218 BOTANICAL GAZETTE [MARCH 
about 85 per cent. The mechanical composition of the soils em- 
ployed is shown in table IT. 
Table I shows the arithmetical mean of each series of determina- 
tions, together with the probable error of the mean and the probable 
error of a single observation. The term “probable error”. is used 
in its usual mathematical sense, that is to say, in the case of the 
series of determinations in sand, the chances are even if the series 
were repeated with the same soil that the mean would lie between 
the values 2.56 and 2.62; and if the experiment were repeated 
with a single pot, the chances are even that the non-available 
moisture in this pot would fall between 2.48 and 2.70. The prob- 
TABLE IL . 
MECHANICAL ANALYSES OF SOIL SAMPLES USED IN THE DETERMINATIONS GIVEN IN 
4 
. i i Jery fine . 
Fine Coarse | Medium Fine Very Silt Clay 
gravel | sand mage = net | 6. 
snide ‘ald Meme 0.25 mm. | o.1 mm. 0.05 mom, \2°5 ™™ 
percent | percent | percent | per cent | per cent | per cent | Per cent 
Fine sand....... 0.4 9.8 17.0 50.1 {4.3 4:7 3-9 
Fine sandy loam. [2 ee § ES 1.2 Io.2 49.6 30.2 6.9 
ay loam.... o.1 He 
2.3 2.0 6.6 13.9 52.6 2 
* Mechanical analyses made by the Bureau of Soils. 
able error of the mean is about 0.005, and that of the single 
observation 6.02 of the actual non-available determination in the 
loam and clay soils, while the corresponding probable error is about 
twice as great in sand. It appears, therefore, that while there 1s 
considerable variation in individual pots, the mean of a suitable 
series represents the non-available moisture content of a given soil 
with an accuracy fully comparable with the accuracy with which 
the soil itself can be defined through its physical properties. 
Summary 
All soils upon which the plant cover has wilted through lack of 
water will still be found to contain moisture, varying in amount 
from less than 1 per cent in coarse sands to 25 per cent in the 
heaviest clays, and even more in the peat soils. The available 
water in the soil at any time is represented by the difference betwee? 
the actual water and the non-available portion. It is consequently 
