Over street and Dean 85 



crements of displaced soil solution. The effects hold for acid soils of 

 the humid region as well as for calcareous and other soils of the arid 

 regions. Mattson et al. (j2) have found neutral salts to increase the 

 uptake of phosphorus by plants. 



Movement and leaching 



The movement of anions in soils is dependent upon the species of 

 ion involved, the extremes being a comparison of the highly mobile 

 nitrate ions with the much less mobile phosphate ions. Losses of ions 

 through leaching may be reasonably considered as a factor influencing 

 availability. The upward movement of ions in soils should also be con- 

 sidered. 



The very restricted movement of phosphate ions in soils may be a 

 restriction on availability. This becomes increasingly apparent if the 

 distribution of phosphorus is thought of as a mosaic throughout the 

 soil mass. With a lack of migration of ions the effectiveness of a given 

 root system would be reduced. 



THE RETENTION OF ANIONS BY SOIL 



From the foregoing discussion on the factors influencing the availa- 

 bility of soil anions, it may be seen that these factors are all in some 

 way connected with the reactions that take place at the liquid-solid 

 interface of soils. This section will deal with the kinds and mechanisms 

 of these reactions. 



Retention of phosphate ions by soil 



The retention of phosphate by soils can be readily demonstrated by 

 introducing soil into a solution containing phosphate ions and noting 

 the decrease in concentration. This phenomenon is frequently termed 

 "phosphate fixation." A review of this subject is available elsewhere 

 (/j). The mechanism of phosphorus retention has not been adequately 

 established. Apparently no single mechanism is applicable to all soil 

 conditions. 



The older theories of phosphate retention by soils envision chemical 

 precipitations. In acid soil systems iron and aluminum appear to be 

 most likely agents, while in neutral or alkaline soils calcium and other 



