68 Dilute Acids and Phosphorus Compounds m the Soil 



a fact which indicates that something more is concerned than a mere 

 mixture of phosphates. One of us has shown, also, that Hall and Amos's 

 results are better explained on the view developed in this paper^. 



Schloesing^ states that the amount of phosphorus dissolved from 

 soils is constant whatever the concentration of the acid between certain 

 limits and de Sigmund^ supported this claim. As his paper is not 

 generally accessible to agricultural chemists his curve is reproduced 

 here (Fig. 1). With very dilute acids the action is only slight: as the 

 concentration increases more phosphate comes into solution. Then 

 for a period the action is constant whatever the strength of the acid; 

 finally, with stronger acid the action again increases, and goes on 

 increasing with each addition to the strength of the acid. These 

 results, if correct, would afford strong indication of the presence of an 

 easily soluble phosphate which was being dissolved out during the 

 period of constant action (the horizontal part of the curve), and of less 

 soluble phosphates, which are only brought out by the stronger acids. 



•15 -20 



Initial acidity equivalents of HNO3 per litre 

 de Sigmund's curve Our results 



Fig. 1. Amounts of PoOj extracted from soil by HNO3 of varying concentration. 

 Laboratory temperature. 



We have carefully repeated these experiments but failed to obtain 

 the same results. Both Schloesing and de Sigmund seem to have carried 

 out their extractions at laboratory temperature, which in practice is 

 somewhat variable. The improved method now available for estimating 

 phosphoric oxide in solution enables us to plot the results on a larger 

 scale than was possible for the earlier investigators. Working under 

 their conditions the results are very variable, and fit neither a curve 

 nor a straight line (Fig. 1, Table II), although if one reduced the scale 



1 J. A. Prescott, Proc. Chem. Soc. 1914, 30, 137-8 



2 Compt. Bend. 128, 1004. 



3 J. Amer. Chem, Soc. 1907, 29, 929-36. 



