320 
JAMES GEERE DICKSON 
under constant conditions is a more or less accurate criterion of total 
growth. Many external as well as internal factors, however, influence 
the rate and amount of transpiration, making the water requirement 
of any plant very variable. Any particular crop will show a varying 
water requirement dependent upon temperature, light intensity, con- 
centration of soil solution, and other factors. In a study of the rela- 
tion of nutrient elements to water requirements, therefore, the neces- 
sity is evident of having all other conditions as nearly constant as 
possible. Great care has been taken in the work heie reported to 
eliminate all environmental variations except the concentration of the 
mineral nutrients. 
In geneial, a decrease in the normal concentration of the soil 
solution or in that of any essential nutrient element therein causes an 
increase in the water requirement of the plants growing upon the soil 
in question. Hellriegel (1883) finds that the water requirement for 
oats is doubled by the absence of potassium and trebled by a deficiency 
in nitrogen. He adds that in general an abnormally high water 
requirement is to be expected of a plant growing in a soil deficient in 
any essential element, because growth would be arrested while trans- 
piration would still continue. The experiments herein described and 
many others of a similar nature show that in general the water require- 
ment of crops is in inverse ratio to the amount of the limiting element 
in the culture solution. 
Table 8 
The Average Water Requirement, Based Upon the Dry Matter Produced, of Plants 
Grown in the Normal Solution and in Solutions with One Nutrient Element 
in Each Case Reduced to One Tenth Normal 
No. Det. " 
per Yr, 
Solution, Deficient 
Element Given 
Cubic Centimeters of Water Required to Produce a Gram 
of Dry Matter 
1915 Crop 
19 16 Crop 
1917 Crop 
Average 
4 
Normal 
516 ± 12 
440 ± 8 
401 ± 2 
452 
2 
Mg 0.1 
400 ± 2 
2 
Ca 0.1 
5C2 ± 2 
407 ± 2 
494 ± I 
468 
2 
K 0.1 
614 ± 7 
471 ±2 
478 ± I 
2 
P 0.1 
717 ± 5 
741 ± 2 
832 ±8 
763 
2 
N 0.1 
839 ± 15 
1330 =t 14 
644 ± 8 
938 
In this series of experiments, the water requirement, based upon 
the dry weight of plants at maturity, is enhanced by a decrease in any 
one of the mineral nutrients with the exception of magnesium (table 8). 
