338 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1913. 



lime is derived from a limestone formation or is a concentrate of 

 the vegetation. On the rich bottom-land islands of the upper Poto- 

 mac the autumn leaf fall barely lasts through the following summer, 

 so rapid is its decay. These bottom lands have an alkaline flora, and 

 they are found to have an alkaline reaction, caused by the lime 

 brought to them in the flood waters. 



The acceleration of leaf decay by an alkaline substratum is due to 

 prompt neutralization of the acid leachings of the leaves and also 

 to the fact that such a substratum harbors with great efficiency many 

 of the most active organisms of decay, from bacteria to earthworms. 



It must not be understood that in a state of nature the decomposi- 

 tion of leaves is always so simple and uniform a process as has been 

 described, or that it always results in the formation of an alkaline 

 leaf mold. The chief factors that contribute to the acceleration of 

 leaf decay have already been enumerated, but there are other condi- 

 tions of nature that obstruct and retard this process. Under certain 

 conditions the progress of decomposition may be permanently sus- 

 pended long before the alkaline stage is reached. The soils thus 

 formed, although high in humus like a true leafmold, have an acid 

 reaction and a wholly different flora. 



Examples of such suspensions of leaf decay are found in bogs, 

 where the deposited vegetation is protected from the organisms of 

 decay by submergence in nonalkaline water, and on uplands where 

 the soil is derived from sand, sandstone, granite, or schist, in which 

 there is not enough lime or other basic material to neutralize the 

 acidity of the decaying leaves. 



There is, of course, a supply of lime in the leaves themselves, and 

 as a new layer of leaves is added to the soil each year it might be 

 expected that there would result an unlimited concentration of lime 

 in the surface soil and that all surface soils that supported a growth 

 of vegetation would ultimately become alkaline. Such an indefinite 

 accumulation of lime is prevented, however, by another factor which 

 requires consideration. As soon as each successive layer of leaf 

 litter is sufficiently decayed to permit the roots of plants to enter it 

 and feed upon it, the lime it contains, together with other mineral 

 constituents, begins to be absorbed. This loss of lime from the de- 

 caying leaves is sufficient, under many situations in nature, to pre- 

 vent the decaying mass from reaching the alkaline stage. Decom- 

 position is suspended while the leaf litter is still acid. True leaf- 

 mold, with an alkaline reaction, is never formed under such condi- 

 tions. The leaf deposit remains permanently acid and such areas 

 bear an acid flora. In the vicinity of Washington one often sees hills 

 of quartz gravel, wind swept and rain washed, where the soil con- 

 tained little lime in the beginning and none could be brought by 

 flood waters or by the dust of the atmosphere. Characteristic plants 



