86 Mineral Nutrition of Plants 



divalent bases prevail. In acid soil systems solubility and plant growth 

 studies point to possible inadequacy of a precipitation theory. In cal- 

 careous soil systems, however, a precipitation theory seems adequate. 



Since the amount of phosphorus taken up by soils is proportional to 

 the concentration, phosphorus retention has been ascribed to adsorption 

 reactions. Adsorption is the tendency to concentrate at the interface. 

 The term is general and may include several kinds of surface reactions. 

 It does not carry any implication relative to the binding forces. Obser- 

 vations have shown that some of the phosphorus retained by soils is 

 more tightly held than others. For example Mattson and Karlsson (]i) 

 have distinguished between colloid-bound phosphorus, a nondiffusable 

 structural unit, and saloid-bound phosphorus, a diffusable ionic atmos- 

 phere held as compensation to ions of opposite charge. 



The substitution of one ion for another or a metathetical reaction 

 involving chemical forces and affinities is a means by which anions in 

 solution may become associated with the solid phase or adsorbed. Thus, 

 phosphate retention may be considered as the exchange of H 2 P0 4 ions 

 in solution for OH ions associated with the solid phase. An increase in 

 pH of the system is associated with this reaction; conversely, phosphates 

 of soils may be displaced by hydroxyl, fluoride, or arsenate ions (12, 



Isotopic exchange studies (jo) between P 32 4 ions introduced into 

 the soil solution and P 31 4 ions associated with the solid phase have 

 indicated that only a small proportion of the phosphate ions retained by 

 soils are readily exchangeable with similar ions in the liquid phase, 

 whereas similar studies (6) have shown that the exchangeable calcium 

 ions were all in equilibrium with the calcium ions in the liquid phase. 



The phosphorus retention by soils, for the most part, is restricted to 

 the clay fraction. However, a large number of minerals common to 

 soil exhibit, when finely ground, a capacity to fix phosphate. 



In acid soils particular attention has been given to the hydrous oxides 

 of iron and aluminum and to the clay minerals. Maximum retention 

 by these materials usually occurs in the pH range of 4 to 6. Soils treated 

 to remove the iron oxide invariably show a reduction in their capacity 

 to fix phosphorus (47). 



