtained from the 

 soil is shown by the following table. Only one pound in a 

 hundred of turnips comes from the solid matter of the soil, 

 and a little over 3 per cent of the grain of corn: 



PROPORTIONS OF DIFFERENT ELEMENTS IN PLANTS 



60. Elements Likely to Be Deficient in Soils. Since 

 ten elements are absolutely necessary for plant growth, 

 if any one of these does not occur in sufficient quantities, 

 the crop will suffer. Hydrogen and oxygen (in the form of 

 water), nitrogen, phosphorus, potassium and sometimes 

 calcium, are not always available in sufficient quantities 

 for the production of good crops. The other elements 

 are practically always present in abundance. 



Water is most frequently the factor that limits the size 

 of the crop. It is increased or conserved by irrigation, 

 tillage and other farm operations. 



61. Functions of the Different Elements. Some text- 

 books of botany mention iron as the element necessary 



