496 SOILS. 



ample of the effective modification of vegetative features by 

 physical and chemical soil-conditions. 



It would be difficult to find a more striking exemplification 

 of the effect of lime carbonate, not only upon the vegetation 

 but also upon the physical and chemical characters of the hope- 

 lessly unproductive soil of the sand hammocks and pine mead- 

 ows; no longer brown and sour, but jet black and neutral, 

 modifying favorably every physical quality. Humus likewise 

 nowhere shows its benefits more strikingly. 



Table of Lime-Percentages. The table below shows the 

 average lime percentages observed in most of the several vege- 

 tative areas mentioned above. To meet the objection some- 

 times made that the vegetative changes noted may be due to 

 the larger amounts of phosphoric acid and potash frequently 

 found in calcareous lands, the percentages of the latter are also 

 given. Considering the origin of limestones, such a connec- 

 tion is not unexpected, but it is far from constant. On the 

 contrary, the frequent co-occurrence of much lime and high 

 production with small percentages of phosphoric acid and 

 potash leads to the conclusion, already discussed ( see chapter 

 19, p. 365), that in presence of abundance of calcic carbonate, 

 smaller percentages of phosphoric acid may be considered ade- 

 quate than when lime is deficient, on account of greater avail- 

 ability. Almost the same may be said of potash ; and it is 

 quite possible that the presence of large amounts of lime tends 

 to prevent the leaching-out of this base, in consequence of 

 greater facility for the formation of zeolites. Illustrations of 

 this kind have already been given (chapters 3, 22). 



Definition of " Calcareous Soils." It will be noted that the 

 very obvious and important changes of vegetation are brought 

 about by comparatively slight differences in lime-content. In 

 fact, only two of the soils enumerated above would, according 

 to the estimates usually given in books on soil composition, be 

 considered as properly calcareous. But the decisive feature 

 in this matter must evidently be the native vegetation, which 

 expresses the nature of the land much more clearly and author- 

 itatively than any arbitrary definition or nomenclature can 

 possibly claim to do. A soil must be considered as being cal- 

 careous whenez'cr it naturally supports the vegetation char- 

 acteristic of calcareous soils. 



