While different authors have suggested many different ways to systematize soil health measures, they all share a common framework. Soil health can be parsed into three primary components : physical characteristics, chemical characteristics, and biological characteristics. Within each of these three components – physical, chemical, and biological – exist many possible soil health characteristics and indicators.
The three major components of soil health: physical, chemical, and biological. For each, we identify what we argue are the most economically meaningful characteristics in the component. By economically meaningful, we mean the characteristics that have the largest and and most easily identifiable impact on a soil’s ability to support life, maximize agricultural yields, display environmental resiliency, and further ecosystem sustainability.
Many of a soil’s physical characteristics are geological in nature and are not alterable by soil management techniques. For instance, a soil’s composition of clay, silt, and sand is the result of millions of years of geological history and not changeable by a farmer. However, there are some physical soil characteristics that can be a affected by soil management. The three most economically meaningful of these characteristics are aggregate stability, soil compaction, and available water capacity.
Aggregate stability is a measure of a soil’s ability to maintain its structure when exposed to different stresses. Soil structure is critical to support plant root systems, provide for air and water movement through the soil, prevent erosion, and provide a stable environment for soil microbiota. A soil that maintains its structure when stressed, specifically by water, is well-suited to supporting robust ecology and agriculture.
Bulk density is a measure of how closely soil particles are packed together, and is measured by dividing a soil sample’s mass by its volume. When soils become more dense (undergo compaction), plants’ roots have a more difficult time growing through the soil. Low bulk densities are thus desirable for farmers. A common cause of soil compaction is tractor use where heavy machinery puts pressure on the soil.
Available water capacity is a measure of how much water a soil can hold between its soil particles. This is particularly important in a soil’s root zone where water availability is critical to plant growth. Not surprisingly, available water capacity and bulk density are inversely related: as a soil becomes ore compacted, its capacity to hold water decreases.
In addition to physical properties, soils have distinct chemical makeups that can change over time. Plant cover, irrigation, and soil additives such as organic or inorganic fertilizer play a large roll in how soil chemistry evolves over time, and this chemistry can have large effects on plant growth and crop productivity. The most significant chemical characteristics of soil include pH (a measure of acidity), phosphorous levels, potassium levels, presence of micronutrients, and soil salinity. (Nitrogen, while a chemical compound, is discussed as a biological characteristic due to the highly biologic nature of the nitrogen cycle.)
A soil’s pH – a measure of acidity – gives information about how available other nutrients are to plants the grow in that soil. Most soils are most productive in neutral to slightly-acidic conditions (pH 6-7). In more acidic soils, there is a low availability of calcium, magnesium, and phosphorus. In more alkaline soils, iron, manganese, copper, zinc, phosphorus, and boron are relatively unavailable. Soil pH can be changed by applying different substances to the soil. For instance, nitrogen fertilizer often acidifies soils, while lime makes soils more basic.
After nitrogen, phosphorus (P) is the most important nutrient limiting crop growth. Phosphorus must be in a particular chemical form to be accessible by a plant, and operates on a long natural chemical cycle. In practice, phosphorus can be added to soils as an inorganic fertilizer. In high-productivity agricultural systems, external sources of phosphorus are often necessary to maintain agricultural yields.
Potassium (K) is another critical macronutrient for agricultural crops. As with phosphorous, potassium is often applied as fertilizer in large agricultural operations. When soils are low in potassium, crops’ leaves can turn yellow at their edges and growth is significantly curtailed. Potash is the most common source of potassium fertilizer.
After nitrogen, phosphorus, and potassium (the “big three” macronutrients N, P, and K), many other chemical micronutrients are important for soil health. Although micronutrients such as boron, copper, iron, chloride, manganese, molybdenum, and zinc are needed in much smaller quantities than nitrogen, phosphorus, and potassium, a deficiency of these nutrients can have as significant an impact on plant growth. Micronutrient deficiencies are highly soil-and crop-specific, meaning that farmers need to be particularly aware of the risks for their particular crops. For instance, many corn farmers know to look for the signs of boron deficiencies in their soils.
Salinity, or the amount of salt in a soil, is another important chemical characteristic of soil health. As a soil becomes more saline, it becomes significantly less productive and can even become infertile in extreme cases. Most soil salinity can be attributed to salt deposited by water. Efficient irrigation methods, such as drip irrigation, can increase soil salinity while flood irrigation can “flush” salt out of the soil. Thus, there is a tension between efficient water use and soil salinity.
Far from being an inert or passive substance, healthy soil is home to a wide variety of life. The biologic characteristics of soil health are some of the most complex and relatively least understood of all. Among the most important biological characteristics are the amount of organic matter, the amount of active carbon, the mineralizable nitrogen, and the microbial ecosystem found in the soil. Of these, nitrogen is the most well understood, and arguably the most important component of soil productivity in agricultural settings.
Organic matter is all material in a soil that is or was at one time alive. This includes plant residues, microbial life, active soil organic matter called detritus, and stabilized soil organic matter called humus. Organic matter provides an environment and sustenance for microbes, while also increasing the soil’s resistance to compaction and erosion. Organic matter can be built up over time by adopting no- or low-till soil management practices, planting cover crops, or using compost. The importance of organic matter to agricultural productivity varies greatly with soil type.
Active carbon is a subset of the organic matter. In particular, active carbon is easily available to microbes and plants in the soil and reacts much more quickly to land management strategies than inactive carbon (e.g. humus). Active soil carbon is beneficial to soil health and simultaneously keeps that carbon out of the atmosphere, thereby presenting a possible mitigation strategy to carbon emissions.
Mineralizable nitrogen is the nitrogen in soil that can be used by plants. Since plants cannot access atmospheric nitrogen, nitrates are critical nutrients for plant growth, and mineralizable nitrogen is often the most important nutrient limiting plant growth. Nitrogen can be considered a biological characteristic of soil because one of the primary natural ways for soils to build mineralizable nitrogen is through nitrogen-fixing bacterial processes. In practice today, however, many farmers use inorganic nitrogen fertilizers to augment soil nitrogen. Another approach is to rotate crops between nitrogen-leaching and nitrogen-fixing species (e.g. corn/soybeans). Nitrogen is a critical contributor to the agricultural productivity of a particular soil, and its dual nature of being both chemical and biological characteristic of soil health makes it particularly notable.
The microbial ecosystem within a soil is one of the least understood characteristics of soil health. Nonetheless, a diverse and thriving microbial ecosystem is associated with increased plant growth, increased agricultural production, and higher resiliency to pests and disease. When other components of soil health are performing well, especially pH, salinity, bulk density, and active carbon, soil microbes are more likely to thrive.
Unlike many production processes, agriculture is particularly dynamic in that future production is highly dependent on past production. Much of this temporal dependency comes from soil health dynamics where future soil health is dependent on past soil health. The nitrogen cycle and carbon cycle are just two examples of complex dynamic systems affecting soils. In addition to natural system dynamics, agricultural practices are also dynamic. Crop rotations, tillage practices, and irrigation all are long-term processes that cannot be reduced to a single-year decision. In this section, I discuss some of the most prominent dynamic considerations affecting soil health.
The two biogeochemical cycles that are particularly relevant for agriculture and soil health: the nitrogen cycle and the carbon cycle. And also matters pest and weed cycles.
As discussed earlier, nitrogen is a particularly important chemical nutrient for many agricultural crops. While atmospheric nitrogen (N2) is abundant, it is not accessible to plants. In order to become a useful nutrient for plant life, atmospheric nitrogen must be “fixed.” The process by which atmospheric nitrogen is fixed into ammonia, then to nitrates, and back to atmospheric nitrogen is called the nitrogen cycle.
Most atmospheric nitrogen is converted to ammonia (N H3) by either lightning strikes or nitrogen-fixing bacteria. These bacteria are found primarily in root nodules of legumes. Different bacteria in soil then convert ammonia to nitrates (N O2− and N O3−) which are consumed by plants. Crops such as corn are particularly nitrogen-sensitive and respond well to plentiful levels of nitrate in the soil. However, once nitrates are leeched from soils by crops, they must be replaced either through natural processes or the application of additional fertilizer. Since natural nitrogen-fixing is performed by bacteria, it follows that soils with healthy biologic ecosystems will be more effective at self-producing nitrogen.
The nitrogen cycle is a primary driver of several agricultural behaviours including fertilizer application and crop rotations. The more a farmer relies on natural sources of nitrogen (optimized crop rotations, etc.), the fewer additional inputs are necessary to maintain yields without taxing long-term soil health.
The carbon cycle is a global-scale process by which carbon moves between soils, oceans, biomass, and the atmosphere. Much of the earth’s carbon exists in “sinks” such as soil and the deep ocean, but incremental increases in atmospheric carbon are having significant impacts on planetary heat retention.
All plant matter acts as a carbon stock while it is alive, and this includes agricultural crops. However, when plant matter is burned, consumed, or otherwise decomposes, much of its carbon is released to the atmosphere. Over long periods of time, soils with high levels of organic matter can successfully incorporate that carbon into the long-term soil carbon stock, but this process takes decades or centuries rather than months or years.
An important distinction must be highlighted: land use practices that capture carbon in the short-term (growing forests, cover crops, etc.) will only have long-term soil capture impacts if such practices continue or are improved over time. Growing a forest for 20 years and then cutting it down to grow crops will only effectively capture carbon for the 20 years the forecast stands. This is not to say there are not benefits to the 20 years of storage; the impacts of atmospheric carbon likely increase nonlinearly with the stock of atmospheric carbon today. It may be optimal to adopt short-term carbon sequestration policies even if long-term carbon sequestration is not achieved. However, determining the “optimal” strategy depends on myriad additional factors.
While the long-term net effect of soil health on total soil carbon stocks is uncertain, we do know that soil practices that support soil carbon, organic matter, and overall soil health will at least weakly increase long-term soil sequestration over practices that fail to build soil organic matter. The unanswered question is whether the societal net-present-benefits of carbon-sequestering practices outweigh the societal net-present-costs.
In modern agricultural systems, especially monoculture systems, pest and weed populations are important dynamic processes. Pests and weeds both require favourable conditions to thrive. Monoculture systems are particularly susceptible to pests and weeds since these systems provide large homogeneous environments with scant support for natural predators in the case of pests. Historically (over the past century or so, at least), the traditional way for farmers to prevent weeds was to till their fields before planting. While this is an effective strategy against weeds, it exacerbates erosion concerns. Cover crops are seen as one possible management strategy to address weed concerns without engaging in the same amount of tillage.
By multi-cropping, farmers limit the opportunities for pests or weeds to gain a foothold in an agricultural field. Similarly, by physically rotating the placements of different crops each year, farmers can prevent a second-generation pest population from emerging in a field with that pest’s preferred environment. These sorts of agricultural management behaviours are a component of “integrated pest management” (IPM) and recognize the important dynamics of both pests and weeds.
The concept of soil health, sometimes called soil quality, is a broad and multifaceted way of summarizing a soil’s productivity, resilience, and sustainability over both short- and long-term time horizons. Over the last several decades, soil health has been successfully incorporated into several fields of study, including soil science, agronomy, plant biology, and ecology. However, since soil health is often defined in different ways by different people, there is no standardized approach to quantifying this concept. As a result, there has been relatively little work done on the economics or public policy of soil health. This report aims to investigate the existing work on the economics of soil health, identify opportunities for future research, and highlight implications for public policy around soil health.