Submitted to: Soil Science Society of America Journal
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: January 30, 1998
Publication Date: N/A
Interpretive Summary: Soil water stored in fragipan horizons is unavailable to plant roots since they cannot penetrate this subsurface layer due to its high strength. This characteristic is also thought to exclude water movement into fragipan horizons during growing seasons. Thus, it is generally assumed that crop growth and yields are determined by the volume of water stored in the soil overlying the fragipan horizon. Our four year study showed, however, that soil water moved in and out of the fragipan horizon in response to water additions or depletions in the non-fragipan horizons. This suggests that fragipan horizons can contribute water to plant roots, and may in fact enhance yields above those predicted for a given depth to fragipan
The resistance to penetration developed during the formation of fragipan horizons is sufficient to exclude plant roots from the water stored in this zone. Thus, plant roots are restricted to the use of water stored above the fragipan. Our objective was to determine how the soil water content of fragipan horizons responds in conjunction with additions and depletions of water in overlying horizons, and if they contribute water to plant rooting zones during the growing season. Twelve experimental field plots were installed on three sites of a Grenada silt loam (fine silty, mixed, thermic, Glossic Fragiudalf) that represented slight, moderate, and severely eroded phases of the soil based on relative depth to the fragipan. Access tubes were inserted to a depth of 30.5 cm below the fragipan surface on one-half of the plots at each site. Water contents were measured weekly by a combination of gravimetric and neutron probe methods during the 1984 through 1987 growing seasons. In the relatively dry years, considerable quantities of water were lost from the fragipan horizon in response to depletions in the upper profile. Conversely, in the wet years the water content of the fragipan remained relatively constant or increased periodically. In all cases, the fragipan at the severely eroded site was most responsive. The low hydraulic conductivities associated with these fragipans appear to control the rate at which they wet up and dry out.