Beyond the Basics: Organic Matter

I've been going back to the first principles of organic matter. The science is changing and the principles I once understood no longer seem to be the case. Here is my distillation of how to understand organic matter.

A Musing on Organic Matter

Soil organic matter is a dance between microbial respiration and carbon inputs.

Put a different way, soil organic matter is photosynthesized carbon (in various forms) that hasn’t been respired by organisms yet.

Getting the full benefits of organic matter lies in the paradoxical flux between two simultaneous processes: building organic matter and microbes consuming it.

Historically in agriculture we have favored the respiration side of the equation - the consumption.

When a new ecosystem is plowed for the first time, 50% of soil organic matter is lost in the first few years after cultivation. The loss slows significantly and reaches a new equilibrium.

BUT, organic matter can rebound FAR faster than it was depleted.

Nature is giving us a window to regenerate soil like it’s giving us a window to use sun and wind to replace coal.

But I’m not here to preach. Admittedly, almost every vegetable garden I’ve ever had I’ve tilled. Annual crops simply thrive in disturbed soil. If consumers want veggies and they want them cheap, then microbial respiration will prevail.

An Unconventional List of the Benefits of Organic Matter

Here is an unconventional list of benefits that goes beyond the repetitious platitudes:

1. Provides the benefits of clay without the disadvantages.

Organic matter provides 10-20x more CEC than clay does per unit of weight. In many soils more than 50% of CEC is from organic matter in the top horizon. OM gives you all the nutrient holding benefits of clay without the (heavy) drawbacks.

2. Increases yield at a lower cost.

Organic matter simply makes crops less susceptible to weather. Water is often the biggest limitation in production, and organic matter smoothes water variability and reduces drought risk.

3. Provides 15-25* pounds of nitrogen per year for each percent organic matter. And large quantities of phosphorus, potassium, sulfur, and boron. Free nutrients. Enough said.

*This number is somewhat debated and difficult to quantity, but everyone agrees it contributes free nutrients.

“Humus” is Dead

If you are an agriculturalist of any kind, it’s time to update your definition of soil organic matter.

Soil organic matter is not simply “stable humic substances”.

It is not “recalcitrant” molecules that cannot be broken down any further by microbes. In fact, it turns out there is a very weak correlation between the age of organic matter and its recalcitrance.

This was the original definition of organic matter that has been flipped on its head in recent years.

Rather, organic matter is a complex flow of carbon molecules between root exudates, microbes, and clay particles. The vast majority of it is actually dead microbial bodies and simple microbial products.

Some of the most stable carbon is relatively new!

And much of the organic matter is made up of smaller, simpler compounds than previously thought.

Instead of imagining stable compounds protected from decomposition, think of a dynamic flux between microbial consumption, carbon inputs from plants, and soil particles and aggregation protecting organic matter at any point in the process. 

The Equation for Building Long-Term Organic Matter

How do you build organic matter that sticks around in the soil for a long time?

The answer is NOT through the application of large quantities of compost or manure. Don’t get me wrong, this is often a great practice for various other reasons. Often times a little can go a long way nutritionally and biologically. But a lot can cause more harm than good in some instances, depending on the quality of the product.

Believe it or not, it is NOT from maximizing the addition of aboveground carbon sources such as crop residue or mulch. (Though this is still a great thing to do to build organic matter and protect the soil.)

LONG-LASTING organic matter is a different kind of alchemy. It’s about two things.

Aggregates.
+
Roots.

The best way to build long-lasting organic matter is to:

1. Protect and build soil aggregates. The organic matter protected in aggregates will be inherently long-lasting if you don’t break those aggregates.

2. Focus on year-round living roots. It turns out that root-derived soil carbon tends to stick around in the soil longer than aboveground-derived soil carbon. For one reason or another, a carbon molecule in a root exudate is more likely to get incorporated into an aggregate than a carbon molecule from a piece of straw is.

3. Maximize the health and photosynthetic rate of those plants. The quantity of root exudates is a function of photosynthesis. Through mineral nutrition, microbial inoculants, and any other technique, get your plants CRANKING!

Don’t Always Trust Your Soil Test’s Organic Matter

Don't trust the organic matter number on your soil test!

Why?

The standard organic matter test is done by igniting a sample of soil and seeing how much weight is lost. All of the lost weight is assumed to be OM. Simple.

But this isn't good enough. Peat moss will test over 80% organic matter, but it brings very few of the benefits of true organic matter (specifically, CEC, nutrients, and a source of microbial food). It's nearly "inert".

A microbial inoculant powder may test at 60% organic matter, but obviously this isn't expressing TRUE soil organic matter.

The point is this: if you are growing in peat-based media, or a soil amended heavily with peat or coco, or a soil with loads of fresh residues or compost, your organic matter number isn't telling you what's actually important.

(If you see that your organic matter is above about 10%, you might as well ignore it completely. Natural ecosystems rarely exceed 10% organic matter).

But this measurement is not TOTALLY worthless. It has value if tracked over time on real topsoil. Just take it with a grain of salt.

If tracking carbon is important to your operation, I suggest thinking about it in a different way...

Couple the standard organic matter test with additional carbon tests such as “ACE Protein” or “Potential Mineralizable Nitrogen”. The Cornell Soil Health Assessment is a good place to start to find more advanced carbon tests such as these.

Fungi

Biology - not additions of organic residues and compost - are what create long-lasting organic matter. 80% of the accumulation of organic matter is from MICROBIAL BIOMASS.

(Residue and compost may feed microbes, which is great, but residues are not most responsible for building organic matter.)

But it gets more interesting...

It turns out that FUNGI contribute more to the creation of soil organic matter than any other microbial group.

I have no idea why this is, but it's fascinating. My guess is that it has to do with the fungi's ability to "glue" organic matter into soil aggregates, protecting it from further microbial decomposition for up to 1,000 years.