The one that seems most readily controlled in ordinary
agricultural practice will be discussed in the following section.
Plant-food and transpiration
It has been observed repeatedly by students of transpiration that
the amount of water which actually evaporates from the leaves is
varied materially by the substances held in solution by the
soil-water. That is, transpiration depends upon the nature and
concentration of soil solution. This fact, though not commonly
applied even at the present time, has really been known for a very
long time. Woodward, in 1699, observed that the amount of water
transpired by a plant growing in rain water was 192.3 grams; in
spring water, 163.6 grams, and in water from the River Thames, 159.5
grams; that is, the amount of water transpired by the plant in the
comparatively pure rain water was nearly 20 per cent higher than
that used by the plant growing in the notoriously impure water of
the River Thames. Sachs, in 1859, carried on an elaborate series of
experiments on transpiration in which he showed that the addition of
potassium nitrate, ammonium sulphate or common salt to the solution
in which plants grew reduced the transpiration; in fact, the
reduction was large, varying from 10 to 75 per cent.
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