21 
• PHOSPHORIC ACID. 
i In the next table are shown the results of the action of heat on the 
Solubility of phosphoric acid.- 
Solubility of phosphoric acid in water and fifth-normal nitric acid. 
iCalculated on basis of dry soil.] 
Soil No. 
Soluble in water (parts per 
million). 
Soluble in fifth-normal nitric acid (per 
cent). 
Air dry. 
Dried at 
100° C. 
Dried at 
250° C. 
Ignited. 
Air dry. 
Dried at 
100° C. 
Dried at 
250* C 
Ignited. 
7-1 
264 
g 
39.8 
24.1 
17.1 
21.9 
22.9 
27.1 
36.5 
54.3' 
23.4 
2.5.6 
25.4 
29.6 
69.6 
36.1 
44.5 
26.9 
36.5 
31.0 
43.6 
52.7 
35.7 
38.7 
24.8 
36.2 
58.7 
16.1 
52.9 
23.8 
25.8 
33.0 
49.2 
46.8 
21.2 
26.8 
27.1 
37.6 
32.6 
15.1 
32.8 
22.8 
27.9 
25.3 
51.4 
39.8 
23.4 
23.8 
27.1 
28.1 
0.007 
.005 
.007 
.058 
.006 
.011 
.005 
.005 
.024 
.011 
.008 
.028 
0.008 
.006 
.007 
.018 
.016 
.014 
.007 
.008 
.030 
.013 
.012 
• .028 
0.024 
.007 
.011 
.020 
.020 
-.027 
.007 
.009 
.064 
.012 
.011 
.036 
0.043 
.00'J 
.013 
2D2 
290 
40.5 
416 
417 
.053 
.031 
.021 
.006 
.005 
406 
.047 
428 
.014 
426 
.013 
44S 
.020 
It may be seen from this table that the solubility of phosphoric 
acid is materially affected by heat, the solubility in water being 
greatest in the soils heated to 100° and 250° C, if it be permissible 
to draw conclusions from the general averages, while it is least soluble 
in the air-dried soils. It is also worthy of note that this element is 
more soluble in the uncultivated than the cultivated soil, the former, 
however, decreasing with increase in heat. In the extractions made 
with dilute nitric acid the average indicates a greater solubility in 
the ignited soils, the solubility tending to increase with increase in 
temperature. 
Phosphoric acid exists in soils in major part combined with iron, 
aluminum, magnesium, and calcium, and is also found combined with 
organic matter, being always present in the so-called humus of soils. 
It may be in the form of basic phosphates, hydrogen phosphates, or as 
complex phosphates in combination with more than one element. 
It is probably combined mostly with iron and aluminum and titanium 
in Hawaiian soils. Considerable work has been done upon the effect 
of heat upon the solubility of this constituent and several attempts 
have been made to draw conclusions from these results as to its state 
of combination; that is, whether organically or inorganically com- 
bined. Peterson ■ using fifth-normal nitric acid found that after 
oxidizing the organic matter with hydrogen peroxid there was no 
increase in the solubility of phosphoric acid when the soil was heated 
to 240° C. He concluded, therefore, that the solubility of mineral 
phosphates in soils is not increased up to 240° C. The author's results 
tend to indicate a decrease in solubility of phosphoric acid at high tem- 
' Wisconsin Sta. Research Bui. 19 
