462 



Journal of Agricultural Research 



Vol. V. No. 



in the soil before and after treating with i per cent of sodium arsenite 

 is also given. 



Table II. — Composition of the extracts {mgm. per liter) 



« Humus I shows the percentage of the humus content of original soil; humus II, that of soil after treat- 

 ment with the I 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 HjO into 

 H4- 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. 



