Organic Matter: what really ‘matters’
Not all OM is created equal, here's why + a list for inspo💫
I’ve noticed many organic farms tend to have a counterintuitive combo of high soil organic matter (SOM%) but poor soil structure… Today, let’s think about how this happens so we can make better decisions for our farms, gardens, and yes, even lawns.
Similar to how not all compost is created equal, neither is all organic matter. To try and get my arms around this I started writing down all the different kinds of OM sources commonly added to soil systems, and then I considered what “services” each of them provide.
I hope to talk with y’all more about this in our Office Hours discussion thread this coming week!
The thread will go live Monday, December 18th and be open for a week to chat more about this and Johnson-Su Bioreactors
SUMMARY
One common way to end up with high SOM% but poor soil structure is by leaning on GOM (most commercial compost) as your main source of organic matter and incorporating it via tilling practices.
Why?
In general, to build soil structure - be it in clay or sand - we need biology and we need to protect that biology by minimizing mechanical and chemical disturbances and maximizing ground cover and living vegetation.
GOM has little microbial life in it to begin with and tilling prevents any beneficial life from gaining a toehold in the soil.
I hope this list reminds you of all the other tools we have available to us outside of GOM to help us inoculate, feed, and build soil 💪
THE LIST
Different kinds of organic matter and their respective functions:
GOMpost: ground-up organic matter (GOM) is a term coined by my friend and colleague, Leighton Morrison, and is what we call the “compost” that tends to come from industrial composting facilities or municipal programs.
Function: a general source of carbon and organic matter to quickly increase soil OM%; provides some nutrients (which can vary significantly from one source and batch to the next)
Notes: GOM does not necessarily contribute to improved soil structure nor does it introduce beneficial organisms in a meaningful way. However, it can be a tool to quickly amend a soil that can then be further improved upon through beneficial soil biology and by employing additional soil health principles.
Biodiverse Compost: this is compost that is made with consideration for microbial populations - the finished product contains stable populations and biodiversity among key microorganism groups (ex. Some main characters of the soil food web are bacteria, fungi, protozoa, and nematodes).
Function: can be used in solid and/or liquid form to inoculate the soil (or GOM!) with beneficial biology to start restoring the soil food web that cycles nutrients to plants, builds soil structure, and protects plants from pests and pathogens.
Notes: This quality of compost can be made successfully using various composting methods (e.g. vermicompost, thermophilic, or static piles like johnson-su bioreactors); success is largely based on the feedstocks that go in and how the compost is maintained and matured before using.
Biochar: another source of carbon-based matter is biochar. In its simplest definition, biochar is a material created from the partial burning of BIOmass (think vegetation or wood) to make a CHARcoal-like material.
Function: biochar can have a pretty quick impact on soil structure as it stimulates aggregation by providing a substrate on which organic matter can start accumulating through biological processes. Of course, soil biology in most cases needs to be introduced and fostered (through good management practices) in the soil for this accumulation to occur.
Notes: in agriculture we always want to use an “activated” or “charged” biochar which means that it was inoculated with microbes (and in some cases nutrients) to ensure that it doesn’t “steal” nutrients from a cultivated soil. For DIY, this can be as simple as adding wood coals from a fire to your actively composting pile - see this post for a deep dive on biochar.
Mulch: mulch is a general term that refers to any material that’s added on top of the soil. Some mulches are even synthetic, but as soil health stewards we are most interested in natural mulch sources which will break down and feed the soil. Common in gardens are wood chips, straw, dried leaves, pecan shells, bark, etc.
Function: mulch serves three main functions - suppresses weeds, protects soil biology and slowly decomposes to provide more carbon and organic matter to the soil
Notes: there are so many different kinds of materials that can be used as mulch and each come with their own considerations (maybe we can do a deep dive on mulch?!) For now I’ll say, regardless of the material, when we mulch plants we generally want to use a mulch that is aged a bit.
For instance, opting for aged straw or woodchips - some that have been sitting around getting rained on for a few months (or years!) - means the mulch will actually breakdown and feed the soil sooner than a fresh mulch source which may have more antimicrobial properties and be more resistant to breakdown. The more established a microbiome is, the less this aging should matter, but for heavily cultivated soil systems or early-stage restoration, aged mulch is a good call.
Humates: humates are basically the end of the road of decomposition - the final molecular structure of whatever didn’t get used by the living organisms that contributed to the breakdown process. Humates can be derived from ancient biological sources like coal or more recent decomposition like composts or fertile soil. My understanding is that “humates" is a catch-all term that refers to both liquid humic acid and solid or pelletized humates - chemically, they’re very similar (ref)
Function: fixes toxic chemicals, makes mineral nutrients more bioavailable, prevents nutrient leaching, indirectly improves soil structure by feeding fungi
Notes: the thing about humates is their molecular structure, with all those versatile carbon atoms, allows them to grab onto many different kinds of chemicals - be it toxins like herbicides and fungicides, or plant nutrients from mineral or synthetic sources (which prevents leaching)
I kind of think of humates as a buffering system for the soil - if there is an excess of something, humic acid will “complex” it into its matrix curbing effects of toxicity, or on the other hand if there is a shortage of something, humic acid can hold onto it until it is needed by plants or other life. Lastly, fungi can feed on humic acid and improving fungal populations leads to more aggregation which equates to better soil structure.
Cover Crops & Green Manure: cover crops aren’t always used as green manure, but I’m lumping them together so this post doesn’t turn into a textbook chapter 😝 the main difference is that cover crops are primarily grown to protect the soil from erosion, while green manures are grown with the intention of incorporating the living green plant into the soil to add nutrients.
Function: one of the biggest services, in my opinion, of cover crops is that they serve as a host for the soil microbiome - their living roots foster plant-microbe relationships before / after crop season or otherwise dormant periods. As vegetation dies back, it functions like a mulch and becomes an additional source of food for microbes and thus other plants. Lastly, living ground cover protects soil from sources of erosion like wind, rain, and sun and keeps weeds at bay.
Manure manure 💩: be it from collecting and spreading or directly from grazing livestock.
Function: provides nutrients and inoculation via the gut microbiome of an animal, especially ruminants
This of course is not an exhaustive list, and if anything comes to mind to add, please comment!
Remember, building structure always requires two things: biology & protection.
How we go about those two things is up to us - the humans, the stewards. As you consider your options, if you feel overwhelmed, slow down, take a deep breath and remember, having options is an invitation to engage in conversation with your land. You are not alone, so many other living beings are communicating and co-creating with you. Together, you’ve got this. 🤎
With Love,
Andie
References:
Kussow, Wayne R. PhD. Humate and Humic Acid. Horticulture Update. https://aggie-hort.tamu.edu/newsletters/hortupdate/hortupdate_archives/2002/jun02/art4jun.html#:~:text=If%20we%20take%20the%20flocculated,acid%20are%20basically%20the%20same.