The flow of energy through the soil determines whether soils will build or degrade.
By Allen Philo, Ken Wacha and Jerry Hatfield
Why do soils degrade? How do degraded soils regain structure and function? Can such changes be predicted? These are some seemingly basic questions regarding soils, and particularly agricultural soils. The answers to these questions can have direct impacts on how we steward our land and grow food. Quantifying and tracking the movement of energy into and out of soils is a new concept that can help answer these questions.
In agricultural systems, soil has four primary functions: to supply water, to supply nutrients, to allow for adequate gas exchange, and to provide physical support for the plant. Soil aggregation/degradation processes are highly dynamic, and the driving mechanisms are not well understood. This uncertainty is due in part to the approach that soil science has taken in addressing these questions.
Until recently, scientists have used mechanistic models to describe physical changes in soil structure, pointing primarily to organic matter content, clay mineralogy or base exchange ratios as the driving factors. It wasn’t until science started treating soils as living systems — and recognized the importance of microbiology and its role in cycling nutrients, in microbial-plant interactions and in the highly interconnected soil food web — that we began to unravel the dynamics of how soil physical, chemical and biological processes interacted.
Even in recognizing soils as a living system, the primary role of microbiology in creating and maintaining soil structure has not been fully recognized or quantified. Surprisingly, even though science has begun treating soils as a living system, we have not considered what all living systems need to function: energy. It is the flow of energy through the soil, and the positive or negative energy balances in the soil, that determine whether soils will build or degrade.
Energy is the basis of any living system. In simple terms, if less energy is flowing into a system than is being used by the system for maintenance and reproduction, that system will go into decline and will eventually cease functioning. If we think of an animal as an example, this net energy deficit would first result in cutting off non-necessary functions, like reproduction, and eventually entering ketosis, where the animal begins to burn fat and its body tissues to survive. If this negative energy balance continues, the animal will begin to starve and will eventually die. If the energetic system is net positive, its results are the opposite: higher states of health, successful reproduction and increases in whatever effects that animal has on the local ecology.
Similarly, in soils with a positive energy balance, aerobic microbes will use energy to create an environment that meets their needs by aggregating the soil to allow for optimal water infiltration, drainage and gas exchange. When such conditions persist, microbes will continue to multiply and die off generation by generation, leaving their bodies to stack up and add to the measurable stable organic matter pools in the form of dead microbes or necromass. This further increases the water storage capacity and overall resilience of the soil system.
When a negative energy balance exists, microbes lack the energy required to build and maintain soil structure. They are unable to produce and excrete the various glues and polysaccharides needed to modify soil structure and improve soil aggregation. If microbes lack the energy, then soils will begin to lose their structure. If energy input is even further in deficit, then microbes will use these carbohydrates (glues) as energy sources, causing an increased rate of degradation of soil structure. These degradation processes can form negative feedback loops, which can result in soil collapse. A soil in collapse is much less productive in terms of agricultural functions and is susceptible to water and wind erosion.
Where Does Energy in the Soil System Come From?
Almost all forms of embodied energy in soils can be traced back to solar energy that was captured by plants using the photosynthetic process. Let’s follow a photon of solar energy that hits the Earth.
As solar energy hits the plant leaves, a large portion is reflected away from the plant. Some solar radiation simply heats the plants and creates the conditions for transpiration to occur. Only around 10 percent of the energy hitting the plant is absorbed for photosynthesis. As we know, the solar radiation is converted into chemical energy in the form of sugars. The plant uses some of the sugars to grow, and the rest is transported or stored in the plant. About half (50 percent) of the energy is transported down to the root system of the plant; 50 percent of that is used by the root system. Of that portion that is transported to the root system, 50 percent is exuded into the soil in the form of root exudates, which are different forms of sugar. This means that about 25 percent of all the solar energy that is being converted to chemical energy by plants is being exuded out of the plant and into the soil.
In an agricultural field, half of all energy that enters the soil system during a year comes from root exudates. The other half of the energy that enters the system comes in the form of plant litter — mainly plant roots. Plant litter on the soil surface provides a valuable function for the microbial community in moderating the soil temperatures to an optimal range for biological activity and maintaining a moist environment to supply water for microbial activity.
There is a major difference, however, in the fate of the energy that enters from surface litter and what enters in the form of roots and root exudates. Some of the plant litter on the soil surface that could potentially enter the soil system simply sits on the surface of the soil and oxidizes away, meaning the energy is lost to the soil system. What does enter the system is largely restricted to the top six inches — the active layer of the soil — where it is mixed into the soil either through tillage or via the action of earthworms and other soil fauna.
The active layer is also a zone of very high biological activity. While the energy that enters this active layer is essential for the continued functioning of the soil, it is less likely to end up as stabilized carbon, or stabilized organic matter, than is energy that is deposited deeper in the soils through root exudates or root biomass. This means that the energy for the maintenance and deepening of the aerobic zone in soils below the active layer is coming from plants pumping the energy into the soil through their roots. This points to the importance of having deeply rooted crops as part of agricultural systems — to maintain functional soils to depth and to build organic matter levels below six inches.
Measuring Energy Flow
Our recent scientific article published about soil energetics was the first to ever broach this subject. It is already being asked how farmers can measure and use soil energetics on their farms. This is a developing science, but there are some interesting paths forward to creating tools using this concept.
Remote sensing systems can measure incident solar radiation down to 10 square meter intervals. Other systems can sense CO2 flux over farm fields. Using such tools in the next few years, it may become possible to accurately measure the amount of energy both going into, and leaving, soils.
One major question that needs greater research is how different plants apportion their energy. Also, it may be possible that certain field conditions or associations with different microbial complexes may increase or decrease the amount of energy exuded into the soil by plants for direct use by microbial populations. Until then, it is possible for some of the major crops to do energy “budgets,” quantifying positive and negative impacts on energy flow in soils and making general predictions about how this will impact the functionality and health of the soils in question.
Energy Losses in Soils
Factors that affect the energy balance of soils in a negative way include raindrop and tillage-induced erosion processes. Rain has a large amount of inherent energy in the falling drops, with larger raindrops falling faster and striking the ground with higher force than smaller ones. This kinetic energy can break apart aggregates. The microbes in the soil must expend energy to reconstruct the aggregates that were broken apart. Further, when soil aggregates dissolve back into disjointed sand, silt and clay particles, these particles can then move down in the soil column, clogging the pores that allow for proper water infiltration and gas exchange. In the short term, this pore clogging can lead to increased soil erosion, as water that can’t drain or infiltrate will pool and begin to move along the soil surface. In the longer term, clogged pore structure can cause soil-system collapse as deeper soil levels lose access to water and oxygen.
Tillage can have similar effects on the soil if not done with care. It also temporarily increases the amount of oxygen that is in the soil and exposes stored energy in the soil to microbial activity as it breaks aggregates apart and inverts soils. But some kinds of tillage can have positive impacts when used to break through compaction layers, resulting in increased air and water flow to parts of the soil that had been cut off from them. Where air and water go, microbial processes follow. In some cases, it can be useful to incorporate residues into the soil so that embodied energy is captured. This can be a delicate balance between what energy is being added to the soil via residues and what must be used to repair the damage being done to the soil in the tillage process. This is a new frontier of soil investigation; what researchers discover will help determine best agricultural practices.
From a practical perspective, the exercise of quantifying energy flows in soils has emphasized the importance of having a living cover on soils as much as possible. It is the connection between the living plants and microbiology in the soil that allows soils to maintain their structure, as opposed to the physical and chemical properties of organic matter, or even the energy that is coming from organic matter. In fact, there is evidence to suggest that the best predictor of whether organic matter or carbon will increase in the soil is whether the energy balance is positive or negative. Having a living cover over as much of the growing season as possible, coupled with minimal tillage disturbance, is the most effective way of creating a positive energy balance.
In a paper that analyzed the energy balances of three different plant-growth regimes —conventionally tilled corn and soybean, no-till corn and soybean, and grass pasture — only the grass pasture had a markedly positive energy balance. This was from a combination of both a lack of negative pressure, such as tillage and exposure to rainfall, and from the fact that the pasture was able to capture more energy during the year than either of the corn and soybean systems. In corn and soybean systems (whether till or no-till), over half of the usable energy that would enter the system (sunlight) never encounters a growing plant that can convert this sunlight into sugar. The addition of a cover crop into the corn and soybean systems expands the amount of time during the year the sunlight can be captured and converted to sugars. This increase in captured energy can allow conventional and no-till systems to become marginally energy positive. Many farmers who have incorporated these practices can attest to the increase in the four primary functions of soils, along with an increase in resilience to variations of weather during the growing season.
This newer concept of soil energetics invites us to think about breeding better cover crops. Over the past century, much effort has been expended in the breeding of our main agronomic crops to create higher yields. From an energy standpoint, what we have been breeding plants to do is to take more of the solar energy they capture and to put it into the harvestable portions of the plants. In most cases, as with corn, this means into the aboveground portions of the plants. As a consequence, we have bred corn plants to have much smaller root systems.
Modern corn has also been bred to minimize the amount of root exudates it produces in order to maximize yield. This has been made possible by our ability to place readily available nutrients close to the root system, which allows the plant to survive and thrive with a minimalist root system and minimal production of root exudates. While this has obvious benefits, like increase in yield, the downside is that the soil is not receiving much energy either from root deposition or from root exudates. However, if it is possible to breed plants to invest less in their root systems, it is most likely possible to breed them to invest more into root systems. Cover crops bred to maximize the amount of energy flowing into the soil could offset the deficit created when growing modern agronomic varieties.
Perhaps the most promising aspect of the idea of soil energetics is its predictive power. Quantifying energy flows through the soil system could allow us to predict changes in soil functionality and carbon sequestration. Moreover, it could allow for the development of reliable recommendations regarding which practices would be best for reversing soil degradation and achieving rapid regeneration of soil functional properties. Soil energetics provides a framework for answering questions about the degradation and regeneration of soils, and over time it can hopefully be developed to accurately predict when either will occur.
Allen Philo owns and operates a 200-acre farm in Dodgeville, Wisconsin, and is the head of product development for BioStar Renewables.
Dr. Ken Wacha is a research hydrologist at the USDA-ARS National Laboratory for Agriculture and the Environment in Ames, Iowa. His research focuses on quantifying and managing interactions between climate, soils, and land-use and their impact on surface hydrology and water quality at field, landscape and watershed scales.
Dr. Jerry L. Hatfield is a former director of the USDA-ARS National Laboratory for Agriculture and the Environment in Ames, Iowa. His research focuses on the evaluation of farming systems and their response to carbon, water and nitrogen interactions across soils.