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passage of time appear to proceed least rapidly with the property of 

 particle-size distribution. 



It is doubtful that any physical property of soil remains uninfluenced 

 in some respect by the proportions which the soil contains of particles 

 of different sizes, since the "ultimate dispersion" obtainable for a given 

 soil (by the arbitrary methods already mentioned) is itself a manifesta- 

 tion of the particle-size distribution, and the extent to which this limit- 

 ing degree of dispersion is approached is of great significance. Cultiva- 

 tion and management practices, the growing plant, microorganisms 

 and the weather, all tend to shift the degree of dispersion of the soil as 

 displayed at any one instant closer to, or further away from, the state 

 of ultimate dispersion. 



The physical properties to be discussed may all be shown, under 

 certain conditions, to be functions of the degree of dispersion of the 

 solid phase. Yet, at present, nothing less than a complete distribution 

 curve and a fundamental expression of aggregation status, together 

 with full information concerning its chemistry, mineralogy, micro- 

 biology, and organic matter, will even approximately specify the physi- 

 cal and physico-chemical behavior of the soil under a given sequence 

 of treatments and changes in water content. The functional relation- 

 ships are extremely complex. 



Role of aggregation 



The tendency of soil particles to form clusters or aggregates which 

 resist dispersive treatment has received much attention by soil physicists. 

 Measurements of approximate aggregate size distribution by sieving 

 in air, and sieving, shaking, settling, or elutriating in water, have been 

 used to provide numerical expressions of soil structure. Although the 

 results are affected by the method of determination used and no stand- 

 ard method of measurement has yet been adopted, aggregate size dis- 

 tributions have been shown to affect other physical soil properties and 

 also plant growth. 



Clarke and Marshall (jo, j/) and others have shown the existence 

 of a significant positive effect of clay on water-stable aggregation. They 

 found that a significant decrease in water-stable aggregation* resulted 



* Aggregation was expressed in terms of the amounts of primary particles 

 smaller than a given size which were present in aggregates larger than that size. 



