Designing a Soil Health Index for Sustainable Agricultural Systems.

Khandoker, Munisath and Ostle, Nick and Haefele, Stephan and Gregory, Andrew (2024) Designing a Soil Health Index for Sustainable Agricultural Systems. PhD thesis, Lancaster University.

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Abstract

Soil health is vital for agriculture and agro-ecosystems. Healthy soils act as a reservoir and cycling system for water, air, organic matter, and nutrients essential for crop growth and as a habitat for a diverse array of organisms, including bacteria, fungi, insects, and worms, contributing towards ecosystem stability and resilience. However, soil health cannot be directly measured effectively with one indicator. Instead, soil health assessments typically rely on a range of measurements of essential biological, physical, and chemical indicators. Due to the complexity and highly integrative nature of soils, it is difficult to develop general soil health indices. The main goal of this thesis was to develop a soil health index that was able to quantify soil health for different agricultural land uses and soil textures across the UK. With the aid of Rothamsted Research’s long-term experiments of known history and land management, we aimed to collect measurements of crucial physical, biological, and chemical soil health indicators using traditional methods. Then, using structural equation modelling (SEM), we hoped to create a robust soil health assessment. When designing a soil health index, selecting the most informative indicators of soil health is essential. To achieve this, we initially focused on enhancing existing soil monitoring methodologies and exploring novel technologies that can streamline this process. In Chapter 2, we evaluated the SLAKES smartphone application as a tool for measuring soil stability. Results from the SLAKES application were compared with the established Le Bissonnais method. Results showed SLAKES could differentiate between different management types on clayey soil, but were less sensitive when tested with sandy soil. Despite this lower sensitivity, we conclude that the SLAKES app can be a legitimate method to measure aggregate stability, providing a faster and easier method for researchers and land managers compared to conventional methods. In Chapter 3, we evaluated the use of extracellular enzymes, N-acetyl-β-glucosaminidase (NAG), acid phosphatase (PHO) and β-glucosidase (GLU), as promising soil health indicators. The objective was to investigate which of these soil enzymes, if any, could be used as a comprehensive biological indicator for soil health by examining the relationships between microbial enzyme activity in a range of soils with contrasting chemical and physical properties. We observed that grass treatments relative to all other plots showed increased levels of enzymatic activity, followed by arable and fallow, respectively. Furthermore, enzyme activity correlated with other observed soil health indicators. Using learnings from the previous two chapters, the Chapter 4 study aimed to create meaningful metrics for soil health that influence agricultural production and other ecosystem services. We included a range of soil measurements relevant to soil health, including physical, chemical, and biological soil indicators under contrasting agricultural land uses and soil types. We found that SEM allows for a comprehensive understanding of the complex relationships within the soil health system. It was particularly beneficial when dealing with multiple indicators and latent variables that contribute to overall soil health. Based on our results, we believe soil scientists can leverage SEM to refine soil health assessment models and improve the accuracy of their measurements, as well as understand the effects of agricultural management practices on soil health. From selecting the most informative soil health indicators to carefully considering the measurement methods used for these indicators, this study demonstrates the multifaceted nature of designing a soil health index.