TSB SOIL PAETWLE 



101 



of the physical agents, exhibit smaller quantities of free 

 quartz. The silica in such soils is held as complex sili- 

 cates, which very largely carry the elements that are so 

 important in plant development. 



Although these data are based on but a few samples, 

 they are so concordant with what would naturally be 

 expected that these general conclusions cannot be avoided. 



75. The chem.ical constitution of soil particles. — The 

 mineralogical examination of soils has revealed a larger 

 percentage of such minerals as feldspars, mica, horn- 

 blende, and the like, in the finer separates. A larger 

 percentage of the important plant-food elements would 

 therefore be expected in those groups. The following 

 data,^ compiled from work performed by the United 

 States Bureau of Soils, substantiate this assumption : — 



Chemical Composition of Vaeious Soil Sepakatbs 





Num- 



Pebcentage of 



Pekcentage of 



Percentage of 



Soils 



ber OF 



Sam- 

 ples 



P2O5 IN 



K2O IN 



CaO IN 





Sand 



Silt 



Clay 



Sand 



Slit 



Clay 



Sand 



Silt 



Clay 



Crystalline residual 



3 



.07 



,22 



.70 



1.60 



2.37 



2.86 



.50 



.82 



.94 



Limestone residual 



3 



.28 



,23 



.37 



1.46 



1.83 



2.62 



12.26 



10.96 



9,92 



Coastal plain 



7 



.03 



.10 



.34 



.37 



1.33 



1.62 



.07 



.19 



,55 



Glacial and loessial 



10 



.15 



.23 



.86 



1.72 



2.30 



3.07 



1.28 



1.30 



2.69 



Arid soils 



2 



.19 



.24 



.45 



3.05 



4.15 



5.06 



4.09 



9,22 



8.03 



It is seen that on the average the soils with finer particles 

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

 those of coarser texture, the only exception in this case 

 being in the lime of the residual limestone soils. The 

 arid soils present a less marked difference in the sands, 



^Failyer, G. H., and others. The Mineral Composition 

 of Soil Particles. U. S. D. A., Bur. Soils, BxiL 54. 1908. 



