A simple model for a soil crust was formulated and experimentally tested. It is based upon the parallel orientation and close packing of clay particles perhaps like that in shale. The physical and chemical factors which disrupt the ordered arrangement of the plate-shaped particles were noted to be those which induce aggregate formation and which result in good soil structure. The factors which induce greater orientation and packing were noted to be those which induce crust formation and poor soil structure. The resultant deleterious effects upon seedling germination, moisture penetration and runoff, aeration, and penetrability to roots are well known.Experimental attempts were made to test the validity of the proposed model. In each instance, the data either provided support for the model or else could be reasonably reconciled with the model. The model appears to provide a reasonable basis for explaining cultural practices which are known to influence soil structure in either beneficial or deleterious ways.