36 
Table XIII. — Solubility and availability of phosphoric acid in. ferruginous soils. 
Soil No. 
Soluble in 
fifth- 
normal hy- 
drochloric 
acid. 
Soluble in 
1 per cent 
sodium 
hydroxid. 
Crop grown. 
Crop 
increase 
from 
phosphate. 
10 
Per cent. 
Trace. 
Trace. 
Trace. 
0. 1128 
Per cent. 
0.0060 
.0219 
.0089 
.1858 
Pineapples — 
.do 
Cotton 
Per cent. 
50 
11 
100 
12 
200 
13 
Soils Nos. 10, 11, and 12 are samples of red soil of the type used in 
the pot experiments, responding to phosphate fertilization, while No. 
13 is a sample of the same soil used as No. 1 in the pot experiments. 
Hence it may be seen that the results are directly in accord with those 
presented in this bulletin. 
Table XII shows clearly the insoluble nature of the phosphoric acid 
in the different types of soil. Fusion with sodium carbonate shows 
the total phosphate content to be very high in practically all types. 
The results of the official method of extraction with hydrochloric acid 
(specific gravity 1.115) throw considerable discredit upon its useful- 
ness as a means of determining the phosphate content of Hawaiian 
soils. This may be attributed to several causes, chief among which 
is the inability of the acid to penetrate to such an extent during the 
period of extraction as to come in contact with any occluded crystals 
or other protected particles of phosphate, and, furthermore, its lack 
of ability thoroughly to decompose the basic phosphates of iron, 
aluminum, and titanium, especially the last. The obvious conclusion 
to be drawn from these results is the uselessness of determining the 
phosphoric acid in the hydrochloric acid extract and the need of 
determining the absolute phosphate content. 
The action of weak solvents upon the various types of Hawaiian 
soils is a means of obtaining data of value regarding the solubility of 
phosphoric acid, but not regarding its availability. Neither is it 
possible to determine in this way whether the soil will respond to 
phosphate fertilization. Stoddard 1 says that, for Wisconsin at least, 
if a soil contains less than 0.015 per cent of phosphoric acid soluble in 
fifth-normal nitric acid, it will respond to phosphate fertilization. 
Snyder, 2 working independently, reached the same conclusion in 
regard to Minnesota soils. That such a relation holds good for 
Hawaiian soils does not at this time appear probable, primarily be- 
cause there are too many other abnormal factors to be considered. 
Soil No. 1 has been shown not to respond measurably to phosphate 
applications, yet it contains only 0.007 per cent phosphoric acid 
soluble in fifth-normal nitric acid, but, on the other hand, contains 
Wisconsin Sta. Research Bui. 2 (1909). 
2 Minnesota Sta. Bui. 102 (1907), p. 
