Abstract: The invention is for an optimum water and soil retention system for use in an agricultural field, which includes watershed units having a basin, borders located on opposite sides of the basin, an ingress and an egress weir located at opposite ends of the basin. The top surface of the ingress weir is below the top surface of the borders at the point where the ingress weir intersects the borders. The top surface of the egress weir is nearly always below the level of the ingress weir. The watershed units are arranged consecutively end to end to form a string, which normally runs from a high point in the field to a low point or drainageway. The field is divided into collateral strings thus creating a complete and systematic field of watershed units.