Monday, November 13, 2006

Soil Structure and Bimodal Pore Size Distribution Influence on Soil Water Retention Curves.

Riley Walton, University of Kentucky-Dept. of Plant and Soil Science, N-139 Ag Sci North, Lexington, KY 40546, John H. Grove, Univ. of Kentucky, Dept. of Plant and Soil Sciences, N122L Ag Sci Ctr - North, 500 S. Limestone St., Lexington, KY 40546-0091, and Ole Wendroth, University of Kentucky, Lexington, KY 40546-0312.

Soil water retention curves (SWRC) are commonly used to describe soil pore size distributions and methods for their rapid determination are being developed.  Dewpoint meter technology, for instance, allows for easy measurement of the dry end of the curve where texture is the main influence.  Although models are readily available for this end of the SWRC, extrapolating the retention curve closer to saturation, based only on the dry end information, may be misleading.  Alternatively, empirical parameters describing SWRC are derived from pedotransfer functions based on soil textural data.  These models result in smooth continuous functions that may not reflect true retention behavior, especially in soils with similar textures but different organic matter contents.  One drawback to these models is that they do not incorporate soil structure, often manifested as a bimodal pore size distribution.  To demonstrate the importance of soil structure and the presence of bimodal pore size distributions, measured SWRC were compared to SWRC derived from simplifying extrapolation methods and pedotransfer functions.  Soil cores were taken from a no-till soil at four depths (5, 10, 15 and 20 cm) with differing levels of organic matter.  Hanging water column and pressure plates were used to determine tensions from close to saturation to 30 kPa.  Pressure plates and dewpoint meters were used to determine tensions from 30 kPa to around 105 kPa.  In addition to laboratory methods, tensiometers and TDR probes were also used in the field, at 5, 10, 15 and 20 cm depths, to determine in-situ soil water retention characteristics.  Almost every curve showed a bimodal response.  Significant differences among the laboratory cores were prevalent at the 5 cm depth, over almost all ranges of tension in the SWRC.  In most cases, laboratory and field measured SWRC exhibited similar behavior.

Handout (.pdf format, 2778.0 kb)