Drainage, tillage, and residue effects on soybean yield and soil characteristics
Barahona, C. F. S.. 2025.
Abstract
With a global population exceeding 8 billion, safeguarding the arable land base is
essential under growing demand for food, feed, and bioenergy. Soybean (Glycine max
(L.) Merr.) is central to this effort because of its high protein and oil content, its role in
biological nitrogen fixation, and its importance in corn-soybean rotations across the U.S.
Midwest. This thesis evaluates how interactions among subsurface drainage, tillage,
residue management, and seasonal weather jointly determine soybean yield, root
architecture, soil properties, and seed composition in a long-term experiment in south
central Minnesota.
In Chapter 2, a 3-year split-split-plot experiment compared subsurface drainage
(drained vs. undrained), tillage [conventional (CT), strip-till (ST), no-till (NT)], and
residue management (retained vs. removed). The dominant response was a drainage ×
tillage interaction, rather than independent drainage or tillage main effects. Yield gains
from drainage were realized primarily where tillage created a warmer, better-aerated
seedbed (CT and ST), whereas drainage provided little advantage under NT, indicating
that soil physical conditions mediated whether drainage-based water management
translated into higher yield. Residue effects on yield and canopy development were also
contingent on tillage and year-to-year weather, with small average gains from residue
removal but context-dependent outcomes. Root traits likewise reflected interaction
patterns: early and reproductive root proliferation was greatest where drainage and more
intensive tillage were combined, while in-season mineral N supply remained relatively
stable across combinations and residue retention chiefly enhanced near-surface PO43- -P.
Chapter 3 uses a 10-year record to quantify how weather-management
interactions shape yield and grain composition. Precipitation and temperature deviations
from the normal were defined as departures from 1991-2020 climatic normals, with
"precipitation extremes" corresponding to May-September totals deviating by ≥75 mm and "temperature extremes" to seasonal departures of roughly ±1 °C. Early-season
precipitation departures from the long-term mean reduced yield but tended to increase
protein, whereas early-season warming increased yield and oil but reduced protein. These
weather effects were strong, yet drainage × weather interactions were not significant,
indicating that, at this site, drainage did not change the sensitivity (slope) of yield or grain
composition to seasonal departures of precipitation and temperature from the long-term
mean, even though absolute yields depended on the underlying drainage-tillage
configuration.
Overall, the work shows that (i) soybean performance is governed by interactions
among management factors, with the agronomic value of subsurface drainage emerging
primarily in specific tillage contexts; (ii) early-season deviations in precipitation and
temperature from the long-term mean interact with those management settings to drive
interannual variability in yield and grain quality; and (iii) residue retention enhances near
surface P supply more than mineral-N dynamics. Apparent nutrient "conservation" under
no-till with residue often reflects lower grain export from reduced yields rather than
improved nutrient-use efficiency, underscoring trade-offs between nutrient retention in
the soil and harvested production within these interacting management-weather regimes.
