318 NATURE AND PROPERTIES OF SOILS 



tracting soils, the idea being in every case to determine the 

 amount of the mineral nutrients immediately available to 

 crops. The scope is thus narrower than in the digestion with 

 strong acids, by which the permanent fertility is sought. 



Two acids have been commonly utilized in the extraction of 

 soils with dilute solvents : one per cent, citric acid proposed by 

 Dyer, 1 and one-fifth normal nitric acid. 2 Dyer adopted the 

 one-per-cent. strength as the result of an investigation in which 

 he determined the acidity of the juices in the roots of over 

 one hundred species or varieties of plants representing twenty 

 different natural orders. The implication is that plants pro- 

 duce a solvent action on a soil in proportion to the acidity of 

 their juices, but an examination of Dyer's figures does not 

 show that the size of the crop ordinarily produced by the plants 

 would in many cases correspond to the acidity of their juices. 

 Thus, of the Cruciferae, the horse-radish has several times the 

 acidity of the Swedish turnip or of the field cabbage, although 

 the crop produced by the former is much less than that of the 

 latter two. 



Dyer's method gave results on Rothamsted soils that en- 

 abled him to estimate their relative productivity. On other 

 soils and in the hands of other investigators, however, the 

 method is unsatisfactory. In soils rich in calcium and low in 

 iron and aluminum, it may often show the amounts of easily 

 soluble phosphoric acid and potash. 



In ease of manipulation, the fifth normal nitric acid is 

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

 tedious to work with. It has been utilized nearly as exten- 

 sively in this country as has the latter in Great Britain. Its 

 use has been confined largely to the determination of the 

 readily available phosphoric acid and potash in the soil, as 



1 Dyer, Bernard, On the Analytical Determination of Probably Avail- 

 able "Mineral" Plant Food in Soils; Jour. Chem. Soc, Vol LXV, 

 pp. 115-167, 1894. 



* Official and Provisional Methods of Analysis; U. S. Dept. Agr., Bur. 

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



