Water Retention of Soft Siltstone Fragments in a Ustic Torriorthent, Central Montana

Some soils in the western USA overlie or contain fragments of porous sedimentary rock. Estimations of plant‐available water that neglect water retained in the rock may be in error. Using pressure‐plate and newly modified tension‐table techniques, we measured the water retained at nine water potentials ranging from 0 to −1500 kPa in fissile siltstone fragments from the Cr horizon of a Ustic Torriorthent developed on the Fort Union formation in central Montana. The siltstone fragments held ≈0.10m 3 m −3 plant‐available water, assuming −1500 kPa (−15 bar) as the lower limit of water availability.