THE EFFECT OF SOIL STRENGTH ON THE GROWTH OF PIGEONPEA RADICLES AND SEEDLINGS

The effect of soil strength on the growth of pigeonpea radicles and seedlings was investigated in cores of three clay soils prepared at different water contents and bulk densities in the laboratory. Radicle elongation directly into soil cores was reduced from 50-70 mm d-1 at strengths less than 0.5 MPa to 0 mm d-1 at 3.5-3.7 MPa. The response to soil strength was affected by the water content of the soil, presumably as a result of reduced oxygen availability in wetter soil. This effect was apparent in soils wet to air-filled porosities less than 0.15 m3 m-3. Radicles were more sensitive to high soil strength (> 1.5 MPa) than were seedling roots which encountered the same conditions at 60 mm in the profile. Radicle growth ceased at 3.5 MPa which reduced seedling root growth by only 60%. Despite a 60% reduction in root length in the high strength zone, seedling roots compensated in zones of loose soil above and below the compacted layer, and total root length and shoot growth were unaffected. There was no evidence of a 'root signal' response which results in reduced shoot growth in some species in response to high soil strength. The proliferation of roots in surface layers and the delayed penetration of the root system to depth in compacted soil are likely to expose seedlings to a greater risk of water-deficit in the field, particularly under dryland conditions where plants rely on stored subsoil water for growth.