44 
the acid extract after oxidizing the iron and organic matter with 
nitric acid. 
In order to illustrate the loss in phosphoric acid, as well as other 
elements, resulting through an evaporation of the soil extract to 
dryness, the following table is inserted. Fifty cubic centimeters of 
each extract, representing 1 gram of soil, was evaporated to dryness, 
taken up in hydrochloric acid, filtered, phosphoric acid determined 
in the filtrate, and the insoluble residue analyzed. 
Table XVII. — Composition of insoluble residue formed on evaporating soil extract to 
dryness. 
Soil 
No. 
1. 
Soil 
No. 
2. 
Soil 
No. 
3. 
SoU 
No. 
4. 
Soil 
No. 
5. 
Soil 
No. 
6. 
Soil 
No. 
7. 
Soil 
No. 
8. 
Soil 
No. 
9. 
Soil 
No. 
10. 
Soil 
No. 
11. 
Soil 
No. 
12. 
Total residue 
P.c. 
0.27 
.23 
C 1 ) 
Tr. 
Tr. 
( l ) 
P.c. 
0.24 
.22 
0) 
Tr. 
Tr. 
C 1 ) 
P.c. 
0.14 
.14 
C 1 ) 
Tr. 
Tr. 
C 1 ) 
P.c. 
0.12 
.12 
Tr. 
Tr. 
0) 
P.c. 
0.12 
.12 
C 1 ) 
Tr. 
Tr. 
C 1 ) 
P.c. 
0.01 
.01 
0) 
Tr. 
Tr. 
C 1 ) 
P.c. 
1.55 
.00 
1.05 
.18 
.10 
.32 
P.c. 
2.66 
.00 
1.85 
.22 
.67 
.59 
P.c. 
2.32 
.00 
1.50 
■ .18 
.20 
.64 
P.c. 
5.50 
.08 
4.30 
.33 
.15 
.71 
P.c. 
3.58 
.00 
2.35 
.18 
.12 
1.05 
P.c. 
3 06 
Silica 
22 
Titanium oxid 
2 00 
Iron oxid 
.18 
Phosphoric acid 
Aluminum oxW.. 
.08 
.44 
1 Not determined. 
While this method does eliminate the formation of the white pre- 
cipitate and indicates the primary source of error to be titanium, 
all possibility of its use as a method for determining phosphoric 
acid in the soil extract is precluded by the results in the above table. 
Attention is called to the remarkable difference in the properties of 
the two chemical types. 
After a thorough trial of several methods and modifications the 
elimination of the white precipitate was successfully effected by a 
precipitation in a nitric-acid solution free from chlorids. The fol- 
lowing method has been adopted : 
To 50 cubic centimeters of the hydrochloric-acid extract, repre- 
senting 1 gram of soil, add 1 cubic centimeter of nitric acid and boil 
to oxidize the organic matter and ferrous iron. Add ammonium hy- 
droxid until faintly alkaline, boil to remove excess of ammonia, filter, 
and wash free of chlorids. Transfer filter and contents to the original 
beaker, add an excess of dilute nitric acid, and heat to boiling on the 
hot plate in a covered beaker. This procedure is necessary to dissolve 
portions of the ammonia precipitate which assumes a colloidal form on 
boiling in an ammoniacal solution insoluble in cold nitric acid. In 
case too large an excess of nitric acid has been added, it should be 
nearly neutralized with ammonia, several grams of ammonium nitrate 
added, the solution diluted to 100 to 150 cubic centimeters, and 50 
cubic centimeters of molybdate solution added slowly while stirring. 
The beaker is then placed in a water bath at 55° C. for four hours. 
Further procedure is the same as that of the official method. 1 
i U. S. Dept. Agr., Bur. Chem. Bui. 107 (rev.), 1908. 
