Slope gradients commonly are 5 to 18 percent but the extreme range include 2 to 25 percent. These soils formed in Late Sangamon paleosols formed in Kansan glacial till, except that the 10 to 20 inches above the pebble band is partly loess or loess and pedisediment. Mean annual temperature ranges from 49 to 54 degrees F, and mean annual precipitation ranges from 30 to 34 inches. Clarinda soils are grayer and finer textured, and Gara soils have thin dark colored surface horizons and are lower in clay content. Drainage is somewhat poorly drained.
Surface runoff is medium or rapid.
Permeability is slow. Armstrong soils, when associated with more permeable loess soils upslope, may develop seepy spots in wet seasons. Occasionally interceptor tiles are placed in the loess soils upslope. In average rainfall years, wetness is usually not a hazard. Depth to an intermittent perched water table is 1 to 3 feet at some time from November through July in most years.