98 SOILS AND FERTILIZERS 



In this case, as in that of the four soils, previously cited, 

 there is a correlation between the productiveness of the soils 

 and the composition of the water extract. 



114. Chemical analysis of soil. — There have been many 

 methods devised for the chemical analysis of soil. The 

 important difference between these is in the solvent used 

 to bring the soil into solution. Most solvents dissolve only 

 a part of the soil, in which case the result of the analysis 

 does not show the entire amount of all the constituents, and 

 does not, therefore, show the total quantity of the plant-food 

 materials in the soils. The figures given in Table 17 are 

 obtained from a complete solution of the soils analyzed and 

 hence show their ultimate composition. 



The advantage of an analysis of this kind is that one can 

 judge of the lasting qualities of the soil, and if any particular 

 constituent is present in very minute quantity that fact 

 is disclosed, and measures can be taken to augment the 

 supply, but nothing, however, as to immediate productive- 

 ness can be learned. A collection of rocks may yield to this 

 analysis as much phosphoric acid, potash, lime, or other 

 nutrient, as a rich soil. Such an analysis is useful only to 

 ascertain the ultimate limitations of a soil, or its possible 

 deficiency in any essential constituent. 



Various solvents have been used with the intention of 

 finding the quantities of food materials that plants may be 

 expected to obtain in a reasonable length of time, or in other 

 words to determine the available plant-food materials. 

 These methods fail because availability, as we have just 

 seen, depends on the conditions to which a soil is subjected 

 in the field, and as these naturally var}' from time to time 

 it is impossible to find any one solvent that will measure 

 such a variable quantity as availabilitj^. 



Chemical analyses of soil are useful in connection with 

 investigations of questions relating to soils but it is not 



