cerned there are many contributing factors not shown by the analysis 

 which affect this as much or more than the relative proportions of the 

 mineral constituents. This matter is treated more fully in Bureau of 

 Soils Bulletins 55 and 57 and Farmers' Bulletin 257. 



When a sample is sent to the bureau for advice as to crop adapta- 

 tion a determination is made, so far as can be done from a small sam- 

 ple, of the nature of the material, to identify the soil series to which 

 it belongs. If the exact locality from which the sample is taken is 

 given and information is furnished regarding the deeper subsoil, 

 the drainage, elevation, slope, location along streams or in the up- 

 lands, the occurrence of hardpan, subsurface gravel or rock layers, or 

 other information bearing upon the origin and mode of formation of 

 the material, an examination of the soil and subsoil samples by the 

 aid of the keys given in Bureau of Soils Bulletin 78 will usually de- 

 termine the soil series to which it belongs. This gives at once the 

 main characteristics of the soil and its crop adaptation under certain 

 physical and topographical conditions which will be referred to later. 



If the soil is found to belong to the Hagerstown, Sassafras, Hous- 

 ton, Norfolk, Knox, Marshall, Dunkirk, Clyde, or other known 

 series, adaptations are advised on the basis of the known normal 

 peculiarities of the series, as determined by the soil surveys. The 

 soil sample or an aqueous extract of the same is then tested by the 

 wire-basket method, described in Bureau of Soils Circular 18, or by 

 the bottle-culture method described in Bureau of Soils Bulletins 36 

 and 70, using carbon black in the soil extract to be sure that there is 

 no obscure substance present that will unfit the soil for normal crop 

 development. 



A sample submitted for fertilizer requirements is subjected to ex- 

 amination by the wire-basket method, using a normally productive 

 soil from the same series for comparison, adding to the soil which is 

 to be tested manure, lime, and commercial fertilizers in approximately 

 the same amounts as used in field culture. 



The requests for information regarding the mineral composition of 

 soils are complied with through the ordinary analytical methods 

 when it appears that the results may throw any light upon peculiar- 

 ities of constitution or character of the soil that may influence its 

 crop production. Many of these cases, it is believed, can more prop- 

 erly and more advantageously be considered questions of fertilizer 

 requirement. It is often found that the more expeditious method of 

 mineralogical examination or analysis gives fuller information re- 

 garding the mineral constitution and composition of the soil than 

 does the chemical analysis. 



All soils sent in for examination from arid regions, or generally 

 where the rainfall is less than 20 inches per annum, are examined by 



[Cir. 26] 



