15 



undoubtedly a result of selective absorption. That is, the arsenic 

 is absorbed and the soda liberated as the carbonate or possibly 

 hydrate. The action of sodium carbonate toward humus is well 

 known, it being responsible for the black alkali lands of the 

 western states. The supposedly neutral salt of sodium arsenite is 

 alkaline in reaction and as long as it is in solution in contact with 

 the soil it remains alkaline, at least in so far as these experiments 

 were carried out. But soon after filtering the soil the solution 

 becomes acid or neutral or possibly may remain alkaline depending 

 on the degree of absorption which took place when in contact with 

 the soil. 



As additional proof of the theory that selective absorption 

 takes place it should be stated that the soil absorbing the highest 

 weight of arsenic was in contact with the solution of highest 

 alkalinity and lost, through solution, the most humus. The soil 

 absorbing the least arsenic lost no humus. 



The extract of the soil treated with arsenic was very dark in 

 color according to the humus dissolved. On acidifying this extract 

 with hydrochloric acid a large per cent of the humus was precipi- 

 tated, carrying with it a large percentage of the iron. This is a 

 well known property of humus as extracted from soils by ammonia. 



The most striking figures are those showing the increase in 

 solubility of iron as effected through the presence of sodium 

 arsenite. The solubility of this constituent of our soils is very 

 low. It is present in a form soluble in water, in only minute 

 traces. 



The interaction of the arsenic and iron apparently depends 

 upon the form in which the latter exists in the soil. Soil jSTo. 1 is 

 a red clay and here the reaction is strongest, soils number 2 and 

 3 are a brown and a black soil respectively and the action is far 

 less marked. What the nature of the action is it is impossible at 

 this time to say as it would involve the establishing of the presence 

 or absence of ferrates or ferrites in the soils. This involves an 

 investigation too technical for a bulletin of this sort. Further 

 work of this nature is contemplated. The reaction with the red 

 soil is primarily chemical, the arsenic replacing the iron and the 

 latter going into solution. It is first necessary to determine 

 definitely the chemical form in which the iron exists in the soil in 

 order to present a theory as to the nature of the reaction. 



