By Caleb P. Goossen, Ph.D., MOFGA’s Crop Specialist
It perhaps goes without saying that one of the most frustrating aspects of gardening impacts from the changing climate is our lack of control in the face of such large forces and systems at work. We must reckon with our usual preparations for previously normal seasonal changes and weather events no longer being adequate. Yet, in many ways the gardening-specific climate challenges are the same as any other for growing — only more so. You need your plants and soil to get enough water, but not too much water. Frost-sensitive crops shouldn’t go out before the last frost of the spring, and you need to at least harvest the bulk of them before the first frost of the autumn.
Adaptation vs. Mitigation
When talking about climate change, it’s helpful to make sure we’re using words in the same manner. Climate adaptation involves adjusting to observed or expected climate effects, i.e., adapting to life in a changing climate. The goal is to reduce our vulnerability. These adaptations also include making the most of any potential beneficial opportunities associated with climate change (for example, longer growing seasons or increased yields in some regions). Adaptation practices might include adding irrigation and/or improving drainage. Climate mitigation involves reducing the flow of heat-trapping greenhouse gases into the atmosphere, either by reducing sources of these gases, or enhancing the “sinks” that accumulate and store these gases (such as the oceans, forests, and soil), i.e., reducing the scale at which human activities are rapidly changing the climate. Electrifying equipment and reducing erosion are both examples of mitigation practices. Reducing single-use plastics (plastic mulch film, drip tape irrigation, etc.) is an example of a climate mitigation practice that would potentially be counter to climate adaptation, as these plastics can make soil moisture management easier to control but release fossil fuel-derived carbon in their manufacture and later disposal and decomposition. Planting more long-lived species (trees, shrubs, etc.), building soil health, and increasing soil organic matter content are all examples of practices that can serve as both adaptation to and mitigation of climate change.
Soil Health: Your Climate Adaptation Foundation
Extremes of precipitation (too much or too little) are probably the most impactful aspects of climate change on gardening. While commercial growers may have protected growing spaces like high tunnels or greenhouses, the average gardener has fewer options to shelter their growing from extreme climate events. This makes usual approaches to organic disease management (e.g., selection of disease-resistant varieties, cultural practices to promote rapid drying, and sometimes fungicides as a last resort) even more important in wet years. Hopefully, you are already exercising these first two approaches! Outside of protected growing spaces, such as greenhouses and high tunnels, most of what is in your control comes back to the soil. Without structures keeping rain off of plants and, importantly, the soil, it is important to be prepared to handle conditions of both too little and too much water. Optimizing your soil’s health will help in both situations.
A soil’s health is often defined as its capacity to support crop growth — in terms of tilth, nutrient-holding capacity, water infiltration, storage and drainage, and the success and diversity of beneficial soil life. Much of that is already determined by your soil’s texture (the proportion of different particle sizes: clay, silt, and sand), and most of us can’t reasonably change our soil texture — but we can sometimes improve our soil’s structure. Healthy, well-aggregated soils (Fig. 1) with an ideal crumblike structure interact with water in ways that maximize their plant growth potential, relative to similar soils with poor health and structure.
Principles for improving soil health over time include reducing disturbance (reducing tillage when possible), maximizing soil cover (with cover crops and/or organic mulches), maximizing presence of living roots (using cover crops, double cropping, intercropping), and maximizing biodiversity (with crop rotations, by feeding soil life different forms of organic inputs, and minimizing pesticide use).
Thinking Through Not Enough Water
Water is the vehicle for all nutrient flow into your plants. It is also needed for evaporative cooling of leaves (important for photosynthetic efficiency) and is the source of hydrogen for photosynthesis (Fig. 2). By the time a plant has begun wilting, its capacity to photosynthesize and grow has already been reduced.
Insufficient rain is often preferable to too much rain. Home gardeners typically have an advantage over commercial farmers, as they can more easily irrigate their entire growing area. However, a healthy well-aggregated soil with higher organic matter content requires irrigation less frequently because it has a much greater rate of infiltration to capture water falling on it that might otherwise runoff. It also has a greater water-holding capacity — both in the diversity of pore sizes created by soil aggregates and in the greater amount of organic matter they contain. Organic mulching materials can reduce soil temperature and evaporation rate, as well as contribute to organic matter in the soil, feeding soil microbes as it is broken down over time.
Too Much Water
A well-structured healthy soil has a greater water infiltration capacity, allowing it to absorb much more water, more quickly. While that helps to make the best use of limited rainfalls, as described above, it also helps to reduce soil erosion from heavy rain events (Fig. 3). That same effect can also improve vertical water drainage into lower soil layers, or to foster horizontal drainage into pathways between raised garden beds, which can serve as temporary drainage ditches. Minimizing the topsoil’s saturation time is particularly important, as the negative consequences of saturated soil conditions are likely to be the most hindersome climate impact we encounter.
Soil is ideally spongelike, with about half of its volume consisting of pore spaces (voids between soil particles and aggregates). As soil pores fill with water, air is forced out, creating anaerobic soil conditions. Very prolonged anaerobic conditions will impair root functioning at best and at worst kill plant roots. Starting in as little as 15 minutes of anaerobic conditions, some soil microbes begin to utilize nitrate, in lieu of oxygen, to continue their metabolic processes. This converts the nitrate to a gaseous form that is easily lost to the atmosphere. The immediate consequence of this is to reduce the amount of readily plant-available forms of nitrogen in your soil, but some of these gaseous forms are also highly potent greenhouse gases, making efforts to improve soil drainage a climate mitigation practice as well as an adaptation practice.
Even without denitrification conditions, large amounts of water moving through soils will take nutrients with it as it leaches into lower soil layers. Particularly susceptible nutrients include nitrate (due to its negative electrostatic charge) and boron (due to the lack of electrostatic charge). This makes organic matter an important reservoir of nutrients to once again be released into the soil water as organic matter is broken down and cycled by soil life.
A Note About Improving Soil Health as Mitigation Strategy and Its Limitations
Sequestering carbon as soil organic matter is a very appealing method to draw down atmospheric carbon dioxide levels. Improving soil and reducing climate risks at the same time is a win-win situation, however, there are unfortunately a lot of devils hiding in the details that I encourage you keep in mind as you consider this approach’s climate mitigation potential.
While every avenue to sequester atmospheric carbon needs to at least be investigated and hopefully pursued, I get very nervous when I encounter proclamations that certain agricultural practices can “solve” or “fix” the climate crisis. Most concerning to me is a risk for complacency regarding additional climate actions that we as a society urgently need to take to give us our best shot at minimizing climate impacts, i.e., ending the burning of fossil fuels as fast as possible.
Every soil type (specific texture and mineralogy) is typically considered to have a “carbon saturation limit” in whatever context it exists in. That context includes influences from climate (annual temperature and rainfall amounts being large factors), plant species composition and presence over time, and levels of disturbance (i.e., tillage intensity). We can change some of these factors, but only to an extent. Eventually, even in the most optimal conditions to build up more soil organic matter, we will encounter a “declining return on investment.” In a soil that is below carbon saturation, new inputs of organic matter, whether they’re being added from elsewhere, like manure or compost, or produced in place, like biomass and root exudates from cover crops, can build up the soil’s carbon content in the “mineral associated organic matter” pool, while also contributing to a more actively cycled “particulate organic matter” pool. Eventually, the amount of mineral surfaces available for organic matter to become associated with become saturated, and further carbon inputs can mostly only build up as particulate organic matter. Higher levels of particulate organic matter are great for soil health and crop production — microbes cycle this organic matter more readily, helping plants grow well but also making this pool of carbon less stable over the long term.
In thinking through soil carbon in the state of Maine, we need to first define our starting point. Many folks, intentionally or not, begin with the contemporary and look forward. While that is important for thinking about ways to draw down atmospheric carbon, I would argue that we should at least look backwards to the 17th century to establish our baseline. Though Indigenous peoples’ land management practices varied across the continent, the most drastic changes to soil carbon in what is now Maine, occurred with the influx of European settlers and the large-scale land clearing that happened since then. The majority of carbon released from the soils of Maine at that time — and since then — is still in our atmosphere. Even if we can figure out how to sequester as much carbon in Maine soil as existed pre-colonization (and more to account for the soil which has been made more or less permanently unavailable for this approach by being buried under pavement, buildings, etc.), we are still paying down the carbon debt of the “original climate sin” of large-scale land use change. Only after repaying that carbon debt can further increases in soil carbon begin to pay down the further debt of carbon released from the burning of fossil fuels. Additionally, that newly sequestered carbon needs to be carefully managed in a way that ensures it will not be re-released for a very long time, as any terrestrial carbon is at much greater risk of being remobilized into the atmosphere than the deep subterranean sequestration that keeps untapped fossil fuel carbon out of the atmosphere.
I do not intend to be discouraging here. We can sequester carbon into soil and timber products, and already have, particularly with the reforestation that has occurred in the past 100 years. Every kilogram of CO2 that is sequestered from the atmosphere today is one less to worry about tomorrow, if well managed. I just don’t want us to be lulled into complacency that carbon credits (whether in an official accounting scheme or not) will allow us to carry forward towards carbon neutrality without being paired with a much more important, concerted effort to reduce the extraction and burning of fossil fuels as rapidly as we can.
This article was originally published in the summer 2025 issue of The Maine Organic Farmer & Gardener and is part of a series on climate change and gardening, and follows a prior article describing observed and expected climate impacts affecting gardens in Maine.
