Over street and Dean 83 



Soil texture 



Soil texture can be shown to have various effects on availability. 

 It is the common practice of European workers in this field (75) to 

 take into consideration soil texture when evaluating soils for available 

 nutrients. In general, light-textured soils need to contain higher 

 amounts of readily soluble phosphorus than do heavy soils in order to 

 have equal available phosphorus. At saturation, a unit volume of a 

 heavy soil contains more soil solution than a light soil. The effect of 

 this factor often has not been taken into consideration. Other things 

 being equal, heavy soils fix more phosphorus. In so far as leaching or 

 lack of leaching affects the amount of leaching and movement of ions 

 in a soil, its texture influences availability. 



Change in pH and lime status 



A change in soil pH brings about changes in the equilibrium be- 

 tween the soil solution and the solid phase of soils. The following 

 experiment by Dean and Rubins (//) illustrates this point well. Six 

 samples of soil colloid were suspended in 200 ml. of water and con- 

 tinuously agitated. Varying amounts of monobasic sodium orthophos- 

 phate were added to the suspensions representing approximately 5, 10, 

 15, 20, 25, and 30 per cent of the saturation capacity. Collodion bags 

 were introduced into each system, and at intervals small aliquots were 

 removed for analysis. Twenty-eight days after the start of the experi- 

 ment an equal amount of sodium hydroxide was added to each of the 

 suspensions. In Table I are given the phosphorus concentration and pH 

 values of the intermicellar liquids. These data show the magnitude of 

 change of concentration with pH. Equilibrium constants for the sets 

 of data are relatively close. 



The effect of change in pH on the phosphorus, boron, and sulfate 

 fixation will be discussed in a later section. 



Liming of acid soils is reported to increase the availability of the soil 

 phosphorus. Salter and Barnes (42) in summarizing the long time 

 field experiments at Ohio concluded "These facts are interpreted as 

 indicating an increase in the availability of native soil phosphorus as 

 the reaction is made more alkaline up to about pH 7.5." 



