Mechanical strength values were determined for dried briquettes formed from soil material taken from the surface horizons of 83 experimental cotton plots located in the North Carolina Coastal Plain and for which cotton stand data were available. A highly significant direct relationship was found between strength and content of clay. Greater strength values were also observed with increasing amounts of fine clay (<.2 μ). A significant inverse relationship between cotton stand and content of clay was also demonstrated. This appeared to be an indirect effect of clay operating through its effect on strength.The effect of clays separated from the above soils on mechanical strength of artificial soil samples was also studied. The relative concentrations of clay-size quartz and alkali-extractable alumina were the mineralogical properties most correlated with the strength of artificial soils. Quartz was negatively correlated whereas alumina was positively correlated with briquette strength.Enrichment of the clay fraction with negatively charged particles increased strength two- to threefold. This enrichment was accomplished by the selective dissolution of some of the noncharged clay components such as gibbsite, a portion of the quartz, and alkali-extractable silica and alumina.