WATER DISTRIBUTION IN A FRAGIPAN SOIL DURING THE GROWING-SEASON

The resistance to penetration developed during the formation of fragip an 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 experim ental field plots were installed at each of three sites situated on Gr enade silt loam (fine-silty, mixed, active, thermic Glossic Fragiudalf s), Based on relative depths to the fragipan horizon, these three site s 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 grow ing season at all three sites. The extent of water loss from the fragi pan 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 resu lts suggest that plants grown on soils with fragipan horizons have ind irect access to water stored in that zone. Consequently, yield data fr om such studies must be carefully interpreted when comparing sites wit h a range of depths to fragipan.