Soil Carbon & Tillage

Wednesday, April 23, 2025

Eat Your Cake & Have It Too

By Scott Gillespie

The assumption in soil health circles has been that farmers must choose between building soil carbon (having the cake) and using tillage (eating the cake) – as if the two are mutually exclusive. The reasoning makes sense – tillage breaks up soil aggregates, releases carbon and accelerates organic matter breakdown. No-till, on the other hand, keeps everything intact, protects against erosion, and builds soil structure.

While no-till has its advantages, it is not a perfect system. Problems like compaction, ruts, gopher holes, and nutrient/pH stratification can make no-till fields less productive over time.

That is where tillage comes in – not as a step backward, but as a tool that can be used in specific situations without undoing years of soil-building progress.

A rigid, all-or-nothing approach to tillage does not serve farmers well. If you have already minimized or eliminated tillage, have you reached the endpoint? No, you still need to use tillage as a tool. There is a big difference between full-field, multi-pass tillage every year and an occasional tillage pass to address a specific issue. The key is understanding when, where, and how tillage can be used without compromising long-term soil health.

One of the strongest arguments against tillage is its effect on soil carbon. The common belief is that tillage leads to massive carbon losses, breaking up organic matter and exposing it to microbes that release it as CO2.

While this is true to an extent, recent research is showing that not all soil carbon is equally affected by tillage.

Soil carbon exists in different forms, and understanding the difference is important. There is heated debate in soil science circles on how organic matter forms, so I am just going to present a top-level overview. Here is a working concept of it as it is understood now:

Particulate organic matter (POM) is the more active, short-term carbon that decomposes quickly. As its name suggests, it consists of tiny pieces of once-living organisms that you would be able to identify under a microscope. Its source is mostly plant tissue, and it can last for years to decades in the soil.

Tillage speeds the process of these particles being found by the soil microbes, but even without tillage, these organisms would have come across it. Earthworms, ants, and other soil fauna slowly move this organic matter into more ideal places for the microbes to consume it.

Mineral-associated organic matter (MAOM) is the more stable, long-term carbon that binds to soil particles. This could be from organisms as their cells rupture from freezing, or it could be after something has eaten and excreted it. Its source is mostly from the exudates of roots, not the plant tissue.

These exudates are the substances that the plants release into the soil to recruit the microbes that they want around them. They don’t do this to be nice – they want something from the microbes: Protection from predators and nutrients they cannot access. Its cycle is far less affected by tillage because it is bound to the clay particles of the soil. This is why sandier soils cannot build organic matter levels that clay soils can. The percentage of clay drives organic matter retention.

Strategic tillage does not necessarily lead to long-term carbon loss (MAOM). While deep, aggressive tillage can release significant amounts of carbon (lots of POM and some MAOM), a one-time, shallow pass might redistribute layers and improve soil structure without drastically reducing soil carbon levels.

The goal here isn’t to till for the sake of tilling. (Unless you need something for Dad to do – in which case an antique tractor, an old disc, and the back 40 can be a viable option.) It is about using it as a tool – selectively and strategically – in a way that supports long-term soil health. A short-term burst of POM loss that leads to a greater accumulation of the more stable MAOM is a win. Better plant growth accumulates more MAOM, which helps with water infiltration, storage, and release, and helps to hold the soil together through better aggregation.

Here's where tillage could be a net gain to your fields:

1. Breaking up compaction

   Over time, no-till can lead to soil hardpan layers just below the depth of seeding. This can restrict root growth and water infiltration. A one-time deep ripping pass can loosen things up without completely disturbing the soil structure. Most of this disturbance is deeper down, so the soil surface stays mostly intact. Making this work is part art and part science. This science comes from measurement of the depth of the layer. A layer that has a resistance of greater than 300 psi will restrict root growth. Cover crops can break compaction, but they have their limits, too. Breaking this layer is the art.

   The greatest effect comes when the moisture is in the Goldilocks zone: The layer shatters but does not smear, and it does not just pull up large clods. If the layer is consistently at the same depth and you can keep the shanks one to two inches below it, you will likely have good success.

   However, compaction is variable, and machines are tough to set in the best of cases. A skilled operator is crucial to success. If you cannot do it right, wait another year. To prevent the soil from recompacting, a living root should be established immediately (cash or cover crop).

2. Resetting stratification

   First of all, test some shallow depths and see if you do indeed have a problem. Instead of combining the zero- to six-inch layers into one sample, divide it out into two or three samples (three-inch or two-inch layers).

   This is time-consuming and expensive, so start with a poor area rather than a random sample across the field. A slight pH/nutrient imbalance between layers might be able to be fixed with tillage.

   Test out an area that combines the layers and observe. This could be a great job for Dad and his antique discs. Don’t do a lot to start – be sure the practice is not leading to greater erosion.

   Deep banding nutrients – specifically phosphorus – can be a way to change the system over the long run. Because this disturbs the soil and takes more horsepower, a two- to four-year supply could be placed deeper. This is more expensive up-front.

   However, it may not be a bad thing to have a good supply of nutrients in the soil in case you cannot access fertilizer some year. Lime is an option when the pH has dropped, but it is often an issue of cost and physically getting the lime into the low pH zone. This is where variable rate application shines. Mapping out the areas by satellite, yield, EC (electrical conductivity), and/or topography – combined with soil testing – can help cut expenses by only applying where needed.

3. Amendment incorporation

   Manures or compost are an excellent addition to any field where they are available. If you have the opportunity, do not pass this up due to concerns of tillage. (However, if your soil test levels of phosphorus are high, then pass on this – please be sure you follow local nutrient management guidelines). Incorporating manures or compost holds onto most of the nutrients that come with it and gets it breaking down in the soil.

   This holds true for cover crops as well. While winter-killed cover crops give us an easy way to kill them, research is indicating that they are more prone to gassing off nitrous oxide in the early spring as compared to soils with only crop residue on top. Additionally, snowmelt water flowing over frozen ground can have elevated phosphorus levels due to the plant cells being burst with nowhere for the nutrients to go.

The real conversation shouldn’t be about whether tillage is good or bad; it should be about how to make informed, research-backed decisions that maintain both soil health and farm profitability.

Every field is different, and so are many areas within each field. The best management system is not about ideology – it is about what works, when it works, and how it fits into the bigger picture of long-term sustainability. BF