CHAPTER X. 



FIG TEEE FOOD. 



In a general way the food of fig trees is the same 

 that other plants consume — oxygen, nitrogen, hy- 

 drogen, carbon, silicon, sulphur, phosphorus and 

 chlorine of the non-metalics, and potassium, cal- 

 cium, magnesium, sodium, aluminum and iron of 

 the metals. Although oxygen forms about forty- 

 two per cent of their substance, it is taken from 

 the air by leaves without effort to find a supply. 

 Most other elements are either similarly inspired or 

 exist in abundance in ordinary soils, so they may 

 be at once dismissed from consideration. Vege- 

 table matter contains forty-five per cent of carbon 

 and six and one-half per cent of hydrogen, both of 

 which are derived from air and water, so there re- 

 mains but about one-fifteenth part of tree food, the 

 supply of which is of practical concern. Nitrogen 

 forms but one and one-half per cent of plant sub- 

 stance, and potash less than five per cent. These 

 three — potash, phosphorus and nitrogen — are the 

 only foods that have thus far become depleted in 

 soils and they constitute less than seven per cent 

 of the total. The practical problem is to maintain 

 these ingredients, when needed, in such forms and 

 proportions as to be most available and nutritious. 

 How plant food is obtained and made into leaf, sap, 



