32 
WATER REQUIREMENT OF PLANTS. 
The jars used (fig. 3) were of glass, 14 inches deep, 9 inches in diam 
eter, and held about 42 pounds of soil. The direct evaporation was 
cut off by the use of a glass plate cemented to the top of the pot, a 
f-inch hole being provided in the center for the plants. A check pot 
without a crop was supplied to determine the loss from evaporation 
through the central opening. The loss thus measured amounted to 
about 3 per cent of that from jars containing plants. This correction 
was not applied, however, Lawes believing "that the indications of 
this experiment should serve rather to prevent any too nice applica- 
tion of the numerical results 
obtained in relation to the 
plants than as j:>rovidmg any 
available means of correct- 
ing them." 
The comparison of the two 
series will show that the 
water requirement was re- 
duced by the use of mineral 
manures. The plants in the 
series in which ammoniacal 
salts were used made such a 
poor growth that Lawes at- 
tributed little significance to 
the results. 
MARIE-DA VY'S EXPERIMENTS. 
Marie-Davy (1875) inves- 
tigated the effect of various 
fertilizers on the water re- 
quirement of wheat for grain 
production. The pots used 
Fig. 3.— Apparat usused by Lawes (1850) in measuring the Were 2-liter flasks with small 
water requirement of plants. ^^ ^ ^^ evaporation 
from the soil being very small. The results of this experiment are 
recorded in Table XXII and represent the mean values obtained 
with six different soils. Each treatment lowered the water require- 
ment as compared with the check series, which received no fertilizer. 
285 
. 
