Comparative Assessment of Soil Health in Organic vs Conventional Farms

Thesis Abstract

Soil health is increasingly recognized as a central determinant of sustainable agricultural productivity, biodiversity support, and ecosystem services, yet empirical comparisons between organic and conventional farming systems remain contested due to disparate methodologies and regional contexts. This study addresses the gap by comparing soil health indicators across organic and conventional farms within a representative agricultural landscape, with the aim of elucidating how farming practices influence soil quality and resilience over time. The specific objectives are to (i) quantify physical, chemical, and biological soil health indicators across farming systems; (ii) evaluate temporal trends in soil heritage indicators over a five-year period; (iii) identify key drivers of soil health differentials using multivariate analysis; and (iv) assess the potential trade-offs between yield performance and soil health under organic versus conventional management. The research employs a comparative cross-sectional design complemented by a longitudinal observation component. The population comprises commercial arable farms within the Midwestern region, stratified into organic (n=40) and conventional (n=40) management units. A multi-stage sampling approach selects representative fields (sub-sampling within farms to account for spatial heterogeneity) resulting in a total of 160 soil cores per system per sampling event. Data collection combines standard soil physical measurements (bulk density, porosity, aggregate stability, infiltration rate) with chemical assays (organic carbon, total nitrogen, CN ratio, available phosphorus and potassium, pH, electrical conductivity, cation exchange capacity) and biological assessments (microbial biomass carbon, basal respiration, enzyme activities such as dehydrogenase and phosphatase, and soil microbial community structure via 16S rRNA gene sequencing). Complementary agronomic data include crop yield, input expenditures, and pest and disease incidence. Instruments were calibrated following ISO/IEC 17025 guidelines, and data collectors were blinded to farming system classification to reduce measurement bias. Statistical analyses proceed in a hierarchical framework. Descriptive statistics summarize central tendencies and dispersion; mixed-effects ANOVA tests detect system effects while accounting for repeated measurements and nested field structure. Multivariate techniques, including principal component analysis (PCA) and redundancy analysis (RDA), identify principal soil health axes and their associations with management practices. Regression models examine the relationships between soil health indicators and yield, while controlling for soil type, climate variables, and crop rotation length. The analysis explicitly tests hypotheses derived from provisioning- and sustainability-based theories, including the Soil Health Hypothesis and Ecosystem Services Theory, complemented by a functional diversity framework to interpret microbial and enzymatic activity patterns. Expected findings anticipate that organic farms exhibit higher soil organic carbon stocks, greater aggregate stability, improved soil structure, and enhanced microbial biomass and enzyme activity, reflecting longer-term soil building processes under organic amendments and diverse rotations. Conventional systems may show higher short-term nutrient availability and faster mineralization in certain contexts, but with comparatively lower microbial diversity and potential compaction impacts. Temporal analysis is expected to reveal converging soil health indicators in years of perennial cover crops within conventional systems, while organic systems demonstrate sustained advantages in soil biological indicators. The study will also identify context-specific modifiers, such as soil texture and climate, that mediate the magnitude of health differentials. The study contributes to knowledge by providing a rigorous, field-based, side-by-side assessment of soil health under organic and conventional management, integrating physical, chemical, and biological metrics with yield outcomes to illuminate trade-offs and synergies. It advances methodological standards for cross-system soil health comparisons, including an integrated indicator framework and a robust mixed-model analytical approach. The discussion will inform policy and advisory services on soil management, highlighting scenarios where organic practices yield superior long-term soil health and resilience without compromising productivity. The concluding recommendations advocate for targeted integration of organic amendments, cover cropping, and judicious input management to optimize soil health across farming systems, while identifying areas for future longitudinal and regionally diverse research to generalize findings.