Soil organic matter: what you don’t know

This article first appeared in the Wyoming Livestock Roundup in September, 2014

In one of my first talks as extension soils specialist at the University of Wyoming I began to describe the importance of soil organic matter and how, unlike many soil properties, it is a component of soil quality we can really affect with management. Then a farmer in the front row said loudly, “Yeah, Yeah, soil organic matter is important. Tell us something we don’t already know.” While the audience voted for me to continue through my introduction, it made me realize how important, and often difficult, it is to provide new information to producers.

The goal of this article is to describe the research view of soil organic matter in order to provide food for thought about management practices that improve soil quality. We’ve long known the importance of soil organic matter for regulating nutrient and water supplying potential of soils. But an ever increasing number of researchers continue to discover and publish new knowledge about this old subject. Much of it is very interesting and some even impacts the way we manage soils and design cropping or grazing systems.

The deep black soils of the corn belt are typically over five percent organic matter, and sometimes 10 percent or more. Wetter soils have higher contents because of greater plant production and slower decomposition in seasonally saturated, oxygen-deprived conditions. It may seem that organic matter is not as important in the west, with less than two percent in soils of the western fringes of the Great Plains and often less than one percent in soils of intermountain basins and deserts. But, as we’ve learned in reclamation, it is crucial for maintaining or restoring sustainable production of crops or forage, even at those low levels.

Soil organic matter is formed through decomposition of above- and below-ground plant residues along with compounds secreted by roots, as well as manure and animal residues. Components of any organic (carbon-based) material cover a spectrum of decomposability, from soft tissues and intracellular materials that break down rapidly, to woody materials that breakdown very slowly. Scientists have categorized the materials into active, slow, and passive fractions that perform different functions in the soil. The fractions are quantified in different ways, including simple sieving into different sizes, floating in dense liquid to separate density fractions, and treating with strong acids to identify chemical fractions. But generally they behave similarly, whether differentiated by size, density, or chemistry.

Active soil organic matter typically turns over, or completely decomposes, on an annual basis. It consists metabolic tissues and soluble carbon and nitrogen compounds that are rapidly consumed by soil microbes that cycle nutrients and make them available to plants. This fraction is important for meeting immediate plant nutrient needs, but much of it is converted to gases including carbon dioxide and nitrous oxide during microbial respiration. Disturbance like tillage accelerates turnover of active organic matter and loss as gases.

Slow organic matter turns over on the order of decades and  is made up of less decomposable plant parts, secondary compounds created by rapid decomposition of active materials, and active materials that are protected from microbial activity. Protection from decomposition occurs with materials are locked inside soil structural units, or peds, or because soils are wet, cold, or otherwise poor  environments for microbial activity. Soils of wetlands or alpine meadows often have higher proportions of slow organic matter because environmental conditions suppress decomposition. Undisturbed grassland and rangeland soils have much higher proportions of both active and slow fractions than cultivated soils because disturbance breaks up soil peds, destroying structure and exposing protected organic matter to oxygen and rapid decomposition.

Passive soil organic matter is often known as humus turns over and on order of centuries or longer. It consists mainly of stable compounds made up of large, complex organic molecules that are distilled from plant and animal residues during microbial decomposition of active and slow organic materials. Humus does not directly supply plant nutrients like active and slow fractions, but has huge impacts on soil quality by sticking soil particles together to form strong structure, porosity, and water holding capacity, and by providing many negatively charged broken ionic bonds that hold and release plant nutrients.

This graph, modified from one in an introductory soil science text by Nyle Brady and Ray Weil, shows how cultivation affects the proportions of organic matter (OM) in the active, slow, and passive fractions. It indicates that before cultivation begins over half the total soil organic matter consists of active and slow OM, along with plant residues decomposing on the soil surface. Disturbance has a drastic effect on those fractions, especially in the first 10 years after cultivation begins. After 50 years passive OM declines by a small amount, but the proportion of total organic matter made up of fractions that actively supply plant nutrients drops to approximately 13 percent.

Organic matter history graph

This graph has two strong implications for sustainable management practices, whether for farming or grazing: first, reducing disturbance can drastically improve the nutrient-supplying power of soils by preserving active and slow organic matter. Cultivation and other forms of disturbance cause brief surges of decomposition that use up much of the organic matter without making released nutrients available to plants. Second, returning as much plant residue to the soil as possible builds the active and slow fractions. This can have rapid impacts on soil quality.

In research on Wyoming dryland wheat production, we concluded that years of wheat-fallow rotations with intensive tillage resulted in loss of nearly 70 percent of the original soil organic matter. Modern farming practices over the last two or three decades have restored much of that through combinations of reduced tillage and increased biomass production, along with diverse plant residues from more complex crop rotations.

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