Towards A Hawaiʻi Soil Health Index: Identifying Sensitive And Practical Indicators Of Change Across Land Use And Soil Diversity

Abstract

Soil is an important natural resource and has the potential to provide food security, mitigate climate change, and protect coastal and inland ecosystems from degradation if it is in, or is restored to, a healthy and resilient state. Measuring soil health allows land managers to track improvements or degradation over time and optimize their management strategies for long term benefits of the land by use of a soil health index. However, such soil testing methods and analysis necessary to monitor changes are not currently established for the unique soils of Hawaiʻi. Ten soil series samples within six soil orders were collected from 22 field sites on Oahu, Molokai, and Maui, spanning a range of soil conditions and land cover including cropland, forest, and grassland to capture high diversity. A suite of 30 potential soil health parameters measured physical, chemical, and biological soil properties. Multivariate analysis identified those parameters which could be used as indicators of soil health based on their sensitivity to changes in the soil characteristics as well as their practicality for routine soil testing. Current and previous management based on land use history showed the greatest association to the variance in soil data and created an associated potential gradient of soil health. Nine indicators from a reduction process using quantitative and qualitative criteria comprise the recommended soil health indicators for detecting differences in management practices across a spectrum of soil health and include: water holding capacity, water-stable mega-aggregates, percent total organic carbon, C:N ratio, 24 hour CO2 burst, β-glucosidase, β-glucosiminidase, hot water extractable organic carbon, and potentially mineralizable nitrogen. The proposed indicators were effective in detecting differences in management across the full landscape as well as qualitative differences in soil management within soil order, which highlighted soil taxonomy as an important inherent contributor to data variance. These most practical and sensitive indicators of soil health will be used in further field trials which will be necessary to determine the effectiveness of implementing new soil health management strategies as well as to identify the quantitative thresholds used in developing scores in a Hawaiʻi soil health index.