To serve as a broad introduction to the course, today’s readings and activities aim to orient us towards seeing soil as a material always in relation to its user and thus shaped by how it is cared for or neglected with that use in mind.

Soil is a substrate viewed and evaluated in different ways depending on who is using it for what: it might be a source of medicine, a media for growing crops, a place to store carbon, a filtration system for water, or where to obtain clay for bricks, adobe or pottery, among many others. Soil is weathered from rock, influenced by plants, soil biota and climate over time. Its properties depend on the interplay of all these factors in a way that it is difficult to say whether the soil determines the plant or microbial community or vice versa. Rock-derived minerals such as phyllosilicates (smectite, vermiculite, etc.) or quartz are the inorganic building blocks of soil that are separated by size classes (texture) into rocks (>2mm), sand (0.02-2mm), silt (0.002-0.02mm) and clay (<0.002mm), organic matter is mostly composed of decomposition products of plant and animal residues remaining from the activity of soil biota, and the resulting soil structure distinguishes soil from other substrates. Soil structure is the formation of aggregates as organic matter, especially from microbial products, which glues together clay-, silt- and sand-sized minerals. Most plant nutrients are released by weathering rock, except for nitrogen that evolutionarily is a result of microbial fixation of atmospheric N₂ gas.

Historically, organic matter in soil has been called humus. Humic substances were understood to be created by microorganisms through humification, a process where recalcitrant material such as dead leaves and roots break down and become part of the soil. The level of organic matter is often seen as a barometer for soil health and the “fuel” feeding soil’s fertility. Organic matter tends to make up about 2-10% of soil but it determines its ability to retain moisture, sequester carbon, decompose pollutants and retain nutrients. A quick look online reveals it is easy to buy a bag of “humus” to improve your garden’s soil. However, when scientists use the term humic substances (HSs) they are referring to something more specific: a particular transformed substance that for centuries now has been theorized as fundamentally different from what it was derived from. More recently, soil scientists have moved away from such a model and towards understanding soil organic matter through a soil continuum model. In this model, plant residues are decomposed and the microbial residues of this decomposition persist in soil longer than predicted from their molecular properties, because they exist within and even interact with a complex soil architecture. This also means that plant materials that decompose more rapidly generate more persistent organic matter if they generate more microbial matter.

Metabolomics, or the science of what substances microorganisms produce, can tell us more about whether greater diversity of metabolites in a given soil lead to persistence and whether decomposition generates a more diverse molecular composition. In this feedback loop, fast decomposition of organic matter creates more diverse metabolites and which then in turn slows decomposition. For example, if a microorganism requires a different enzyme to eat each different organic compound around it, then it has to make a choice whether it is worth making that costly enzyme to eat that one compound. Imagine you have to have a different spoon for each food item on your plate, and making those spoons cost you more energy than the food contains that you can eat with it, what would you do? More organic carbon therefore accrues not by accumulation of what cannot be decomposed because it is inedible, but by accumulation of what is produced as a result of decomposition. These materials are eminently edible, but are not eaten for a variety of reasons, including spatial heterogeneity (because the microbe cannot invest the energy to swim around the corner in the hope to find some food), temporal variability (because getting used to cold or warm, wet or dry conditions takes time, and if conditions change very quickly and every day, microbes that could eat organic carbon cannot adapt) and molecular diversity (as explained above) that also affect how molecules interact with mineral surfaces (e.g., organic matter adsorbed to clay minerals are more difficult to eat by microorganisms) and whether they are located in aggregates (e.g., organic matter within aggregates are not reachable by microorganisms), both of which that makes them less accessible to microorganisms that would eat these molecules to produce carbon dioxide.

The turn towards a metabolic understanding of soil compositions also opens up possibilities for thinking about the metabolic flows shaping soil in relation to human history, in particular due to technological changes and industrial processes. Alterations in the material world following the emergence of synthetic chemistry and its manufacture of new chemical compounds, as well as the adaptive re-use of industrial by-products (i.e “waste”) are but a few ways these anthropogenic histories show up in the properties and make-up of soil. As knowledge systems change, so do the assessment of the dangers and harm these histories entail, generating new regulations that unequally affect communities and users of soil.

The problem of industrial metabolites and synthetic compounds in soil as an historical remainder and reminder of globalized industry and science leads to different kinds of practices of problem-solving depending on the sites of the soil. In some cases, the proposed solutions may involve remediation practices, removal and containment strategies, manufactured ignorance, or a combination of approaches. These histories and approaches generate what (following Tsing et. al) we might call “patches” in the landscape. They also shape the kinds of designed interventions and regulatory frameworks put in place for the future. Particular regulatory frameworks that differ by nation or state may delimit what sorts of uses are now safe, or designate particular clean-up strategies that may involve new approaches to landscape’s substrate that harness the metabolic properties of certain plants to help “clean” that soil.