s
very much increased, in the case of soil covered with vegetation, by the
transpiration of the plants. The climate and the season of the year will
affect the extent of this upward movement. Where there is a heavy
rainfall it will be very much less than in dry climates. After a long
period of drought the nitrates will be found to be concentrated in the
top few inches of the soil; and in hot climates this sometimes takes
place to such an extent that the surface of the soil has been actually
covered with a saline crust, caused by the rapid evaporation of
soil-water under the influence of a burning tropical sun. From this
point of view it will be seen how very much less powerful a single
shower of rain is--even although at the time it is heavy--in causing
loss of nitrates by drainage, than a continuance of wet weather. In the
former case, where the showers are separated by an interval of dry
weather, the nitrates washed down into the lower layers of the soil are
slowly brought up again by the capillary action caused by evaporation.

_Amount of Loss by Drainage._

What the actual amount of loss is which takes place in this way it is
wellnigh impossible to say. What it amounts to under certain definite
circumstances has been discovered by actual experiment at Rothamsted.
Taking the circumstances most favourable to extreme loss--viz.,
unmanured fallow land--the highest amount registered at Rothamsted for a
year is 54.2 lb. per acre from soil 20 inches deep, while the smallest
amount is 20.9 lb. In the former case, the drainage-water was equivalent
to 21.66 inches, while in the latter, to 8.96 inches. The average for
thirteen years on unmanured fallow soil has been 37.3 lb. (for 20
inches), 32.6 lb. (for 40 inches), 35.6 lb. (for 60 inches). The point
of especial interest in this connection is that an annual loss of
nitrogen, equal to over 2 cwt. of nitrate of soda, may take place from a
comparatively poor arable soil lying fallow.

The loss on cropped soils is of course very much less--in short, should
amount to very little--especially in permanent pasture, where it is
reduced to a minimum. Taking an average, Mr Warington is of opinion that
the loss in England may be put at 8 lb. per annum per acre.[86]

_Loss in Form of Free Nitrogen._

The other chief natural source of loss of nitrogen is due to its escape
from the soil in its "free" state. This source of loss is very much less
important than that by drainage, and