316 



NATURE AND PROPERTIES OP SOILS 



give information regarding the availability of the plant nutri- 

 ents within the soil. They may be listed under three heads: 

 (1) digestion with strong acids, (2) digestion with dilnte 

 acids, and (3) extraction with water. These methods will be 

 discnssed in the order mentioned. 



168. Digestion with strong acids. — While surfuric, ni- 

 tric, and hydrochloric acids have all been nsed as solvents,^ 

 the one most commonly employed is hydrochloric acid of 

 1.115 specific gravity.^ It has been used to such an ex- 

 tent that it may be considered the standard solvent, and a 

 statement of a chemical analysis of a soil in this country may 

 be considered as based on this solvent unless otherwise stated. 



An analysis by this method is supposed to show the propor- 

 tion of nutrient materials in a soil that is in a condition to 

 be used ultimately by plants at the time when the analysis is 

 made. The nutrient materials that are not dissolved by 

 treatment with hydrochloric acid are assumed to be in a 

 condition in whch plants cannot use them. The difficulty 

 with this assumption is that, while treatment with hydro- 

 chloric acid of a given strength marks a definite point in the 

 solubility of the compounds in the soil, it does not bear a 

 Tmiform relation to the natural processes by which these 

 compounds become available to plants. 



This method is not only arbitrary but it is artificial as well. 



^The following analyses of the same soil quoted from Snyder are 

 interesting m this regard. Snyder, H., Soils; Minn. Agr. Exp. Sta., 

 Bill. 41, p. eQ, 1895. 



Extract 



HCL.. 

 HNO3. 

 H2SO4. 



Total 



K.0 



CaO 



MgO 



P.0„ 



% 



% 



% 



% 



% 



18.80 



.42 



.55 



.40 



.23 



16.55 



.30 



.30 



.32 



.23 



19.55 



.52 



.53 



,52 



.26 



SO, 



% 

 .08 

 .08 

 .10 



® Official and Provisional Methods of Analysis; XT. S. Dept. Agr., Bur. 

 Chem., Bui. 107 (revised), pp. 14-18, 1908. 



