What is Soil Organic Matter?

As a soils guy, I have a lot of discussions with clients and colleagues about the quantity of organic matter in soil. Perhaps surprisingly, there is a lot of confusion and misinformation around this issue. What is organic matter, and where does it come from?

Organic matter in soil comes from different sources and is constantly being modified by the soil food web. It starts out as relatively simple carbohydrates produced by the plant and gradually changes into more complex and stable carbon-based chains.  The process from raw leaves or decaying roots takes many decades to be transformed into something loosely defined as humus.  Compost, even good quality compost, and other processed organic soil additives are relatively unstable. This simply means that they are still in an active early decay process.  If compost still produces heat within the stockpile, it is a long way from being a truly stable organic additive.

Soil organic matter (SOM) in a natural soil comes from three sources:

(1)  Dead plant parts and animal droppings deposited on the soil surface.
(2) Roots that die and decompose.
(3) Liquid carbohydrates that flow out of living root tips into the soil called exudates.

Exudates are the least understood of these three but are considered to be quite important to overall SOM, and in particular to supporting the soil food web immediately around the root in the zone called the rhizosphere.  All three carbon sources are processed, over time, in to more complex and stable forms of organic matter by the soil food web, which uses the energy contained in the carbohydrates to process soil chemicals and make them available to plants.  The soil food web also serves an important role in forming soil structure by sticking soil particles together into larger groups of particles and opening up passages for air and water — both of which are critical for root growth — in the soil.  SOM is also subjected to leaching, which can move it deeper into the soil or contribute to it being eroded away in soil that is exposed to surface runoff.

Compost that is added to soil should not be equated to soil organic matter, as it is understood in classic soil science.  This is because it is relatively unstable, meaning it is still changing from plant matter into more stable carbon forms.  This process is called respiration.  During the composting process from green waste to mature soil organic matter, more than half of the carbon it contains is released into the air as CO2, reducing the available carbon in the soil.  This process is still in progress when compost is added to the soil.

Due to issues a few issues, among them flawed soil testing protocols (I’ll address those in another post) the continuing respiration of compost in the soil, and the fact that compost is mixed by volume but then measured by dry weight, the measured amount of SOM in a soil mix containing compost is far lower than the amount of compost added to the soil.  Adding 10% by volume of compost to a soil mix will typically raise the tested dry weight of SOM by only 1-2%.  Compost added to soil is usually large particles that are separated from the smaller soil particles even when well mixed.  With time the compost gets distributed into closer proximity to soil particles by the soil food web.  By contrast, SOM in harvested field soil actually coats soil particles, staining them brown and black and sticking them together.  This organic matter is much more stable.

It may surprise you, but SOM by dry weight is surprisingly low in normal soils.  Most old agricultural field soils may have SOM between 1.5 and 3% dry weight.  Good forest soil in humid regions may be 5% or higher in the Pacific North West.  Peat soils in the South Eastern United States may also have elevated SOM. Undisturbed desert soils in the southwest have lower SOM of 2% or less.  Contrary to what you may have heard, it is not necessary to raise tested organic matter in soil mixes above these levels.

In fact, adding too much compost to a soil mix can be counter productive.  At levels of 15% by volume or higher, significant soil settlement should be expected due to loss of carbon and reduction in soil volume.  At levels of 25-30% by volume or above, the settlement from loss of soil volume can be quite dramatic.  The other drawback is that the compost keeps respiring, giving off CO2 into the soil.  If soils with large amounts of compost are over watered or develop a drainage problem, this CO2 can actually reduce growth rates in plants or quickly kill them.

Image: williumbillium

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