144 SOIL CONDITIONS AND PLANT GROWTH 



Iron Oxide. The iron oxide is present in quantities comparable 

 with those of alumina, but no close relationship is observable, nor does 

 the amount of iron oxide afford any indication of the fertility of the 

 soil. Light soils, good or bad, contain about I to 2-5 per cent., good 

 loams and poor clays contain 3-5 to 5 per cent. Larger amounts of 

 iron oxide are not common. Soils containing ferrous compounds are 

 generally of no great fertility. 



Lime and Magnesia. About -I to -5 per cent, of magnesia is found 



in the soils we have examined, and in general the 



lime 



ratio falls 



magnesia 



between I and 3, but ratios of 4 and 5 are not uncommon, while on 

 chalk soils they may rise very high. No connection could be traced 



between the ' :- ratio and the productiveness of the soil, indeed 



magnesia 



Table LVIII. shows that very good and very poor soils may have 

 practically identical ratios. 



TABLE LVIII.. 



LIME 



RATIO IN VARIOUS SOILS. 



Potash. The amount of potash is closely associated with that of 

 alumina, being commonly about one-tenth ; it is, therefore, about one- 

 thirtieth of the clay. Some examples are : 



