36 



SOILS AND FERTILIZERS 



these particles would displace if they were immersed in it, 

 does not necessarily correspond to these terms. Particles of 

 greater and less specific gravity are scattered through both 

 " light " and " heavy " soils and if we are to find the specific 

 gravity of a soil we must have in the sample to be tested 

 enough particles to give an average of all in the soil. 



35. Chemical composition of soil separates. — The fact 

 that one kind of mineral wears down to a small particle 

 more easily than does another indicates that there would be 

 a preponderance of resistant minerals, like quartz, among 

 the coarse particles and a large proportion of the more 

 easily decomposed minerals, like the feldspars, among the 

 fine particles. This is actually the case, and it indicates 

 a chemical difference in the separates. Analyses of sepa- 

 rates made by the Bureau of Soils of the United States De- 

 partment of Agriculture bring out these differences, as shown 

 by the following table : 



Table 11. — Chemical Composition of Some Soil Separates 



It will be noted from this table that, in general, the smaller 

 particles are richer in 'phosphoric acid, potash and lime than 



