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Soil organic matter: keys to building and keeping it

Soil organic matter (SOM) is the single most important soil property that you have influence over through your management. Higher soil organic matter levels translate into better water holding capacity, which is critical in a season like the one we are having so far. It also results in higher and more consistent yields in the long run. Many of the key management practices that help build SOM are well known and effective. What’s keeping you from getting started?

In this article, I will highlight some of the latest understanding of soil organic matter and how to restore and maintain it. What role does crop rotation play in that % organic matter value you get back on your soil test? Where do organic amendments fit into the operation? Finally, can cover crops help build soil organic matter on your farm?

The role of soil microbes in soil organic matter
Soil microbes and larger soil organisms play an important role in cycling organic matter in soil. Microbes decompose plant residue and, in doing so, release carbon as carbon dioxide (CO2). They also use a portion of the carbon for their bodies, which are themselves part of the “soil organic matter pool.” It has recently been found, however, that soil microbes also influence SOM cycling because dead and dormant microbial cells and by-products can be a significant component of soil organic matter itself. An example of a microbial by-product is a carbon-rich substance called glomalin, which is produced by mycorrhizal fungi, typically stays around for 10-50 years in soil, and can account for almost one third of the total carbon in some soils.

Given that soil microbes themselves are such important contributors to SOM, but also play an important role in decomposition, how do you manage your soil so that you’re balancing the two?

The importance of residue quality
Dr. Lisa Tiemann, a soil microbial ecologist from Michigan State University and 2016 Southwest Agricultural Conference presenter, argues that nitrogen is at the heart of the explanation. On average, soil microbes contain 8 parts of carbon for every 1 part of nitrogen (their C: N ratio). As microbes consume plant residue, around two thirds of the carbon is lost as CO2 and one third is taken into their biomass. This makes the C: N ratio of 24:1 critical (since one third of 24 is 8). Residues below 24:1 are broken down quickly and stimulate microbial growth since they provide easily available nitrogen. On the other hand, residues above 24:1 contain more carbon relative to nitrogen than the microbes require. This means that existing microbial populations need to find nitrogen from other sources, such as existing SOM.  

Table 1. C:N Ratios. Available soon on the Field Crop News website.

Two main groups of microbes are important when it comes to soil organic matter. Fast-growing microbes reproduce rapidly, thrive on low C: N residue, and are generally inefficient (gain a relatively small amount of energy per amount of carbon consumed). Slow-growers, on the other hand, reproduce slowly, feed on high C: N residue (think wheat straw), but are energy efficient.  

Dr. Tiemann stated that the best way to build and maintain organic matter is to strike a healthy balance between the two types of microbes. She offered the following advice for doing so:

•    Wake up soil microbes by growing cover crops and applying organic amendments
•    Provide a mixed quality of amendments over a rotation – both high C: N and low C: N residues and manures
•    Diversify your crop rotation – more diverse rotations in general have higher SOM and more active and diverse microbes

Ontario research shows that adding winter wheat to a rotation increases soil carbon over time. When red clover is included as a cover crop, it likely contributes to soil organic matter not only due to its biomass, but also because it provides a high nitrogen residue that balances the high carbon residue of wheat and corn.

Roots vs. shoots
Often the success of a cover crop is judged by its aboveground growth. However, what’s going on belowground may be even more important if it is SOM you’re after. Research has consistently found that carbon from roots is more stable in soil than aboveground residues. In other words, carbon from roots sticks around in soil longer than carbon from shoots. So, if you’re looking to build soil organic matter, achieving consistently good root systems makes a difference.

Figure 1. Root growth on a fall-seeded cover crop. Roots play a very important role in contributing to soil organic matter. Available soonon the Field Crop News website.

What does this mean?
•    Better soil structure and better crop root growth translates into higher yields, and also contributes more to long-term organic matter as those roots break down
•    Maximize root growth with fibrous rooted crops, such as wheat, as well as cover crops
•    Don’t judge a cover crop entirely by its aboveground looks – do a little digging

June 24, 2016  By Jake Munroe soil fertility specialist OMAFRA field crop unit