THE SOIL AS A SOURCE OF MINERAL SALTS 



141 



is quickly rendered insoluble, mainly by precipitation as hydrated 

 aluminium, iron and calcium phosphates. The addition of chelating 

 agents to phosphate fertiUzers reduces the amount of phosphate 

 fixed in non-exchangeable forms, presumably by removing calcium 

 and heavy metal ions from solution. The presence of legumes which 

 tend to absorb calcium rapidly, can under some conditions promote 

 the growth of associated plants, for example, cereals in phosphate- 

 deficient soils, for the same reason. 



Sulphates form the most important source of sulphur for plants. 

 Most of the sulphates present in soil are relatively insoluble, and 



Root 



-CO2+ HgO ^ ^ H'^+HCO 



(a) 



(b) 



Fig. 45. Absorption of cations from soil 

 a. The carbon dioxide hypothesis; b. The contact exchange hypothesis. 



are not immediately available, but gypsum (CaSO^-lHoO) which is 

 sparingly soluble, weathers rapidly to yield sulphate ions. Sulphides 

 tend to be insoluble but they are gradually brought into solution 

 by oxidation through weathering, and by the activity of sulphur- 

 oxidizing bacteria. 



2. Cations 



a. Carbon dioxide hypothesis. Plants possess the capacity to 

 facilitate cation release from the solid state, as is shown by the 

 well-known ability of roots to etch the surface of marble (Sachs, 

 1875). An early suggestion was that respiratory carbon dioxide 

 released from roots reacts with water to produce carbonic acid, which 

 diffuses to the surface of the soil particle (Fig. 45a). Here exchange 



