CHAPTER VI 



SOIL COMPOSITION 



SOILS IN GENERAL 



ASIDE from the organic matter, any soil material (excepting 

 quartz sand, but including granitic sand) will commonly contain 

 all of the elements found in ordinary silicate rocks, but, of course, 

 in very varying proportions. Soils contain large amounts of silicon 

 and much aluminum and sodium, none of which are essential to 

 plant growth, also very large amounts of oxygen, an element which 

 as plant food is supplied in the carbon dioxid taken into the plant 

 through the leaves. This means that about 85 per cent of the solid 

 crust of the earth has no value as plant food. This includes not 

 only silicon dioxid (as quartz sand), aluminum silicate (as pure 

 clay), and aluminum sodium polysilicates, but also these elements 

 when present in other combinations. 



The remaining abundant elements, iron, calcium, magnesium, 

 and potassium, are all essential as plant food. Of these four, iron is 

 the most abundant in the earth and the least abundant in plants, 

 and, so far as the writer is aware, soil has never been known to be- 

 come deficient in iron as measured by crop requirements. 



Calcium and magnesium are somewhat less abundant than iron, 

 and are required by crops in very much larger amounts, and on 

 some soils crop yields are appreciably increased by the application 

 of one or both of those elements in suitable compounds, but in 

 many or most such cases the increase in crop yields is not due to 

 the direct effect of the calcium or magnesium as plant food, but 

 rather to the indirect effect their compounds may produce in 

 increasing the availability of other less abundant plant-food 

 elements. 



In the average crust of the earth, potassium is slightly more 

 abundant than magnesium but less abundant than iron or calcium. 

 Of these four elements, potassium is required by plants in greatest 



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