150 EXPERIMENTS WITH PLANTS 



to the soil: such crops are called green manures (see 

 page 384). 



Certain other bacteria living free in the soil take 

 nitrogen from the air and render it available to plants, 

 while still others act on the humus and set free nitro- 

 gen in soluble form (see pages 383 and 384) . 



Most forest trees have their roots covered with 

 fungi (small colorless plants), which absorb and de- 

 compose the humus in which such trees grow and 

 render it available to the roots. 



How may phosphorus be supplied to the soil ? Phos- 

 phorus as fertilizer is chiefly obtained from bones. 

 If we soak a bone in muriatic acid it retains its form 

 but becomes like gristle, because the mineral con- 

 stituents are dissolved out: if, on the other hand, we 

 burn it, it also retains its form but becomes brittle 

 because the gristly substance has been burned away, 

 leaving the earthy constituents, which consist of phos- 

 phoric acid and lime. The burned bone may now be 

 pulverized and spread on the land as fertilizer, where 

 it is so very slowly dissolved by the carbonic, humous 

 and other acids in the soil- water as to be scarcely 

 at all available for absorption by the roots. By 

 treating bone with sulphuric acid, we obtain what are 

 known as superphosphates, which dissolve readily 

 in water: when placed in the soil the superphosphates 

 are changed back into the ordinary "insoluble" phos- 

 phates, but they are divided into very fine particles 



