CHEMICAL ANALYSIS OF SOILS 319 



has the citric acid method. It is obvious that some materials 

 are more readily soluble than 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 strength of acid. According to 

 Praps,^ it dissolves iron and aluminum phosphates to only a 

 slight extent, thus distinguishing between these forms of phos- 

 phorus and calcium phosphate. Fraps finds also that no 

 potassium is removed from orthoclase and microcline, that less 

 than 10 per cent, is dissolved from glauconite and biotite, and 

 that from 15 to 60 per cent, is dissolved from muscovite, 

 nephelite, leucite, apophyllite, and phillipsite, minerals known 

 to be rather easily available. 



There are several factors, however, that make the use of 

 one-fifth normal nitric acid an uncertain guide to the avail- 

 able phosphoric acid and potash in the soil. "When a soil is 

 treated with the acid, some of it is neutralized by the reac- 

 tions that result and thus its strength is lessened. This may 

 have no relation to the quantities of phosphoric acid or potash 

 dissolved. Some analysts correct for the neutralization and 

 some do not. Again, as with concentrated hydrochloric acid, 

 the degree of solubility of the soil constituents in the nitric 

 acid may not correspond with the ability of the plant to ob- 

 tain these substances. "With this, as with the other methods 

 discussed, the objection holds that the results cannot be taken 

 as an infallible guide to the productiveness of a soil, or to its 

 fertilizer needs. The artificial extraction of a soil in the 

 laboratory cannot be expected to simulate the action of a 

 crop even for one year. 



170. Extraction, with, water. — ^As carbon dioxide is a 

 universal constituent of the water of the soil, and without 



^ Fraps, Gr, S., Active Fhosphorio Acid and Its delation to the Needs 

 of the Soil for Fhosphoric Add in Fot Ma^periments ; Tex. Agr. Exp. 

 Sta., BuL 126, pp. 7-72, 1909. 



Also, The Acttm Potash of the Soil and Its Eelation to Pot Experi- 

 ments; T6X. Agr. Exp. Sta., Bui. 145, pp. 5-39, 1912. 



