College of Agriculture & Natural Resources Department of Plant, Soil and Microbial Sciences

Dissertation Defense Crop and Soil Science Ph.D. Degree: Benjamin Kwadwo Agyei

November 17, 2025 12:00PM - 1:00PM

---

Room A 271 PSSB

Members of the Examining Committee and their Department:

1. Dr. Maninder Pal (Manni) Singh - Plant, Soil and Microbial Sciences

1. Dr. Jeffrey Andresen- Geography

1. Dr. Christine Sprunger– Plant, Soil and Microbial Sciences
2. Dr. Brook Wilke- Kellogg Biological Station AG Nat Res

 ABSTRACT

The corn (Zea may L.)-soybean (Glycine max L.) rotation dominates northern U.S. cropping systems. These systems, typically under conventional tillage, are very productive but can lead to potential negative impacts such as soil health depletion and high greenhouse gas (GHG) emissions. Additionally, the frost-free growing season length (late April to mid-October) in this region is relatively short compared to regions like southern U.S. One primary aim of my research was to evaluate the impact of various conservation agricultural (CA) practices on crop productivity and soil health. The second aim was to identify optimal planting date (PD) and hybrid maturity (RM) selection to maximize corn yield, kernel dry down, and profitability.

For the first aim, we collected samples from farmer fields and conducted trials at three research stations (one at Michigan State University Agronomy Farm and two at Ohio State University sites in northwest and west Ohio) to evaluate impact of different conservation practices on soil biology, crop yield, and GHG emissions. In the on-farm study, we collected soil samples from 310 grower managed fields across MI and examined the impact of tillage (no-till, NT vs conventional tillage, CT) and crop rotation diversity (continuous monocrop, corn-corn or soybean-soybean; two crops, corn-soybean; and three crops, corn-soybean-wheat, Triticum aestivum L.) on soil health. The soil health indicators measured included microbial population, organic carbon (OC), permanganate oxidizable carbon (POXC), and water-stable aggregation (WSA). Fields under CT had significantly lower microbial populations, POXC, OC, and WSA compared to NT fields. Increasing crop rotation diversity significantly enhanced arbuscular mycorrhizal fungi and eukaryote populations as well as POXC. In trials at three research sites, we examined different combinations of biochar (BC), cover crop (CC), and nitrogen management (NM) under NT and their impact on yield, soil health, and GHG emissions. Yield responses and GHG mitigation were mostly driven by site-specific characteristics. During the first year of corn, NT+BC+NM resulted in 9% yield improvement compared to NT+BC in the sandy loam soils at MSU while NT+BC increased yield by 5% compared to NT+BC+NM in the clay soils at northwest Ohio. NT+BC significantly reduced cumulative CO₂ at MSU and northwest Ohio while NT+BC+NM+CC reduced cumulative N₂O across all locations during corn phase. NT+CC was effective in reducing cumulative N₂O emissions at MSU during soybean phase.

For the second aim, one of the studies estimated optimal corn RM for maximizing yield and profits at typical PD (early to mid-May) across MI locations by using data from corn hybrid trials conducted between 2006 and 2022. Averaged across years, optimal RM for maximum yield was lowest (ranged from 84 to 95) in northern locations (latitude > 44° N) and highest (ranged from 104 to 109) in the south (latitude 42° N). The optimal RM for maximum partial returns was up to 3 RM lower than that maximized yield for central and southern locations but showed variability in northern locations. The second study examined how RM should be adjusted based on PD to maximize yield and profits using four PDs (late-April to early June) and five RMs (89-109 RM) at MSU Agronomy Farm from 2021-2023. Under early planting, grain yield and profit benefits increased by using hybrids with 5-10 RM longer than the typical RM. When planting is delayed to late May or early June, maintaining the optimal or switching to 5 RM shorter hybrids, respectively, is necessary to maximize profit and reduce the risk of frost damage. Kernel dry down rates were also measured three times a week after black layer formation and ranged between 0.3 to 1.0% per day dependent on PD, RM, and weather conditions. Overall, this research provides innovative sustainable agronomic solutions for northern growers that can help increase cropping system productivity and profitability, while building or maintaining soil health.

---