SOIL ANALYSIS AND ITS INTERPRETATION 339 



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 infertile.^ 



Lime and Magnesia — About "i to '5 per cent, of magnesia 

 is found in the soils we have examined, and in general the 



— ratio falls between I and 3, but ratios of 4 and 5 are 



magnesia 



not uncommon, while on chalk soils they may rise very high. 



No connection could be traced between the ratio 



magnesia 



and the productiveness of the soil ; indeed, Table XCIII. shows 



that very good and very poor soils may have practically 



identical ratios. 



Lime 



Table XCIII. 



'Magnesia 



Ratio in Various Soils. 



Potash. — In the south of England soils 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 given in Table XCI V. 



The " available " potash ^ shows no kind of regularity, 

 but varies between 5 and 50 per cent, of the quantity ex- 

 tracted by strong acids. In deciding whether or not sufficient 

 is present, attention must be paid to the soil, the crop and the 



^For the effect of soil iron compounds on soluble phosphates see A. C. 

 de Jongh, Int. Mitt. Bodenkunde, 1914, 4, 32-45. 

 2 I.e. extracted by i per cent, citric acid. 



22 * 



