104 
verified, and that the quantity of potassa indicates their 
relative productiveness. The quantity being 0.250 in the 
two superior, and one-half of it in the inferior soils. 
The depth of the Norton and Womersley soils examined, 
can have little influence, because they are both above 
three feet deep, and therefore the depth is only a criterion 
within certain limits, that is to say, if the depth to the 
rock in one place be only five or six inches, and at another 
fifteen to twenty inches, it will be found that the latter 
soil, whatever may be its constitution, will be the more 
productive. It is however probable that the fertility of the 
land in the vicinity of Vesuvius is not wholly owing to the 
quantity of alumina, and therefore of alkali contained in it, 
as stated by Liebig, but principally to the genial temperature 
maintained by the heated lava, and also from the evolution of 
ammoniacal gas, as proved to be the case by Dr. Daubeny. 
The quantity of alkali in the soil of Virginia and in all 
newly cultivated lands, is not to be ascribed to the original 
quantity in the rock from which the soil originates, but from 
the accumulation, by the periodical decomposition of vegeta- 
ble matter grown on the spot. In fact the same thing occurs 
in old swarth by the decay of the roots, &c. of the grass. It 
is also highly probable that the inability of the limestone soils 
to produce clover oftener than every eighth year is due to 
the exhaustion of alkali, and not to the want of gypsum, for 
there being so small a portion of alumina, and therefore of 
potash in the rock, little or no alkali is supplied by the soil 
itself, or by the manures applied ; for it is on boned lands that 
this crop more particularly fails. It is probable that several 
causes afi'ect the growth of clover ; want of firmness or con- 
solidation of the land is one ; after teazles it never fails ; 
again on headlands, which have been carted upon and trodden, 
it always grows — on the stiffer lands east of the limestone, 
when drained, if the clover-seed grows, it never sickens in the 
spring. 
