319 
are the best fitted for watering meadows ; but most 
of the benefits of irrigation may be derived from 
any kind of water. It is, however, a general prin- 
ciple, that waters containing ferruginous impreg- 
nations, though possessed of fertilizing effects, when 
applied to a calcareous soil, are injurious on soils 
that do not effervesce with acids ; and that calca- 
reous waters, which are known by the earthy de- 
posit they afford when boiled, are of most use on 
siliceous soils, or other soils containing no remark- 
able quantity of carbonate of lime. 
The most important processes for improving 
land, are those which have been already discussed, 
and that are founded upon the circumstance of 
removing certain constituents from the soil, or 
adding others, or changing their nature ; but there 
is an operation of very ancient practice still much 
employed, in which the soil is exposed to the air, 
and submitted to processes which are purely me- 
chanical, namely , fallowing. 
The benefits arising from fallows have been much 
over-rated. A summer-fallow, or a clean fallow, 
may be sometimes necessary in lands overgrown 
with weeds, particularly if they are sands which 
cannot be pared and burnt with advantage ; but 
is certainly unprofitable as part of a general system 
in husbandry. 
It has been supposed by some writers, that cer- 
tain principles necessary to fertility are derived 
from the atmosphere, which are exhausted by a 
succession of crops, and that these are again sup- 
plied during the repose of the land, and the expo- 
sure of the pulverised soil to the influence of the 
