CHEMICAL NATTJEE OF SOILS 355 



According to Hilgard/ the substance which, assumes com- 

 manding importance as controlling the fertility of a soil, aside 

 from physical conditions, is lime, in the presence of which, in 

 adequate proportions, smaller percentages of the other plant 

 foods will suffice for high and lastmg productiveness, than 

 would otherwise be the case. Since lime is the essential con- 

 stituent of the rock limestone, it follows that, other things 

 being equal, a ''limestone country is a rich country." As else- 

 where noted, however, a limestone soil may have become so 

 leached of its lime, through prolonged decay, as to be benefited 

 by artificial applications of this same constituent. Lime is, 

 moreover, so generally distributed throughout the great majority 

 of rocks that few soils would be lacking in this constituent 

 were even a small proportion of the original amount left in the 

 residue from rock decay, instead of being so largely removed 

 in solution. 



It would follow from this that the composition and fertility 

 of a soil is dependent not more upon the character of the rock 

 mass from which it is derived, than upon the prevalent climatic 

 conditions under which it originated, the general average tem- 

 perature and the amount and distribution of the rainfall being 

 particularly important factors. This branch of the subject has 

 also been considered in some detail by Hilgard, to whom we are 

 indebted for the only satisfactory resume. Concerning condi- 

 tions of temperature, this author says : 



''Within the ordinary limits of atmospheric temperatures all 

 the chemical processes active in soil formation are intensified 

 by high and retarded by low temperatures, all other conditions 

 being equal. This being true, we would expect that the soils 

 of tropical regions should, broadly speaking, be more highly 

 decomposed than those of the temperate and frigid zones. 

 While this fact has not been actually verified by the direct 

 comparative chemical examination of corresponding soils from 

 the several regions, yet the incomparable luxuriance of the 

 natural as well as the artificial vegetation in the tropics, and 

 the long duration of productiveness, offer at least presumptive 

 evidence of the practical correctness of this deduction. In 

 other words, the fallowing action, which in temperate regions 

 takes place with comparative slowness, necessitating the early 



1 The Eelation of Soil to Climate, BuU. No. 3, U. 8. Weather Bureau, 

 1892. 



