Transpiration as a Factor in Crop Production. 129 
ferent seasons. For example, the total dry matter produced in 
soil 50 per cent saturated was much below that in optimum soil 
moisture in 1913, while in the season of 1914 this moisture 
content resulted in a yield not far below the optimum. Climatic 
conditions were such that the free-water-surface evaporation 
during July and August in 1913 was only 60 per cent as great as in 
1914. The difference in the climatic conditions of the two years is 
shown in Table 34, page 109. In 1910 the maximum total plant 
weight was attained at 80 per cent relative saturation, while the 
maximum ear weight was had with 60 per cent relative satura- 
tion. Indications are that a range of optimum moisture content 
may exist within the limits of which a variation does not ma- 
terially affect the growth of the plant. A reduction of soil- 
moisture content to 45 and 35 per cent relative saturation greatly 
reduced the ear and total dry matter, while an increase to 95 
per cent also resulted in a marked reduction in total weight and 
weight of ear. From these data it was concluded that a mean 
saturation of 70 per cent of the water-holding capacity of the 
soil (70 per cent relative saturation) may be regarded as an opti- 
mum condition for this soil, and was used as such in the experi- 
ments which followed. 
In 1910, five different degrees of relative saturation were 
maintained, namely, 35, 45, 60, 80, and 100 per cent of the 
water-holding capacity of the soil. 
During 1913 and 1914 three degrees of water content were 
compared, representing less than optimum, optimum, and above 
optimum soil moisture. These were 50, 70, and 95 per cent of 
the water-holding capacity, with 70 per cent as the optimum. 
During 1914, an unmanured and a manured group of potometers 
were conducted at each of these three degrees. In addition, one 
group of both manured and unmanured potometers was started 
at 70 per cent relative saturation, the same as the normal 70 per 
cent group, but each day only two-thirds as much water was 
added to each potometer as was transpired on an average by the 
eight plants continuously kept in 70 per cent saturated soil. 
The results of all three years' tests follow in detail in Tables 
53, 54, and 55, and are summarized in Tables 56, 57, and 58. 
DISCUSSION OF RESULTS. 
The results of the three years' tests have been summarized 
in Tables 56, 57, and 58. In Tables 57 and 58, the data have 
been assembled into three general classes, according to whether 
the plants were grown in optimum, less than optimum, or above 
optimum soil-moisture content. This necessitated averaging 
together for 1910 the 35, 45, and 60 per cent relative saturation 
5 
