A discussion on soil organic matter, hummus is the tasty chickpea dish.
By Caleb Goossen, Ph.D.
The word “humus” has been used to describe soil organic matter since the late 18th century, deriving from the same word in Latin, which simply meant soil. Beyond referring generally to the layer of a soil profile rich in organic matter, humus has been taught to many of us in more specific terms for the purposes of soil science.
To cut to the chase (spoiler alert), the purpose of this piece is to describe why the “classic” humification model might be relegated to scientific history and how the newer understanding of soil organic matter influences the way we think about managing it. You don’t have to worry too much about keeping all of these older soil science terms straight – there will not be a quiz!
The Classic Humification Model
You may have heard soil organic matter divided into living, dead and “very dead” components, with humus, or humic substances, used to describe the very dead components. This is grounded in soil science’s traditional humification model of the creation of soil organic matter. The humification model describes humus as being composed of distinct fractions of organic matter that have undergone many biological and chemical changes over long periods of time, and become large, complex molecules which are highly recalcitrant (resistant to decomposition). The notion that these fractions are recalcitrant has led to the belief that they are relatively stable in soil and they are often thought to be of ancient origin, ergo being called the very dead fraction of soil organic matter.
But what are humic substances? Humic substances are operationally defined based on a method using strong chemicals to extract organic components from mineral components of soil. This method was developed in the 1790s and used for centuries. Some continue to use it today. Soil samples are first washed with strong alkali extractants (often sodium hydroxide, also known as lye, which can have a pH as high as 13). The resulting organic components, called “humic substances,” are further divided into an insoluble fraction, which is labeled “humin,” and a soluble fraction. The soluble portion is then subjected to a very strong acid. The fraction that remains in solution when acid is added is labeled “fulvic acid,” and the fraction that is insoluble (i.e., forms a precipitate) is labeled “humic acid.”
The trouble with defining soil organic matter fractions by their extraction methodology is that the extraction procedure was devised before there was much of a theory of soil organic matter to begin with. I relate it to teaching someone, with no prior jigsaw puzzle experience, to complete a puzzle by taking the pieces out of the box and sorting them by colors and shapes – without any concept of the picture that the finished puzzle will make. Except our analogy must be further complicated to fully capture what the extraction procedure is doing. By exposing soil organic matter molecules to bases and acids at pH extremes far beyond what would be encountered in most soils, bits and pieces of soil organic matter are being ripped apart and recombined in new ways. That means that each time the puzzle novice attempted to figure out the jigsaw puzzle, they would be trying to make meaningful groups of the pieces after those very pieces had been ripped apart and reassembled – often in enormous clumps and potentially in different combinations of the properties that previously defined the pieces’ shapes and colors.
With newer technology, scientists can observe soil organic matter in undisturbed forms without these harsh extractions. Emerging evidence suggests that soil organic matter likely consists of much smaller and simpler molecules, composed of dead microbial cells and other microbial byproducts (i.e., proteins, lipids, etc.) — not the very large and complex molecules suggested by the humification model. Evidence also suggests that the physical protection of organic matter bound to soil mineral surfaces, or within soil aggregates, is more important for stabilizing soil organic matter than the complexity of its chemical structure. The exciting aspect of a new mode of understanding soil organic matter is that it allows research to more freely suggest theories that better fit observed data and trends. For example, stable soil organic matter can be created more quickly than previously believed, and the pathway for creation likely involves plant root exudates feeding soil microbes to a greater extent than was previously given credit for. In other words, the soil-enhancing properties of cover crops and perennial forages that sometime seem magical make more sense with emerging understandings.
Soil Organic Matter and You
So does this discussion matter in your garden or on your farm? Most of us are looking at our soil organic matter on a coarse level, i.e., what percent of our soil is organic matter generally. That’s the same under any theoretical model. And all of the proven benefits of soil organic matter still hold true! Organic farmers have always cared about the soil and used practices such as cover cropping, rotating crops and composting. The emerging understanding highlights the importance of year-round, living roots in the soil and reducing soil disturbance to increase organic matter. This is important and exciting for farmers and gardeners who don’t have access to large supplies of compost or manure.
Even if this doesn’t change your management, I believe it’s still a good discussion to engage in as a reminder that science is constantly evolving and being reevaluated. The debate over soil organic matter models is ongoing, and the language surrounding soil organic matter will likely remain a bit of a confusing mish-mash of newer descriptions and older terminology as the emerging understanding filters down from peer-reviewed research articles in scientific journals to college textbooks and, hopefully, on to high school and middle school textbooks. If you have any doubts regarding the complications being created by this change in theoretical framework, which is simultaneously innocuous (to most folks) and yet a seismic shift (to soil science), just try to keep your head on straight as you read through the Wikipedia entries for humus, organic matter and humic substance!
With all that being said, you can bet I’ll still refer to the top layer of topsoil in my woods as “humus-rich” – it will just be a use of the older, more general definition of humus, meaning soil organic matter in general.
One final note, that is separate from the main discussion for simplicity’s sake, is about products sold as humates, humic acids and humic substances. These products are typically extracted from mined deposits of truly ancient organic matter – often peat or a low-grade form of coal. When extracted similarly to the traditional soil organic matter extraction method described above, the material is divided into fractions of the same names. Some greenhouse studies have shown that these products can have a positive effect on plant growth, though results are mixed. There are few documented benefits to their use in field soils where soil organic matter is typically providing the same functions. These substances should not be confused with what is actually present in soil organic matter, and they are definitely not a cost-effective way to build organic matter in soil.
This piece was greatly informed by “The Contentious Nature of Soil Organic Matter” by Johannes Lehmann and Markus Kleber (published in Nature in 2015), as well as Natalie Lounsbury’s podcast, “Priming for Production Podcast,” which features Johannes Lehmann and other soil scientists.