56 Di' Seller on the Nutrition of Plants. 



Our next step is to examine what kinds of data exist for a 

 calculation of the present amount of organic matter, or, for 

 greater simplicity, of the present amount of carbon, in the 

 organic compounds of the soil of the earth, to set against the 

 annual demands supposed to be made upon it. The most 

 devoted partisan of the theory of the organic nature of the 

 food of plants must regard, I think, the following admission 

 as even extravagantly favourable to his opinion. Notwith- 

 standing the immense tracts of oui* planet covered by the 

 ocean, inland seas, lakes, rivers, sandy deserts, rocks, rocky 

 mountains, and the eternal snows of Alpine and the Polar 

 regions, I assume one-fifth of its surface to be covered with 

 soil to the depth of one foot ; one-tenth, or ten per cent, of 

 this soil to be organic matter, and three-fifths of this organic 

 matter to be carbon. On these data, taken in round numbers, 

 I find there would be nearly three billions and a half of tons 

 of carbon in the organic compounds of the soil of such a por- 

 tion of the earth's surface. 



This limit the quantity of soil cannot possibly exceed ; and 

 it would probably bring us nearer the truth to make the esti- 

 mate of its amount on such data as the tenth part of the 

 earth's surface, six inches of depth and one-twentieth, or 

 five per cent, of organic matter. If these last data be as- 

 sumed, the first quantity, namely, three billions and a half 

 of tons of carbon, will become reduced to one-eighth, or to 

 something more than four hundred thousand millions of tons. 



Between these two limits the truth, I think, must lie. 



If the first estimate be adopted, the soil would bear an 

 annual drain of 600 millions of tons of carbon for nearly six 

 thousand years, or for a period equal to that which seems to 

 have elapsed since man appeared upon the earth. On the 

 second estimate, the soil, at the same rate of consumption, 

 would be completely exhausted of carbon, and, therefore, of 

 organic matter, at the end of 740 years. 



^.ccordlng to the probable data assumed in the paper, the conversion of 

 the whole carbon of the organic matter of the soil, and of living plants 

 and animals into carbonic acid, would not more than double the small 

 proportion of that gas existing at present in the atmosphere, 



