moulds, the earths seem to be in combination with 
a peculiar extractive matter, afforded during the 
decomposition of vegetables : this is slowly taken 
up, or attracted from the earths by water, and 
appears to constitute a prime cause of the fertility 
of the soil. 
The standard of fertility of soils for different 
plants must vary with the climate $ and must be 
particularly influenced by the quantity of rain. 
The power of soils to absorb moisture ought to 
be much greater in warm or dry counties, than in 
cold and moist ones ; and the quantity of clay, 
or vegetable or animal matter they contain, greater. 
Soils also on declivities ought to be more absorbent 
than in plains or in the bottom of vallies. Their 
productiveness likewise is influenced by the nature 
of the sub-soil or the stratum on which they rest. 
When soils are immediately situated upon a bed 
of rock or stone, they are much sooner rendered 
dry by evaporation, than where the sub-soil is of 
clay or marie ; and one cause of the great fertility 
of some lands in the moist climate of Ireland is 
the proximity of the rocky strata to the soil. 
A clayey sub-soil will sometimes be of material 
advantage to a sandy soil ; and in this case it will 
retain moisture in such a manner as to be capable 
of supplying that lost by the earth above, in con- 
sequence of evaporation, or the consumption of it 
by plants. 
A sandy, or gravelly sub-soil, often corrects the 
imperfections of too great a degree of absorbent 
power in the true soil. 
In calcareous countries, where the surface is a 
