PHYSICAL DEFECTS IN SOILS. 121 



mals, though largely produced by them, is so ne- 

 cessary to plants that they may almost be said 

 to subsist upon it ; absorbing it from the air, and 

 decomposing it by their leaves in the sunshine, 

 in such a manner as to retain the carbon and 

 liberate the oxygen, which thus becomes fitted 

 for the purposes of combustion and respiration. 

 Were it not for the absorption of carbonic acid 

 by plants, the air would become unfit for respir- 

 ation, and all animal life would perish from the 

 face of the earth. 



Since carbonic acid is so important as food for 

 vegetables, it is interesting to know how much 

 of it is available in the atmosphere in other 

 words, to estimate the stoclc on hand. The 

 amount in weight is nearly the one-thousandth 

 part of the weight of the atmosphere, and accor- 

 ding to the calculation of Liebig, the amount of 

 carbon contained is more than 3081 billions of 

 pounds a weight exceeding that of all the veg- 

 etation and all the strata of mineral and brown 

 soil on the face of the earth. Nor has this vast 

 amount been perceptibly lessened since the me- 

 thod of estimating its amount was discovered, 

 because it is being constantly supplied from the 

 sources previously stated, as fast as it is appro- 

 priated by vegetation. 



The physical condition of soils, and their ca- 

 pacity for retaining heat and moisture, is a sub- 



