Ch. 30] SAMPLING FOR CHEMICAL ANALYSIS 545 



to place it may be desirable to determine the pH of each sample of soil 

 as an aid in interpreting the data. No serious interference has been 

 reported from artificial contamination of soils if normal care is taken 

 in obtaining clean samples. In the east Tennessee zinc district where 

 the fields have been limed with zinc tailings from the concentrating 

 plants, no noticeable effect was observed (Hawkes and Lakin, 1949). 



Where the soil is free of rocks, a hand-operated soil auger is satis- 

 factory for sampling for depths up to 20 feet. For rocky soils a hole 

 may be driven with a crow bar, and the sample collected with a spoon 

 attached to the end of a pole (Sergeev, 1941, Fig. 18). For large-scale 

 surveys, power-driven equipment will probably be desirable. Surface 

 soils, of course, can be sampled with no special equipment. 



Alluvium. In sampling alluvium, the same principles may be fol- 

 lowed as in alluvial prospecting for heavy minerals (Raeburn and 

 Milner, 1927). Under certain conditions experience has shown that 

 sieving the samples and rejecting all but the fines materially reduces 

 the number of erratic and inconsistent data (Lovering, Huff, and 

 Almond, in preparation) . 



Till. Any sampling of glacial material should be done with due re- 

 gard for the glacial history of the area and the resulting structure of 

 the glacial deposits. In general, the same precautions should be taken 

 in till sampling as in soil and alluvium sampling, except that sieving of 

 the sample may be a more critical factor, especially with subaqueous 

 tills that have been diluted with a variable amount of sand. 



Vegetation. In sampling and analysis of plants as a prospecting 

 method, it should be borne in mind that the metal content of the plant 

 parts cannot give a better picture of the metallic dispersion pattern 

 than the metal content of the nutrient solution available to the root 

 system. The life processes of the plant may further modify the relative 

 concentrations of the various inorganic constituents of its structure 

 (Thyssen, 1942) . The outstanding point in favor of the method is that 

 the root system of a large plant or tree draws its moisture from a large 

 volume of inaccessible soil or underlying rock and concentrates the con- 

 tained mineral matter in the leaves, where it can be more readily 

 sampled. 



The uptake of a given element by a growing plant depends on the 

 availability of that element in the nutrient solution rather than on the 

 total concentration in the soil. The ratio of available to total element 

 in a soil is primarily a function of the pH, organic content, and colloid 

 content of that soil (Jacks and Scherbatoff, 1940; Erkama, 1947). 

 Many plants have a selective action whereby they can take up cer- 

 tain elements and reject others (Thyssen, 1942) ; furthermore, those 



