33^ 



SOIL CONDITIONS AND PLANT GRO WTH 



Alumina. — In general, in the much weathered soils of 

 the south of England the alumina is approximately equal in 

 amount to one-third of the clay fraction, indicating that the 

 acid treatment breaks down some definite group of silicates 

 associated mainly with the clay fraction in the soil.'' Ex- 

 amples are given in Table XCI. 



Exceptions to the rule occur when much fine silt is 

 present, the alumina then being markedly less than one-third 

 of the clay (Table XGIL). 



Table XCII. — Alumina and Clay Contents of Silty Soils. 



Formation. 



Percentage of fine silt in soil 



„ clay in soil . 



AI2O3 . 



., . ALO3 



Ratio — r-^ 



clay 



Iron Oxide. — The iron oxide is present in quantities com- 

 parable 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. F. V. Button finds no 

 more iron in the fertile red soils of Devon than in the infertile 

 grey soils. Light soils, good or bad, contain about i to 2*5 



^ The percentage of AljOg given here represents the amounts extracted by 

 HCl and zCre not comparable with those on p. 102 obtained after treatment with 

 ammonium fluoride. 



