Iowa farmer Jeremy Swanson lists residue management as the top priority of a fall tillage pass (with the mitigation of soil compaction from the year’s fieldwork as the second main focus). For this article, Jeremy breaks down residue management into its various components, from initial mechanical breakdown to the microbes that must go to work on your residue to make its nutrients available for next year’s crop

Growing Through God’s Design

Take the forest, the old prairies. No one was fertilizing them. And certainly no one was doing tillage. The grass would die off at the end of the year. The leaves would fall from the trees. But guess what? The trees would grow again the next year. The grass would grow again next year. What happens to all that stuff? Well, the soil biology feed on it. All of this is the way God designed it. Those systems are self-sustaining. Everything’s in balance.

From the FCLG: The idea of looking to nature for inspiration is a common theme on our Farmer Sharing Calls, where we delve into a variety of agronomy topics. Take Jeremy’s example of an old-growth forest. These ecosystems have sustained themselves, in God’s design, for hundreds of years. As farmers, the more we can mimic nature in our practices, the closer we can get to long-term profitability while also safeguarding the health of our soil for future generations.

Carbon/Nitrogen Balance Essential for Breakdown

Obviously, the type of residue you’re dealing with is going to be the main factor in how you need to deal with it. The soil biology need energy and nitrogen to be able to process the carbon in the leaves, the stalks, and any other residue. Bean stubble has far less carbon in it, so it takes less nitrogen and energy to process it. The biology is able to break down bean stubble quicker and easier and get those nutrients processed into an organic state where the plants will be able to take advantage of them. 

With corn residue, there’s so much more carbon there in sheer quantity alone. But corn stalks also have longer, harder carbon chains for the biology to break down. Hence they need more energy, more nitrogen to do so. That’s what residue management boils back down to: carbon to nitrogen ratios. 

From the FCLG: According to the USDA (Soil Tech Note 23A, if you’re keeping score), the carbon to nitrogen ratio is a quick way to evaluate the balance between two elements present in the soil that are essential for crop growth and microbial health. When organic material, such as manure, crop residue, etc., is added to the soil, the increased carbon triggers microbial growth. If the amount of nitrogen in the materials is not enough to support the increased growth of microbes, the microbes will instead absorb nitrogen from the soil. This will deprive plants of nitrogen they need for immediate growth. As carbon breaks down, larger microbe numbers decrease and nitrogen is now released again into the soil. 

Why Corn Residue is the Toughest

With the corn plant being taller, it has to build a stronger stalk to be able to hold all that weight up. Lignin is one of the tougher carbon chains in the corn stalk; you’ll also see lignin in tree trunks. It’s a really long carbon chain that gives those tree trunks strength, and it’s the same compound that ends up in the corn stalk to help it stand up tall all season long. Like I said, that’s a really long carbon chain that takes a lot of energy to break down. 

Plus, a waxy, protective layer develops on the outside of a corn stalk. Here again, that wax is a really long-chain carbon. The bugs have a hard time getting through that waxy layer and into the inner parts, the cellulose and stuff in the middle, that’s easier to digest. What you see in some of the biological products on the market today, these residue degraders, will be biology that prefer the waxy, outer layer. It’s the biology that feed on those harder compounds, such as the lignin. These products are designed to get rid of that layer so that more digestible stuff can get eaten quicker. 

Residue Breakdown: Essential for Plant Availability

Always remember that you’re dealing with a multi-stage process to go from residue to plant-available nutrients. (We touched on this earlier.) If you can get it [the nutrients] into those first few soil layers, it will start to break down into something plants can use. Your organic matter also becomes almost like a glue that helps hold the soil together and bind it to the point where it’s less likely to wash away or blow away in the wind. 

From the FCLG: A healthy soil structure is vital to protecting your farm’s profitability. Well aggregated soils have a variety of soil particle sizes with plenty of space for air and water (and beneficial organisms such as earthworms) within the soil. The structure of compacted or over-tilled soil will break down, become much finer, and be pressed into a solid “brick.” In these types of soils, wind and water erosion can become a serious problem, and the soil’s water- and nutrient-holding capacity is much decreased.

How Small You Should Size Your Residue?

The smaller, the better. This is where the modern vertical tillage machines have a slight advantage. Let’s take corn stalks again. The more times you can slice them, the more openings (or ends) you’ll have for the biology to get in. The biology can then more easily work from both the inside and outside to process that residue. If your residue is really large, as in a two-foot-long piece of corn stalk, it’s going to be much harder to break down. Why? Well, you only have two ends. The more you can chop that up, the more access points you’re giving the organisms to go to work. Chopping corn heads help a lot with this problem, too. 

There is one challenge with those finer, smaller pieces: they blow a lot easier in the wind. This is especially true if you’re really dry and don’t get any rain during fall. All of a sudden a portion of the residue could wind up in your ditch or your neighbor’s fence line. And with it goes a portion of your nutrients. 

Use Biological Products to Aid in Breakdown

There are more and more products on the market every year designed to assist with residue breakdown. I think we’re learning more and more about how to use them as we go. First, you have to have some way to manage the biology in them. A lot of the products we have available now are bacteria-based, but we also know that fungi are also extremely helpful in breakdown. Bacteria are a lot easier to work with in a jug—they’re easier to manage and keep alive. 

From what I’ve been told, fungi are really hard to manage and you can wind up with products that aren’t effective by the time you can get them onto the fields. As we learn more about formulating these products, I think that those that combine bacteria and fungi are going to be really effective. I would say, for the most part, this type of product is still in development. 

And if you start trying to apply bacteria, you have to think about a food source—sugar, molasses, even getting into types of protein. You have to think about how you’re going to get the bugs from the jug onto the field and get a colony established. Since these are liquid products, we’re going to obviously be spraying them, so you have to look at the quality of water in your spray tank. If your water has too much chlorine, calcium, magnesium, or other salts, they can kill the bugs in the spray tank before they even land on your fields.

Most water has a pH over seven. All aerobic bacteria need hydrogen energy as an energy source. I’ve tested water all over northwest Iowa, and every single water source I’ve tested has a negative oxygen level to it. So now we’re putting aerobic bugs in a thousand gallons of water that has a negative oxygen level. How long would we survive if we sucked the oxygen out of the room right now? Not very long. So we need to get the water in the spray tank right before we go adding our biological product. Like I said, we need to give them a good environment and some food so they can get established and go to work. 

Does Ammonia Application Harm Soil Biology?

Anhydrous will knock them [soil biology] out, for a period of time. But you’re also knifing that anhydrous in probably six to eight inches deep. So you’re getting it down away from where a lot of the residue management and breakdown is happening. So I don’t know that there’s necessarily a huge detrimental effect in terms of residue management. 

The other aspect that could be a positive is the physical effect of applying the anhydrous; you’re opening up the soil a bit, throwing some dirt, and letting some oxygen in. As I mentioned earlier, this can increase activity among the biology and be a benefit in terms of residue management from that perspective. That said, I don’t know that you’re going to see a drastic difference either way. I don’t know if anyone has ever tried to quantify this or not.

Looking Ahead: Spring Tillage Success

As Jeremy says, residue management and breakdown should be one of your most important considerations as you work through your fall tillage plans. In the end, your goal is a solid start for next year’s crop. In our next article in Jeremy’s tillage series, we’re going to explore his thoughts on spring tillage strategies for building a seedbed that will give your crop its best start.