AVAILABILITY OF PLANT NUTRIENTS 339 



from glauconite and biotite, and that from fifteen to sixty 

 per cent is dissolved from muscovite, nephelite, leucite, 

 apophyllite and phillipsite. 



There are several factors, however, that make the use 

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

 available phosphorus and potassium in the soiL When 

 a soil is treated with the acid some of it is neutralized by 

 the reactions that result and thus its strength is lessened. 

 This may have no relation to the quantities of phosphorus 

 or potassium dissolved. Some analysts correct for the 

 neutralization and some do not. Again, as with strong 

 hydrochloric acid, the degree of solubility of the soil con- 

 stituents in the nitric acid may not correspond with the 

 ability of the plant to obtain these substances. With 

 this, as with the other methods discussed, the objection 

 holds that the result cannot be taken as an infallible guide 

 to the productiveness of a soil, or to its fertilizer needs; 

 but each of the methods affords some information in 

 regard to a soil, and is thus of value. 



242. Extraction with an aqueous solution of carbon 

 dioxide. — As carbon dioxide is a universal constituent 

 of the water of the soil, and without doubt a potent factor 

 in the decomposition of the mineral matter, it has been 

 proposed to use a solution of carbon dioxide as a solvent 

 in soil analysis. The amounts of soil constituents taken 

 up by this solvent are much less than are taken up by 

 any of the others heretofore mentioned, but all mineral 

 substances used by plants are soluble in it to some extent. 

 The amount of phosphorus is so small as to make its 

 detection by the gravimetric method difficult. Like 

 other methods employing very weak solvents, this method 

 is open to the objection that the extraction fails to remove 

 a considerable portion of the dissolved matter that is 



