“Soils are developed; they are not merely an accumulation of debris resulting from decay of rock and organic materials…In other words, a soil is an entity—an object in nature which has characteristics that distinguish it from all other objects in nature.”
- C.E.Millar & L.M.Turk, 1943
When the average consumer thinks about agriculture, particularly crop production, most focus on a crop’s yield, its pleasing aesthetics and possibly even the brand of nutrients used to produce it.
In soil that is not self-cycling, a major expense every year is the nutrient brands and additives that help a cultivator get around the necessary soil cycling processes. Little do most people know, in a properly designed cropping system, with a healthy cycling soil, you can toss the brands and rely on the real soil rock stars: organic matter and microbials.
When a soil’s organic matter and the microbial herd working tirelessly to decompose it are in tune, they will enrich your soil with a diverse herd of biology and a special substance called humus, which due to its color is also known as Black Gold of the Earth.
What is Soil?
Soil is not dirt.
Dirt is what you sweep into a pile on the floor and throw away in a trash can. Dirt is the shameful name we give to soil that no longer has the properties to support life.
Soil, meanwhile, is beautiful.
Soil is intoxicating. Soil is biologically rich. Soil is alive!
Soils are like anything else alive—they have parents, are born, need time to mature, require nutrients and respiration and can get sick.
If not properly maintained, soils can even become tired and die.
How is Soil Born?
Soil is born from parent material.
This material is where soil gains its physical and chemical features. Over time, these acquired characteristics help define whether a soil is capable of supporting a productive crop or not.
Soil parent material comes in two primary forms: inorganic and organic matter.
Pebbles, sand, silt, clay and minerals are all examples of inorganic matter, which, in nature, come from large rocks being mechanically and chemically weathered over time. In a managed soil, inorganic matter is added via rock dusts, perlite and pumice.
Organic matter in nature, on the other hand, is composed of dead plants and animals, along with the various living microbes and fungi which slowly decompose these dead plants over time into nutrient rich soil.
3 Types of Organic Matter
Organic matter itself is broken down into three primary forms: the living, the dead and the very dead.
The living portion of organic matter consists of plant roots, worms, arthropods, protozoa, microbes, fungi and other living creatures. The primary function of the living portion of organic matter is to be the beasts of burden during soil cycling by decomposing plant and animal detritus.
The dead portion of organic matter is referred to as the active portion of soil organic matter because it consists of the recently dead debris such as leaves, stems and dead micro-organisms being actively decomposed by the decomposers from the living portion of organic matter.
Decomposition in these first two pools of organic matter creates microbial glues.
These decomposition byproducts glue together soil particles. Once bound together, these soil particles become aggregates and come in all different shapes and sizes in a soil.
Proper soil aggregation is crucial because aggregates give soil its structure, in the same way framing and walls give a building structure and help define its open spaces. As the aggregates pile up, a lot of nooks and crannies form around individual aggregates.
These defined spaces between aggregates are called macro pores in soils.
These macro pores help ensure proper water infiltration and gas exchange at soil surfaces, as well as proper percolation once water works its way down into a soil’s profile.
Aggregation also gives microbes and fungi a home so they don’t wash away further down into the soil profile with rain and irrigation water, where the lack of oxygen will slow or stunt their decomposition abilities. Additionally, aggregates provide a place for microbes to hide from microbial soil predators, called protists, who help keep microbial populations in check. If any single part of a soil population becomes too large and out-competes other microbes, it will create a biological imbalance—which can have serious negative consequences for a crop’s health while growing in the soil.
In the soil food web, these protists are single-celled eukaryotes that hunt microbes by following the smell of the organic compounds that microbes release to communicate with one another. Micropores, the small pore spaces found within an aggregate, provide this place of refuge for microbes to live and reproduce in peace, out of the reach of larger protists.
Finally, this brings us to the very dead portion of soil organic matter, also referred to as humus.
The very dead humus is considered to be the most stable part of organic matter because it can no longer be broken down any further by decomposition.
In managed soils, humus is formed from organic matter that is usually added as already decomposed and unrecognizable material called compost. Compost is chopped and dropped green manure which is left on a soil surface for microbes to compost in place.
Some of the plant matter that makes it to the humus stage gets broken down into microscopic pieces of matter that become soil colloids.
Once organic matter reaches the colloidal stage of decomposition, the colloids slowly work themselves into soil aggregates where their chemical properties allow them to hold and exchange nutrients. This exchange process is called a soil’s cation exchange capacity (CEC); it operates much like a preloaded credit card account.
Mineralization: Seasonal Nutrient Repayment
Every time a production crop turns a full cycle in a soil and is harvested, each harvest will slowly remove nutrients from the soil.
Think of this as chipping away at a preloaded credit card balance. In order to keep plants healthy, every seasonal debt needs to be paid off in full with a deposit before the next round. In a living soil, this credit card debt deposit happens during the decomposition of the dead portion of organic matter by microorganisms.
For organic matter, growers make “deposits” using cover crops like alfalfa, clover, hairy vetch, or other decomposable materials like rye, compost, worm castings, shredded wood or newspaper. If this redeposit doesn’t happen in a living soil, you won’t have an available balance in your soil prepaid credit card account to grow plants with in the next cycle.
This soil nutrient repayment process is called mineralization.
In this instance, the bank would be the soil colloids that worked their way into your aggregates and hold onto these extra available nutrients for your plants to access later.
The chemical properties of soil colloids derived from decomposed organic matter possess one last hidden benefit that often gets misattributed to microbes: the ability to buffer soil pH.
pH buffering is a soil’s ability to resist swings in pH. The common refrain with living soil growers is, “I don’t pH my feeds, my microbes control pH for me.” In reality, a soil's water and carbonate content, alongside the large surface area of soil colloids, satisfies most of a living soil's pH buffering needs.
A colloid's large surface area means increased CEC docking sites, which allows more available hydrogen ions to be absorbed out of a solution. This process helps buffer soils against the soil acidification abilities of available hydrogen.
The ability to buffer soil PH is important because the only way a plant can access a soil's stored nutrient pool from the colloids in aggregates is to have the proper soil PH.
In the spirit of continuing our banking metaphor, this would be akin to needing a pin code to complete a deposit or withdrawal.
Organic Matter and Soil Water Content
The last crucial advantage of soil organic matter, and perhaps even the most overlooked, is its effects on soil water content. Without organic matter, soil can’t cycle nutrients—and plants can’t grow.
The greatest factors in a soil’s water holding capacity are its soil porosity and the concentration of fine particles (colloids).
Soil water holding capacity is the ability of soil to hold plant-available water long-term in soil pores against the forces of gravity, which are always trying to pull water down further into the profile. Water gets stored in the larger spaces between individual aggregates, called macropores, through a process called adhesion, which is the ability of different molecules to stick to the surface of one another. This is how water resists the downward pull of gravity by clinging to soil aggregate surfaces.
Water also gets stored in the micropores and on colloids found within soil aggregates.
When plants consume all of the easy-to-access water held by the macropores around a soil’s aggregates, it can then access the more tightly held water inside an aggregate’s micropores that are adhering to the surfaces of colloids. The size of the pore spaces inside and outside an aggregate is what defines how tightly the water will be held in soil by adhesion. These unique traits reduce overall water usage long term, and aid in keeping microbes and fungi alive—while also providing a reaction medium for a soil’s chemical processes, like nutrient cation exchanging between roots and soil particles.
Too Much of a Good Thing
How much organic matter is enough, and how much is too much? When does it reach the “too much of a good thing” point?
Keeping soil organic matter percentages around 10-20% is my target for my living soils indoors, as well as in raised beds and containers outdoors.
If I’m planting directly into the ground outside, I’ll aim for roughly 10% organic matter content. This slight difference between the two soils is because my outdoor soils will receive more natural organic matter over the course of the growing season.
Anything over 30% has caused problems for my living soils.
There is nothing more important a farmer could do than manage his soil's organic matter content, especially if you’re trying to find ways to cut overhead in a production setting.
Without organic matter, soil is just a bunch of inert ingredients. Some might even call it dirt. Humus production, the final form of organic matter, is the alpha and the omega of a soil's health.
“To be a successful farmer one must first know the nature of the soil.”
- Xenophon, Oeconomicus, 400 B.C.
C.,, Weil. Ray R. Brady, Nyle, and Ray R. Weil. The Nature and Properties of Soils. 15th ed., Pearson, 2016.
Crouse, D.A. 2018. Soils and Plant Nutrients, Chapter 1. In: K.A. Moore, and. L.K. Bradley (eds). North Carolina Extension Gardener Handbook. NC State Extension, Raleigh, NC. https://content.ces.ncsu.edu/extension-gardener-handbook/1-soils-and-plant-nutrients