Water distribution in a fragipan soil during the growing season

The resistance to penetration developed during the formation of fragipan horizons effectively excludes plant roots from the water stored in this zone. Thus, plant roots are believed to be restricted to the use of water stored in the profile between the fragipan and soil surface, This study was conducted to determine if fragipan horizons contribute water to plant rooting zones during the growing season. Twelve experimental field plots were installed at each of three sites situated on Grenade silt loam (fine-silty, mixed, active, thermic Glossic Fragiudalfs), Based on relative depths to the fragipan horizon, these three sites represented slight, moderate, and severely eroded phases of the soil. Access tubes were inserted to a depth of 30.5 cm below the fragipan surface on six of the plots at each site. Water contents were measured weekly by a combination of gravimetric and neutron probe methods from the 1984 through the 1987 growing seasons. Results indicate that the fragipan horizons contributed water to the overlying profile each growing season at all three sites. The extent of water loss from the fragipan was determined by its proximity to the soil surface, For the four growing seasons, the fragipan horizon contributed on average 15.6% of the water lost at the severely eroded site, compared with 5.5 and 6.1% at the slightly and moderately eroded sites, respectively. These results suggest that plants grown on soils with fragipan horizons have indirect access to water stored in that zone. Consequently, yield data from such studies must be carefully interpreted when comparing sites with a range of depths to fragipan.