Abstract:
Many agricultural sustainability issues are related to biological processes which
are central to the ecological function of soils. Soil physical properties are the
architecture in which these processes are carried out. Cover crops hold promise as one
of the techniques which can ameliorate poor soil structure and improve bulk density and
water intake. In addition, integrating cover crops into the production cycle may
improve cash crop quality and yield. A multi-disciplinary research project was initiated
in July 1996 to compare the effects of winter fallow and winter cover crops in
Willamette Valley vegetable production. This thesis addresses soil physical properties
which are important for plant water relations, root growth, and microbial habitat.
Responses in soil physical properties to these treatments were measured in six farm
fields and two research stations. At a seventh farm site, conventional tillage was
compared with minimum tillage. A third component of this research was to identify
early indicators of change in soil quality trajectory. Lower bulk densities and enhanced
water intake were observed in research plots and farm fields with cover crops when
compared to fallow. As part of this research, a procedure was developed to pre-treat
soil samples to equalize water content before determining aggregate size distribution. A
simple technique was developed to obtain a subsample of specified mass that contained
the same percentage of aggregate size fractions found in the parent sample. The dry
aggregate size distribution procedure measured aggregate size fractions (1.00 - 2.00,
0.50 - 1.00, 0.25 - 0.50, 0.106 - 0.25, and < 0.106 mm) on soil samples pretreated to
equalize soil sample water content at -1300 kPa. Cover cropping increased 1.00 - 2.00
mm aggregates (P = 0.05) in farm fields. Water stable aggregation improved at the
Oregon State University Vegetable Farm (Corvallis, OR) research plots where cover
crops have been part of the management plan since 1993. Aggregate size increase
occurred in the farm fields after one winter cover crop and appeared to precede an
increase in water stable aggregation. The results suggest that dry aggregate size
distribution may be a useful early predictor of a change in soil quality.
