28 
VAPOR PRESSURE. 
Comparatively little work has been done on the effect of soluble 
salts on vapor pressure of soils, i. e., the rate of evaporation. Theo- 
retically salts should increase the surface tension of solvents and 
thereby lower their vapor pressure and hence increase the water- 
retaining capacity of the soil. The study made of this property indi-. 
cates that this theory apparently apphes to Hawaiian soils. 
As a means of measuring this property of soils ordinary weighing 
bottles were used. Twenty gram lots of soil with which the various 
salts had been mixed were placed in these bottles, 12 cubic centime- 
ters, or 60 per cent, of water was added to each and then allowed to 
stand one week in the open air. Weighings were made at this stage, 
following which all the samples were placed in the same desiccator with 
calcium chlorid and weighings were again made after one week. The 
results are given in Table XII. 
Table XII. — Effect of salts on vapor pressure. 
Salts and fertilizers. 
Sodium carbonate 
Sodium nitrate 
Magnesium oxid 
Calcium carbonate 
Calcium phosphate 
Calcium oxid 
Potassiimi phosphate 
Potassivun sulphate 
Ammoniimi sulphate 
Potassimn chlorid 
Superphosphate 
Acid phosphate 
Cottonseed meal 
Blood 
Acid phosphate and blood . . 
Potassium sulphate and 
acid phosphate 
2(NH4)-14-2i 
l(NH4)-9-li 
2 (N03)-14-2 1 
Check 
Amoimt 
added. 
Gm. 
0.10 
.09 
.04 
.10 
.23 
.06 
.21 
.17 
.13 
.08 
.10 
.10 
.10 
.10 
.10 
.10 
.10 
.10 
.10 
Soil No. 530. 
Water retained 
In air. 
Perct. 
65.3 
71.2 
64.2 
66.2 
52.8 
58.6 
51.4 
62.7 
73.9 
50.2 
63.3 
77.0 
32.5 
42.6 
44.0 
58.2 
36.5 
37.8 
59.6 
48.0 
In des- 
iccator. 
Perct. 
46.0 
55.5 
43.6 
42.7 
21.0 
29.7 
26.9 
42.9 
25.6 
10.2 
10.8 
41.3 
24.6 
Soil No. 573. 
Water retained. 
In air. 
Perct. 
39.4 
50.0 
45.7 
41.0 
42.7 
44.8 
41.3 
48.1 
41.7 
45.8 
44.7 
45.8 
41.5 
36.1 
31.2 
40.3 
39.3 
35.7 
45.3 
36.3 
In des- 
iccator, 
Perct. 
20.9 
33.2 
26 
21 
15 
24, 
19 
33 
26, 
21, 
27, 
28.5 
9.7 
7.2 
4.3 
16.2 
8.3 
9.6 
25.6 
15.3 
Soil No. 428, 
Water retained. 
In air. 
Perct. 
39.2 
44.1 
44.4 
44.3 
43.0 
39.8 
42.4 
39.5 
43.0 
40.2 
42.8 
41.3 
43.3 
48.7 
41.4 
49.2 
41.2 
38.7 
57.2 
37.0 
In des- 
iccator. 
Per ct. 
31.9 
38.8 
38.2 
35.3 
30.7 
31.5 
25.4 
31.2 
31.7 
40.5 
25.7 
15.3 
23.1 
30.3 
19.3 
17.5 
22.8 
17.8 
Soil No. 516. 
Water retained. 
In air. 
Per ct. 
Perct. 
43.7 
29.5 
46.8 
31.4 
42.9 
24.4 
44.9 
26.9 
45.7 
44.8 
35.9 
37.8 
35.6 
44.8 
38.4 
37.3 
43.6 
43.8 
40.7 
44.1 
40.6 
37.2 
42.4 
35.4 
In des- 
iccator. 
28.3 
15.2 
17.5 
17.4 
33.8 
25.6 
22.7 
27.4 
29.9 
22.5 
20.4 
20.9 
28.6 
22.1 
1 Fertilizer mixtures containing nitrogen, phosphoric acid, and potash in the order and proportions 
(percentages) indicated; nitrogen from ammonium sulphate in the first two, from sodium nitrate in the 
third. 
These figures indicate that the effect of salts upon vapor pressure 
in soils is one of considerable importance. Salts act upon Hawaiian 
soils more or less according to theory. The major part of them increase 
the water-holding power in all four soils. Organic substances in- 
creased evaporation in soils Nos. 530 and 573, but had the opposite 
effect upon Nos. 428 and 516. These results also show that the form 
of nitrogen used in mixed fertilizers bears a definite relation to the 
vapor pressure. Those in which sodium nitrate was used show a 
