CHARACTERIZATION OF THE LEAST LIMITING WATER RANGE OF SOILS

The supplies of O-2 and water and the mechanical impedance of soil are determined by soil structural form and soil water content. The range in soil water content in which limitations for plant growth associated with matric pressure, aeration, and mechanical resistance are minimal was defined as the least limiting water range (LLWR). This study was carried out to evaluate the LLWR as an index of the structural quality of soils. Undisturbed soil cores were taken from the 5- to 10-cm dept h of a silt loam and a loamy sand, cropped to corn (Zea mays L.) and r ed clover (Trifolium pratense L.). Soil water retention, soil resistan ce, air-filled porosity, and bulk density (D-b) were measured. Water c ontents at critical limits associated with field capacity (-0.01 MPa), wilting point (-1.5 MPa) air-filled porosity (10%), and soil resistan ce (2.0 MPa) were predicted and the LLWR calculated for each measured D-b. The natural variation in D-b on both soils gave rise to a wide va riation in LLWR. Values of LLWR varied from 0 to 0.14 cm(3) cm(-3) for the silt loam soil and from 0.05 to 0.13 cm(3) cm(-3) for the loamy s and soil. At D-b above 1.36 g cm(-3) for the silt loam and 1.43 g cm(- 3) for the loamy sand, the LLWR declined sharply with increasing bulk density. Further research relating LLWR to crop response is required b efore LLWR can be recommended as a soil structural quality index for c rop production.