Soil physical properties play a crucial role in agricultural productivity and environmental sustainability. This study investigated the seasonal dynamics of soil physical properties under different tillage and drainage practices in poorly drained soils of Ohio, focusing on their interrelationships with soil structure, crop growth, and climate across various agricultural production stages. Soils were sampled under long-term tillage (no-till, NT vs chisel-till, T) and drainage (drained, D vs non-drained, ND) management. Intact soil cores and undisturbed bulk samples were collected at 3 depths in different seasons. The research examined a. soil hydraulic properties, including saturated hydraulic conductivity (Ksat), unsaturated hydraulic conductivity (Ks) and water diffusivity (Ds) at field capacity, field infiltration, and plant available water capacity (AWC), b. soil structure, including bulk density (BD), pore size distribution (PSD), water stable aggregates (WSA), mean weight diameter (MWD), penetration resistance (PR), and aggregate tensile strength (TS), c. soil physical quality index (SQI) with the selected indicators, and d. crop development and climate factors, including crop height, crop yield, ground coverage, air temperature, daily precipitation, and rainfall intensity.
Specifically, this thesis aims to:
a. Investigate the dynamics of soil hydraulic properties (Ksat, Ks, Ds, field infiltration, AWC) under different tillage and drainage practices.
b. Assess the dynamics of soil structure (WSA, MWD, PSD, PR, TS) under different tillage and drainage practices.
c. Determine the yield-oriented SQI across different seasons of crop production.
In terms of the first objective, results in Chapter 2 showed that NT generally maintained higher Ksat (0.675-0.898 cm/hr vs 0.307-0.572 cm/hr, p=0.001-0.124) in fallowing seasons, especially in surface soils, due to better soil aggregation and ground coverage. However, during growing seasons, NT (0.326-0.774 cm/hr) sometimes showed lower Ksat than T (0.402-0.934 cm/hr, p=0.263-0.985) soils. Drainage effects on Ksat (D 0.415-1.302 cm/hr vs ND 0.249-0.645 cm/hr) were more pronounced in surface soils but diminished with depth. Field infiltration rates were generally higher in NT soils (NT 0.733-1.221 cm/hr vs T 0.458-0.640 cm/hr, p=0.013-0.040), while drainage effects were inconsistent. NT (0.097-0.193 m³/m³) soils typically had slightly lower BD (NT 1.41-1.46 g/cm3 vs T 1.42-1.47 g/cm3, p=0.030-0.869) and AWC compared to T (0.100-0.177 m³/m³, p=0.001-0.219) soils, attributed to differences in pore size distribution and soil aggregation. NT practices benefit crop height in growing seasons and total ground coverage (NT 15.2-54.5% vs T 9.7-44.5%, p=0.001-0.089) through the experimental period. The variation in unsaturated hydraulic conductivity (Ks, 0.62-36.42 × 10⁻⁴ cm/hr) and water diffusivity (Ds, 0.001–0.0167 cm²/hr) across soil depths is influenced by seasonal changes, soil depth, and management practices (for example, NT: 12.80-27.59 × 10⁻⁴ cm/hr; T: 2.34-11.56 × 10⁻⁴ cm/hr, p = 0.002-0.046).
In terms of the second objective, Chapter 3 revealed NT improved soil aggregation, showing higher WSA (NT 65.6-92.3% vs T 48.5-73.1%, p=0.001-0.044) and MWD (NT 0.98-3.31mm vs T 0.57-1.98mm, p=0.001-0.051) compared to T, especially in the top 30 cm of soil. Drainage practices had a minor impact on soil aggregates, with some improvements in WSA (D 53.1-69.6% vs ND 52.0-61.5%, p = 0.014-0.027) and MWD (D 0.92-1.15 mm vs ND 0.61-0.94 mm, p = 0.001-0.020) at certain depths and times. Soil aggregates were positively correlated with residual pores (r = 0.12 to 0.66, p < 0.05) and negatively correlated with storage pores (r = -0.38 to -0.15, p < 0.05). PR showed seasonal variability, with higher values in summer (1513-2247kPa) and lower values in spring (657-894kPa, p=0.001). NT systems showed lower PR at the surface (NT 1140-1591 kPa vs T 1280-1900 kPa, p = 0.031-0.163) but higher PR in subsoils (NT 885-1608 kPa vs T 699-1342 kPa, p = 0.019-0.097) compared to T. Drainage practices did not significantly impact PR at any depth. Rainfall intensity showed positive correlationship with PR. Soil aggregates generally showed a negative correlation with TS (r = -0.30 to -0.15, p < 0.05).
In terms of the third objective, in Chapter 4 a yield-oriented SQI was developed, which correlates positively with crop yield during growing seasons (r = 0.350-0.496, p = 0.002-0.036) but not during fallow periods (r = -0.142-0.235, p = 0.101-0.713). The correlation between SQI and crop yield varies with soil depth and crop growth stages. NT practices significantly benefit crop development, both for soybean (NT 9.3-64.3cm vs T 6.0-62.2cm, p = 0.001-0.010) and maize height (NT 196.1-196.4 cm vs T 158.7-179.3 cm, p = 0.001). Drainage practices showed minimal impact on soil aggregates, led to varying effects on crop growth and yield. The importance of specific soil indicators (e.g., BD, PR, and MWD) varies by crop type and growth stage. NT improved soybean yield (NT 4.21 vs T 3.80 Mg/ha, p = 0.002) through better soil aggregation, while ND fields enhanced maize yield (ND 7.96 vs D 6.62 Mg/ha, p = 0.001) by reducing PR.
In conclusion, while NT practices generally improved soil structure and aggregation, they also presented challenges such as potential subsoil compaction. The variable effects of drainage practices highlight the need for site-specific management strategies. The SQI developed in this study proved to be both purpose-oriented and dependent on temporal and depth factors. The SQI showed positive correlations with crop yield during growing seasons but was less reflective of soil physical functions related to yield during fallow periods. This emphasizes the need for dynamic soil quality assessments that account for crop growth stages and seasonal variations. This study identifies key factors for soil health management, emphasizing that no-till practices improve soil aggregation but require drainage management, and that optimal soil sampling occurs during crop growth phases with increased depth as crops mature. The findings contribute to the understanding of sustainable soil management particularly in poorly drained soils, and highlight the importance of considering temporal dynamics in soil quality assessments.