The Composition and Use of Fertilizers. 19'7 



with any other elements, isx familiar to us in several different 

 forms; the most common of these are (1) diamonds; (2) graphite, 

 which is used in the manufacture of lead-pencils; (3) ordinary 

 wood-charcoal; (4) lamp black; (5) animal charcoal; (6) mineral 

 coal. Excepting diamond these forms of carbon are more or 

 less impure, containing some other things mixed with the carbon, 

 (c) Relation to Fertilizers. — We know that carbon must be 

 an important element in plant food, since it forms nearly one- 

 half of the solid portions of plants. Notwithstanding the fact 

 that carbon forms so large a portion of plants, it has no import- 

 ance as an active food constituent of direct fertilizers. This state- 

 ment may appear strange and the question may be asked, " why 

 is not carbon to be regarded as an essential constituent of direct 

 fertilizers? " The answer is that the carbon of plants comes from 

 the carbon dioxide (carbonic acid gas) of the air, and the air 

 furnishes an inexhaustible and available supply of this substance. 

 We do not, therefore, need to add carbon to the soil in order to 

 supply the needs of plants. However, some recent investigations 

 seem to indicate that some carbon is probably taken into the plant 

 through the roots and that forms of carbon other than carbon 

 dioxide may be found to be useful in supplying plants with their 

 carbon food. 



Carbon dioxide is present in the atmosphere to the extent of 

 three parts in 10,000 parts of air. Though present in so small 

 proportions, we have in the atmosphere resting upon one acre 

 of ground more than 20 tons of this gas, an amount sufficient 

 to supply 16 times as much carbon as any crop would ever be 

 likely to need. The supply of carbon dioxide in the air is kept 

 up by the burning of fuel, from the breath of all animals, and 

 from the natural processes of decay of organic matter. A ton 

 of good coal, in burning, produces over three tons of carbon 

 dioxide. The people in a town of 10,000 inhabitants breathe 

 out every twenty-four hours not less than five tons of carbon 

 dioxide. This amount would furnish enough carbon to grow 

 several acres of corn with an extremely large yield. 



Carbon dioxide performs a service for plants in another way 

 when it is carried into the soil in rain water, by means of the dis- 

 solving action it has upon some of the mineral plant food consti- 

 tuents of the soil. In this way it acts as indirect fertilizer. 



(d) Specific Functions in Plant Growth. — Carbon is a con- 

 stituent of nearly all the compounds, except water, found in 



