The concept and future prospects of soil health

Abstract

Soil health is the continued capacity of soil to function as a vital living ecosystem that sustains plants, animals and humans, and connects agricultural and soil science to policy, stakeholder needs and sustainable supply chain management. Historically, soil assessments focused on crop production, but today soil health also includes the role of soil in water quality, climate change and human health. However, quantifying soil health is still dominated by chemical indicators, despite growing appreciation of the importance of soil biodiversity, due to limited functional knowledge and lack of effective methods. In this Perspective, the definition and history of soil health are described and compared to other soil concepts. We outline ecosystem services provided by soils, the indicators used to measure soil functionality, and their integration into informative soil health indices. Scientists should embrace soil health as an overarching principle that contributes to sustainability goals, rather than only a property to measure.

Toc blurb: Soil health is essential to crop production, but is also key to many ecosystem services. In this Perspective, the definition, impact and quantification of soil health are examined, and the needs in soil health research are outlined.

Figure 1. Soil fertility, quality, health, and security.

The concepts vary by what relevant spatial scales, functions, ecosystem services, and stakeholders they capture (listed as nested concepts on the right of the figure). The concepts also differ in the view of soil rights and assessments. Soil health encompasses a broad range of ecosystem functions, services and actors, impacting a wide array of sustainability goals. The five functions listed here impact overall soil ecosystem services^,,.

Figure 2. Soil health and global ecosystem services.

Soil health affects human and planetary health through crop production, quality, storage and transportation; food quality and taste; soil contamination, or through climate change, recreation, and culture. Immediacy of soil health effects on plants and soil biota facilitates assessment of causality (for example, soil nutrient availability affects crop production). Cascading effects (such as soil nutrient availability affecting human health indirectly through crop quality and food storage) require causalities to be demonstrated for which in some cases science still needs to be established.

Figure 3. Soil ecosystem services management.

Four important roles of soil (plant production, water quality, human health, and climate mitigation) are listed at the top of the figure. Various management strategies, and their impacts on key soil properties and ecosystem services, are listed below.

Figure 4. Soil health indicators and relevance to assessments.

Soil health indicators ideally are informative, sensitive, effective and relevant^,. Some do not fulfill all criteria but are still relevant (such as TEXTURE or SOIL DEPTH that do not change readily and are not managed, therefore also called capability indicators). Bold black text denotes indicators that expand the utility of soil health quantification beyond crop production towards sustainability and planetary health; the white arrow outline encompasses indicators that should be further developed to be effective and practical. Note, diversity includes biota in soil, diversity of soil types in landscape, molecular/structural in soil organic matter and plants growing in soil, some of which may not be readily quantified through analytical or modeling approaches. C, carbon; CEC, cation exchange capacity; EC, electric conductivity; GHG, greenhouse gases; N, nitrogen; TOC, total organic carbon

Figure 5. Biological, chemical and physical indicators included in soil health assessment schemes

a| Number of indicators and proportion of each type (biological, chemical or physical). Size of the circle represents the number of indicators in the assessment scheme. b| Year of each soil health assessment scheme. Only the last two digits of the year are shown (values in the 80’s and 90’s are from the 1980s and 1990s, values from 00 to 20 represent years 2000 onwards). Currently proposed soil health indices utilize mostly chemical and physical indicators. The proportion of biological indicators is typically lower than either chemical or physical indicators, which did not change over time as the methods were published, likely reflecting the historic focus of soil health indices on crop growth. The number of indicators in the proposed schemes does not relate to the proportion of biological indicators. A comprehensive soil health index may consider a balanced set of indicators that represent at least 20% biological, physical and chemical measurements (dashed red triangle). However, indices designed to quantify different services may require a different set of indicators: a soil health index for crops may require more chemical indicators (inside the yellow ‘Plant’ triangle), for water more physical (blue triangle), for biodiversity more biological (green triangle), and for climate more physical and biological indicators (orange triangle).