Chemical indicators tell us more about how particular reactions and processes occurring in the soil may be impacted. Soil pH for example, controls the solubility of nutrients and chemicals in soil water and thus their availability to be taken up by plants and exposure to microorganisms. Maintaining a good balance with pH is important as very acidic or alkaline soils can have variable implications for the health of the soil. More acidic soils generally favour plant growth due to more micronutrient availability [6], however in strongly acidic soils (pH <5), higher metal concentrations can cause toxicities alongside deficiencies (calcium, magnesium) [7]. Contrary to this, alkaline soils show a big reduction in macronutrient availability such as phosphorus – an important nutrient for younger plants as it aids in plant growth. Alongside this, many metals like cadmium and nickel are far less mobile and are more likely to be retained in the soil [8]. Another important chemical indicator is the occurrence of trace pollutants and other contaminants. This could be as simple as heavy metals that are naturally occurring but may have differing concentrations due to anthropogenic activity, to more complex chemicals like per- and polyfluoroalkyl substances (PFAS), and dioxins through pesticide applications or microplastics. Substances like these can influence soil processes and have toxicological effects on microorganisms—some of which are well studied, while many other emerging pollutants remain largely unexplored.

Biological indicators are often the more costly and labour-intensive aspects of soil health. There are multitudes of biota within the different layers of the soil which all play different roles in the ecosystem, these all adapting from the stressors in the environment. Because of this constant dynamic change, it is difficult to interpret what constitutes a ‘healthy level’ of organisms and how diverse it should be. As a result, properties like organic matter fractions, respiration and mineralizable nitrogen as a surrogate for biological processes mediated by organisms [9]. Mineralizable nitrogen refers to how much of the organic nitrogen is being converted (often by microbes) to mineral forms which are more available to plants. Other measures like total organic carbon and nitrogen can tell us more about the sources of organic matter in the soil as well as their productivity [10].

Te Ao Māori looks at the health of soil a little differently, not only looking towards what functions the ecosystem provides but also the interconnectedness between the environment and people. Manaaki Whenua | Landcare Research, currently employs 4 provisional Māori soil health indicators, mauri, mana, mahinga/maara kai and oranga ora [11]. Mauri relates to the essence and life force behind the soil, with the idea that healthy soil should be fit for sustaining life, health and prosperity. This means there should be well-functioning ecosystems full of biota and acknowledgment of the interconnections between people and the physical/chemical/biological components of soil [12]. Mana relates to prestige or authority, recognising management rights to resources, land and soil. This can include honouring the Treaty of Waitangi and other iwi/hapu/whanau land ownership interests. Mahinga/maara kai principles refer to the food bearing properties of the land and the ability to provide sustenance, food sovereignty and prosperity [11]. Evidence of this indicator can come from the production of healthy kai (food). The last of the provisional indicators is oranga ora, measuring the safety of food that’s grown from the soil. Healthy soil will yield healthy food, which in turn leads to healthy people; thus, oranga ora follows tikanga (practices/guidelines) to prevent harmful contaminants and waste from affecting good-quality soil.

Other principles that are important include whakapapa, wairua and Taonga Tuku Iho [13]. Whakapapa shows respect for ancestral lineage, connecting soil back to ancestors and origins. It also establishes a metaphysical place, helping us to understand the source of where soil may have come from. More spiritually, wairua provides balance between mauri and mana, ensuring there is a healthy interconnectedness between the spirit of a person and the physical state of soil [12]. Wairua allows for a balanced ecosystem as kaitiakitanga (guardianship/protection) and karakia (prayers) can be implemented. The last principle, Taonga Tuku Iho, relates to sustainability of soil as a resource. Soil must be used equitably and responsibly, ensuring that this vital resource is maintained and ideally enhanced, benefiting future generations.

Understanding soil health should require a holistic approach, integrating both the conventional scientific indicators and Te Ao Māori standpoints. Whilst physical, chemical and biological indicators may be able to easily measure functionality of the soil, Te Ao Māori emphasizes deeper and important connections that people have with the environment. Whether it is assessing nutrient availability or soil’s ability to sustain life, maintaining the integrity and health of soil is essential for sustaining ecosystems and creating resilient communities.