.■j^^re J.— Expectation 

 -vjjr. water retention 

 -,.,'2 -unction of Ph. 



1/3 bar 



15 bars 



\ - 



Pb 



2.78 



fine particles occupy large pores, increase the specific surface (and consequently 

 balk density) and tend to form smaller pores at higher F-levels. Added increments of 

 fines begin to favor a weak crumb structure due to interparticle binding, formation 

 of larger pores, and a decrease in bulk density. The data suggest this phenomenon 

 ha-^ a strong effect in the range F = 20 to 30 percent by weight. 



The expectation for retention over bulk density (Pb) poses an interesting problem. 

 Logically, at Pb = and Pb = PS (where PS is solid phase density; approximately 2.78 

 for our parent materials), porosity and therefore retention is 0. Retention therefore 

 has two minima and is assumed to have one maximum at some intermediate value (fig. 3). 

 The value of Pb where this assumed maximum occurs is a function of the pore size dis- 

 tribution in a polydispersed soil system. The total porosity, E, approaches a maximum 

 where Pb approaches zero, but this sheds no light on maximum retention at energy levels 

 (tensions) greater than zero (where zero is saturation percentage). The increase in 

 retention from very low Pb values to this maximum is probably attributable to increased 

 specific surface. Interparticle binding by OM or F to form a weak crumb structure 

 diminishes this effect, as previously pointed out. 



Changes in the foregoing main effects were e.xpected to occur as the three soil 

 factors interacted and the combined expectation would be as shown in figure 4. Actual 

 data points would be distributed over the ranges of the soil factors in a pattern 



hulk density, and MATTER 

 percent fines. (Percent) 



5 



