Can I till and maintain (or even build) soil organic matter

Old ideas die hard. Especially ideas explaining processes or events we cannot easily see with our own eyes. For example, the idea of a sun-centered solar system originated in 1530 by Nicolas Copernicus. The concept was hotly debated for over a hundred years. Yep, a hundred years. Even after Galileo used his telescope to prove Copernicus correct with visual evidence, it took still another hundred years for the idea of an earth-centered solar system to finally fall into the dustbin of outdated and incorrect concepts. Like I said, old ideas die hard.

The practice of tillage appears to be on a similar path. For generations, farmers believed tillage improved soil organic matter levels. Intuitively, it made sense. Decomposing plant residue is a big source of soil organic matter, so getting more of that plant residue into the soil should increase soil organic matter. But much like observers before Copernicus and Galileo who were only looking at the sun and stars from one perspective, so too did early soil scientists and farmers look at only one side of the soil organic matter equation. Yes, more plant residue will help build soil organic matter. But, what happens to existing soil organic matter when tillage occurs? Microbes in the soil, now exposed to oxygen, go on a feeding frenzy, digesting existing soil organic matter and, in the process, releasing nutrients that quickly are lost without readily available growing plants to take up those nutrients. The process of tillage is almost always a net loss in soil organic matter, regardless of soil type, when we consider the loss through decomposition. Throw in soil erosion that intensifies with tillage, and soil organic matter loss can quickly get out of hand. 

Still, growers may be wondering just how much SOM can be gained or removed through various practices? For example, if a grower uses tillage only before corn and no-tills their soybeans, will their soil organic matter levels rise or fall over time? Same with adding manure? How about adding cover crops? Growers can generate a thousand different combinations of practices that can impact soil organic matter trends. But here are some general rules of thumb:

These practices will definitely build soil organic matter:

• Continuous no-till with some cover crops and/or some manure added periodically
• Perennial crops in the rotation with some manure added 

Likely to slowly build soil organic matter:

• Perennial crops with annual crops in the rotation with tillage for the annual crops

Likely to slowly lose soil organic matter:

• No-till with residue removal
• Some tillage with a high biomass crop (such as corn) and low biomass crop (such as soybeans) 

Will definitely lose soil organic matter:

• Fall and spring tillage every year
• Annual tillage with crop residue removal
• Crop rotation with 50% low biomass crops and periodic tillage

Put another way, the more you add cover crops, perennial crops, or manure, the more tillage a grower can do while still protecting or building soil organic matter. Failure to add these other practices will result in a decline of soil organic matter. Removing residue acts like another tillage pass in that the net impact is a decline in soil organic matter. 

To fully answer the question, yes, you can till, but growers must offset tillage with soil protecting and building practices to offset the soil organic matter loss caused by tillage. Many of the practices that build soil organic matter also improve carbon sequestration.

By Ryan Stockwell, Grower Engagement Senior Manager and Wisconsin farmer