462 
Journal of Agricultural Research 
Vol.V, No. 11 
in the soil before and after treating with 1 per cent of sodium arsenite 
is also given. 
Tabi^E II .—Composition of the extracts {mgm. per liter) 
Constituent. 
Soil No. 1. 
Soil No. 2. 
Soil No. 3. 
Water 
extract. 
Arsenic 
extract. 
Water 
extract. 
Arsenic 
extract. 
Water 
extract. 
Arsenic 
extract. 
Fe s 0 3 . 
Trace. 
716 
Trace. 
121 
Trace. 
90 
CaO. 
11. 2 
84 
13. 6 
124 
74. 6 
126 
MgO . 
3-6 
20 
10. 8 
44 
7-4 
26 
AS2O3. 
2, 060 
6, 000 
4, 480 
Mg.As20 3 fixed by 100 gm. soil. 
2 , 64.O 
600 
2, 120 
Humus I°,per cent... 
2. 
77 
1. 
68 
8. 
75 
Humus 11 °, per cent. 
1. 
56 
1. 
80 
8. 
40 
a Humus I shows the percentage of the humus content of original soil; humus II, that of soil after treat¬ 
ment with the 1 per cent sodium-arsenite solution. 
Table II shows a replacement of and a solvent action toward iron, 
calcium, magnesium, and humus, and suggests several theories as to the 
nature of the reaction. The soil absorbing the largest amount of arsenic 
lost through solution or replacement the most iron and humus. The 
soil absorbing the least arsenic lost the least iron and no humus. Appar¬ 
ently the absorption of arsenic by soil No. 3 is largely a mechanical 
fixation, as the data show a high absorption, but a low replacement. 
In sodium arsenite we have the combination of a strong base with a 
weak acid. A well-known property of such salts is to react alkaline 
when dissolved in water. This is due to the faint dissociation of H 2 0 into 
H -f and OH — ions. Here the chemical and physical phenomena involved 
in the fixation of sodium arsenite are directly or indirectly a result of 
hydrolysis. The latter term as used herewith is intended to convey the 
increased dissociation in a solution of sodium arsenite, which itself is only 
faintly dissociated. ' This results in an increase in the concentration of the 
hydroxyl ion and the formation of the highly dissociated electrolyte 
sodium hydrate, which in the soil would probably be rapidly converted to 
bicarbonate. In this form it would have a solvent action toward the 
iron and humus and more or less toward the magnesium and calcium 
through the formation of slightly soluble bicarbonates. Magnesium 
bicarbonate is very unstable as compared to calcium bicarbonate and, 
hence, is precipitated following the solvent action of the sodium bicar¬ 
bonate. The calcium is more soluble even in the soils containing much 
higher amounts of magnesium. These reactions leave the arsenic free as 
the negative ion to combine with the dibasic and tribasic metals to form 
slightly soluble arsenites or arsenates, thereby fixing the arsenic in the 
soil. 
