There is no simple definition of the “right way to farm” because any answer would have to reflect the constraints of each growing region (soil type, weather, infrastructure…) and the resources and markets available to the farmers. To make things even more complicated there are many possible combinations of specific farming practices to consider including different tillage systems, crop rotations, cover cropping or double cropping, livestock integration, fallowing, controlled wheel trafficking, fertilization strategy, and more. For any given combination of practices, assessing whether it constitutes a “good way to farm” still requires consideration of the following five questions:
1. Does it work for farmers, practically and economically?
2. Does it make responsible use of limited resources such as land, water, energy…?
3. Does it minimize environmental impact?
4. Does it optimize soil health in a way that increases resilience in the face of climate change?
5. Can it help mitigate climate change thorough agriculture’s special potential to sequester carbon in soil?
There are measurable “outcomes” that can help to answer most of these questions. These include things like crop yields, energy and water use, fertilizer and crop protection, inputs, environmental monitoring and other “above ground” data. But the last two questions about soil are more difficult to address because it is very difficult to fully characterize what is going on underground. For many plants there can easily be 1.5 times as much “dry matter” below the ground as above. The roots and all their associated organisms comprise a complex system that changes over time and across three dimensions with considerable variability. Ideally we can look to agriculture to both provide the crops we need and simultaneously sequester carbon in the soil to help mitigate climate change, but to optimize those two goals it is important to know what is going on in the soil even to considerable depth.
Arguably the best way to get the big picture about what is going on under the ground is to “ask the roots.” There is an agronomist named Michael Petersen who has spent much of his career doing just that. Michael got his degree in Soil Science from the University of Nebraska in 1972 and went to work for the Soil Conservation Service (which later became the Natural Resource Conservation Service or NRCS). After 34 years in that role he spent more than a decade in a research role the farm equipment manufacturer, Ortman. He has also worked as a consultant – a role he continues today from his home in Colorado.
5 years, Petersen has used a backhoe to excavate 1,760 “root pits” in farm fields in order to observe rooting profiles as deep as 12 to 15 feet. Any plant biologist would acknowledge that roots are extremely important, but very few researchers ever do this kind of work. Thus, Petersen’s observations are a valuable “window” into the hidden but extremely important plant world below the ground. Much of his work has been with corn in the Midwest, but Petersen has also looked at many other crops in many geographies including the Pacific Northwest, South Africa and Europe. He is currently working on a book to summarize his many years of research and he hopes that it will encourage young people to get involved in this kind of research. This article will discuss just a few of the highlights from his experience.
Rooting Depth: Row crop roots can grow quite deep if there are not barriers like a plow-pan in the soil. Soybeans penetrate to around 4 feet, wheat 5 to 6, corn to 8 feet, sunflower to 9 and alfalfa to 15 or even 30 feet. Earthworm activity can extend to 6 or even 8 feet and that is important for carbon sequestration since they transport plant material from the surface down to those depths. Mycorrhizae are beneficial fungi associated with roots and they can be active down to two feet and the glomalin they make is an important contributor to carbon sequestration.
There are now several initiatives which offer to pay growers to farm in ways that can sequester carbon in the soil (e.g. no-till, cover crops,…), but for practical reasons, soil carbon is typically only measured in the top 4-6 inches and there is focus on farming methods that leave a great deal of crop residue on top of the soil. There are also remote sensing approaches that can verify certain farming practices and then use modeling to estimate soil carbon. While these measurement approaches are practical at scale, they don’t capture what is going on at the depths that could reflect the most stable, long-term forms of carbon sequestration as well as soil health and drought tolerance perspectives.
Breeding ramifications: Traditionally, plant breeders have known that certain hybrids or varieties are better suited to different soils or management methods, but they have not utilized something like Petersen’s pits to evaluate how this relates to rooting patterns. That is beginning to change and there now also corn lines specifically developed for deep rooting and interest in developing more. Typical rooting volume per corn plant is around 3,700 cubic inches, but the best cultivars and practices can lead to rooting volumes in the range of 8,000 cubic inches per plant. The deep carbon storage potential of these lines needs to be fully documented.
Corn Rooting Behavior: Corn plants don’t share rooting zones – they stop growing towards each other when they sense a neighbor. Each plant has a distinct sort of box-shaped root structure with a one dimension limited to a few inches by planting density in the row, width determined by row spacing, and depth somewhat dependent on temperature in that in Northern regions the soil below 5 feet or so may never get warm enough to allow roots to grow much further. This has ramifications for optimal planting density.
Tillage systems: no-till is often considered to be a sort of “gold standard” for soil and residue management, but Petersen has frequently observed that top-soil can become increasingly dense in a zero tillage system. There can be a lack the larger pores that can accommodate corn roots since they are often in the range of 5mm in diameter. Early root development tends to be better in a strip-till system which provides the seedling plants with a 10” wide and 12” deep cultivated zone in the planting row. Petersen has found on the order of a 35% higher overall root density in a strip-till system (see example root profile diagrams below). The strip-till soils also have larger pores that favor earthworm activity as well as deeper penetration of rain or irrigation water. If in fact there is an equivalent or greater carbon deposit associated with the growth beyond the tillage zone, it might be an easier sell to growers on shifting to strip tilling rather than to complete no-till.
Cover Crops: In regions with adequate rainfall, cover crops are highly desirable because they continue to feed the soil biome for more of the year, and some are particularly deep rooted. Cover crops are known to be an excellent part of a soil health program, but the economics are only positive in the medium to long term. Once again it is possible that their full contribution to carbon sequestration is under-estimated by shallow sampling.
Compaction Zones: One fairly common farming strategy has been to use auto-steering and GPS in order to make sure that only a small portion of the field is ever compressed by the tires of equipment. If fertilizer is only applied to the non-compacted part of the field, emissions of the potent greenhouse gas nitrous oxide a greatly reduced. However, what Petersen has observed is that these “controlled wheel tracks” or “tramlines do become problematic for overall root growth and yield over time, and that it might be best to shift them every 5 years or so. More research is needed about how that influences stored carbon. Other alternatives include very wide tires which allow the equipment to “float” over the field, or possibly very small, autonomous equipment currently being developed as an alternative.
A Good Example: When asked what sort of overall system elements looked the particularly good from the perspective of root development, Petersen cited strip till, and a 4-year rotation (e.g. corn, soy, small grain, and a year of mixed cover crop). He would add some manure application if that is available because it introduces bacteria that are beneficial for the soil microbiome and aid in the digestion of organic matter and subsequent nutrient release. This sort of system could bring soil organic matter up to the 6% range and keep it there with excellent root growth throughout the soil profile. That is certainly not a one size fits all system, but it is feasible in many geographies.
Closing thoughts: Healthy Roots and the microbiomes that they feed are among the major contributors to the carbon that can be sequestered in farmed soils. They are also the basis for greater drought tolerance, and nutrient scavenging. Obviously digging large root pits can’t be the system used to validate carbon credits, but it does seem logical to use this method in the experimental stage when farming methods and genetics are being compared.