338 SOILS: PROPERTIES AND MANAGEMENT 



241. Dilute mineral acids. — Of the mineral acids 

 in a diluted form used for extracting soils, those that 

 have received the most attention are one-fifth normal 

 nitric l or hydrochloric acid and one two-hundredth 

 normal hydrochloric acid. 2 The methods employing 

 these solvents are admittedly empirical. There is no 

 natural relation between these solvents and the processes 

 by which the plant obtains its nutriment from the soil. 



The solvent that has received the most attention is 

 one-fifth normal nitric acid. In ease of manipulation 

 this is preferable to the one-per-cent citric acid, which is 

 rather tedious to work with. It has been used nearly 

 as extensively in this country as the latter has in Great 

 Britain. Its use has been confined largely to the deter- 

 mination of the readily available phosphorus and potas- 

 sium in the soil, as has the citric acid method. It is 

 obvious that some minerals are more readily soluble than 

 are others, and for that reason the method will distinguish 

 between phosphorus and potassium in different forms. 

 The calcium phosphates are supposed to be entirely soluble 

 in this solvent. According to Fraps 3 it dissolves iron 

 and aluminium phosphates to only a slight extent, thus 

 distinguishing between these forms of phosphorus. Fraps 

 finds also that no potassium is removed from orthoclase 

 and microcline, that less than ten per cent is dissolved 



1 Official and Provisional Methods of Analysis. U. S. D. A., 

 Bur. Chem., Bui. 107 (revised), p. 18. 1908. 



2 Wiley, H. W. Principles and Practice of Agricultural 

 Analysis, pp. 394-396. Easton, Pennsylvania. 1906. 



3 Fraps, G. S. Active Phosphoric Acid and Its Relation 

 to the Needs of the Soil for Phosphoric Acid in Pot Experi- 

 ments. Texas Agr. Exp. Sta., Bui. 126, pp. 7-72. 1909. 

 Also, The Active Potash of the Soil and Its Relation to Pot 

 Experiments. Texas Agr. Exp. Sta., Bui. 145, pp. 5-39. 1912. 



