29 
much greater capacity for holding moisture. In fact, sodium nitrate 
itself has the most striking effect on all types of soils. There is no 
apparent classification of the results according to the changes in sur- 
face tension which should theoretically result through the addition of 
the salts. 
HYGROSCOPIC MOISTURE. 
When a soil has been dried in the air and then is exposed to a moist 
atmosphere it will reabsorb moisture. The amount which it is able 
to take up depends upon several factors, such as mechanical composi- 
tion, presence or absence of organic matter and its state of decay, 
temperature of the air, and presence of colloidal clay and ferric and 
aluminum hydrates. This form of moisture, while it is not in itself able 
to support normal plant growth, may materially assist in sustaining 
vegetation during drought. Some investigators claim it to be of 
absolutely no service to plants. 
Hawaiian soils, owing to their high humus and ferric hydrate con- 
tent, possess a very high hygroscopic coefficient. Of the series used 
in this study the sandy soil was lowest, as would be expected, but 
even in this case the hygroscopic moisture is high in comparison with 
normal sandy soils, due to its high organic content. 
Table XIII shows the comparative moisture-absorbing power of 
the types studied. From these data we are led to conclude that, 
due to the abnormal physical properties of Hawaiian soils, the size 
of particles is not the primary factor in determining its hygroscopic 
properties, although surface exposed is an important factor. 
The data presented in this table were obtained by exposing a very 
thin layer of soil in a saturated atmosphere for 144 hours, after which 
the total moisture was determined. Soil No. 428 having the least 
exposed surface, the highest percentage of organic matter, and the 
highest moisture content in the air-dry soil, has the least hygroscopic 
power. The clay soil. No. 530, has the most exposed surface, the 
largest percentage of ferric hydrate, the lowest moisture content in 
air-dry form, the least organic matter content, and next to the lowest 
absorbing power. But soils Nos. 516 and 573, a" manganese silt of 
high iron content and an organic silt, respectively, soils very dissimi- 
lar in chemical composition and physical properties, have the highest 
absorbing power, with the balance in favor of the manganese soil. 
Table Xlll .—Percentage of hygroscopic moisture absorbed in 144 hours. 
Sou No. 
Hygro- 
scopic 
moisture. 
530 
573 
428 
516 
Per cent. 
19.32 
21.59 
15. .')6 
24. W 1 
1 
