180 DRY-FARMING 
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 tran- 
spired 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 com- 
paratively 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 ex- 
periments on transpiration in which he showed that 
the addition of potassium nitrate, ammonium sul- 
phate or common salt to the solution in which plants 
grew reduced the transpiration; in fact, the reduc- 
tion was large, varying from 10 to 75 per cent. This 
was confirmed by a number of later workers, among 
them, for instance, Buergerstein, who, in 1875, 
showed that whenever acids were added to a soil or to 
water in which plants are growing, the transpiration 
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