Figure 1 . --Expectation 

 for water retention 

 as a function of 

 percent OM. 



OM (Percent) + 



Our initial expectation was for a linear increase in retention with increasing OM 

 values; however, the data strongly indicated the diminishing increase in retention with 

 increasing OM as shown in figure 1. The curvilinear relation has a positive intercept 

 at OM = 0; this value is equal to the matric potential of a polydispersed system of 

 lithic particles having a pore size distribution that would hypothetically be formed for 

 a coarse- textured mineral soil. The amount of increase in water retention diminishes as 

 OM increases, particularly beyond about 8 percent OM. A possible explanation is that ini- 

 tial increments of OM tend to coat coarse lithic particles and the interstices of large 

 pores, maintaining the single grain structure and not altering pore size distribution. 

 However, at higher OM levels, added increments of OM tend to bind the coarse particles 

 forming a weak crumb structure (Emerson 1959) . This would change the pore size distri- 

 bution, and could result in a higher percentage of large pores. This effect on reten- 

 tion should be more evident at the lower energy level (1/3 bar) since most retention at 

 15 bars is by adsorptive (surface active) phenomena. Again the data support this. The 

 retention at 15 bars over OM is more nearly linear than it is at 1/3 bar tension. 



The expectation for water retention as a function of percent fines (F) (fig- 2) 

 follows essentially the same line of reasoning as retention over OM (fig. 1). Again 

 there is a positive intercept at F = 0, equal to the matric potential of a polydispersed 

 soil in which all particles are greater than SOym in diameter. Initial increments of 



