Issue No. 1, Article 4/March 26, 2010
Handling Untilled Fields of Cornstalks
This article was originally published as an electronic Update on March 5, 2010.
Late planting, cool summertime temperatures, and a wet October together made the 2009 harvest one of the latest on record. About 50% of the Illinois corn crop remained to be harvested in mid-November.
Many producers harvested in December, January, and even February, and some fields are yet to be harvested in early March. Remarkably, most are still standing quite well, and the moisture is finally down below 30%. While we hope that "field storage" doesn't become normal, it has worked better this year than it will in most years.
Here are some questions and answers about soil conditions, tillage, and crop residue as we head into the spring of 2010:
1. What are we facing in fields of untilled corn stalks?
Much of the corn was planted into wet soils in 2009, creating a considerable amount of compaction. When this happens we typically try to do deep tillage in the fall after harvest, such as ripping to a depth of 12 inches or more, in order to shatter soils at that depth and restore some air space in the soil that compaction removes. With the wet fall, most were not able to do this, or to do it effectively, so much compaction remains. In addition, many fields had ruts cut into them during harvest. Finally, of course, cornstalks, in many cases large amounts of cornstalks from a large harvest, remain in the field, insulating the soil and slowing rates of drying.
2. Won't freezing and thawing relieve compaction?
Freezing causes ice crystals to form and expand in soil pores, thus increasing the pore space and decreasing compaction. Each freeze-thaw cycle decreases compaction a little more, which is why we often see a "spongy" soil surface in the spring. But at depths of 6 inches or more, soils do not freeze and thaw repeatedly, but usually only once or twice. This winter the soil has remained frozen much of the time, with no significant thaws since November. So we cannot expect much relief of deeper compaction from natural causes.
3. Can't we do "primary" tillage in the spring?
We can do tillage in the spring, but it is not very likely that there will be good conditions, such as those we usually get in the fall, for the soil to be dry enough to get the shattering that it takes to make tillage effective at deeper depths. Soils dry extremely slowly over the winter: there is no crop present to remove water through roots, soil temperatures are low, which slows evaporation, and many fields still have cornstalks present, which insulate the soil and cause even slower evaporation. The soil is thus unlikely to be dry enough at the depth to which we normally do primary tillage.
4. Couldn't things turn around quickly?
Our hopes for an unusually early warm-up and the faster drying that would come with it are fading quickly as we move into March with below-normal temperatures. So it's not hopeless, but it remains unlikely that soils will be dry enough to allow effective tillage well before we want to start planting. Soils don't dry rapidly until soil temperatures move up into the 50s and 60s, usually in April or even May.
5. What do we do if it stays wet through March?
It will be helpful to find ways to at least disturb the soil surface and to cut and move, and perhaps bury, some of the residue. Disturbing the surface soil helps it to dry out, and it also interrupts many of the capillary connections that keep water moving up to the surface around soil particles, which provides the "mulching" effect. The downside of this is that it slows the drying rate deeper in the soil. But that may be necessary if it allows us to plant earlier. Moving some of the residue off the surface can also help drying rates, but a major reason for doing this is to physically make it easier to get seed placement that will encourage uniform and rapid emergence.
6. What tillage implements do make sense in the spring?
It is unlikely that a field cultivator or similar implement will be able to get through standing cornstalks. Some have suggested using a chisel plow, perhaps set to run only 6 inches deep or so. That could work, but the points are usually spaced farther apart than is ideal. Using sweeps instead of chisel points can help, but these will not break up and bury residue very well. Disk-rippers may work better due to the disk blades, but they will not be ideal. It may seem like a throwback, but the offset disk harrow might work better than most alternatives to do shallow tillage of cornstalks. This would not be the heavy disk used for primary tillage, but the "finishing" disk that was once much more common than it is now.
7. Doesn't spring disking produce disastrous compaction?
The disk harrow's reputation as a compaction-producing implement is not very well deserved. Any implement run to a certain depth can relieve compaction only to that depth. In the case of a disk or field cultivator run to a depth of 3 to 5 inches, it is easy to find the "compacted layer" left at that depth and to blame the implement. But it is heavy equipment that produces serious compaction. The only real "blame" that a relatively light tillage implement earns is by being run shallow, making the break between the tilled and untilled soil easy to find. It is often possible to find roots that reach this untilled layer and then grow sideways some, especially if soils are dry. This can produce the "pancake" roots that are often used to demonstrate the drawbacks to this type of tillage.
8. How about "vertical" tillage?
The term "vertical tillage" is a confusing one, applied both to deep tillage (usually in bands where the rows are then planted) and to the shallow-tillage implements that have appeared over the past decade or so. These shallow-tillage implements are typically run at high speeds (often about 10 mph) and consist of rolling blades that chop stalks and cut into the soil, ripple or wavy coulters, rolling spikes of some sort, and in some cases leveling boards or blades. The main advantage given for using these instead of a disk harrow is that they can be run faster and they do not produce a distinct break between tilled and untilled soil like the disk or field cultivator might. If the surface is dry and relatively level, these implements can do a good job of breaking residue and improving the conditions of the seedbed.
9. Can we do strip tillage in the spring?
Very little strip tillage got done in the fall of 2009, for obvious reasons. It is unlikely that leaving the tillage points (knives, usually) on will work well this spring, due to wet soils. It might in some cases work to take the knives/points off and use only the coulters and berm-builders (disks) to move some residue off the row and to do some shallow tillage. Soil in the berms should be loosened to a depth of 3 inches or more to provide a place for seed placement. Berms may not melt down much in the spring, so they should be formed ready to plant. Use rolling baskets if they help. Anhydrous ammonia should not be placed into wet soils during a spring strip-till operation, as it will stay concentrated and may well move back toward the roots if the soil dries out.
10. What about ruts from harvest?
Ruts will be one of the bigger challenges in many fields. To make a good seedbed to plant into, we have to find a way to fill in ruts completely, without leaving pockets that interfere with seeding uniformity. "Rut-mending" is not a very well-developed art form, and some creativity might be needed. In some fields it may be possible to rig up trash movers or disk blades to pull soil into the ruts before tillage is done. Remember that soils that were wet enough to form ruts at harvest probably did not experience serious compaction, since soils need to contain air in order to be compacted, and saturated soils contain little air. Of course, soils that were compacted before harvest likely remain compacted. In the majority of cases, tillage might be needed to do a good job of filling ruts. If the field is a no-till field, we can reasonably deem such an operation to be "leveling" rather than "tillage."
11. Can we do something to make cornstalks break down quickly?
We can do nothing to increase the rate of microbial breakdown, which will be slow until soil temperatures are back in the 60s and 70s. Biological breakdown is typically limited by low temperature and sometimes by low moisture supply, but seldom by lack of nitrogen or lack of microbes. The late maturity and cool temperatures after harvest last fall meant very little stalk breakdown in the fall, and with the stalks "in the freezer" since then, they remain intact in many fields. This will be a challenge, and in most cases the only sure way to get stalks under control to allow planting will be to break them up with tillage operations, assisted by trash movers at planting to move them out of the way. This could be a special challenge in fields where stalk-chopping corn head attachments were used, leaving a mat of chopped material on the surface. This material will at least be easy to move because it's not in long, heavy pieces, but it will also insulate very well if left undisturbed. Likewise, chopping stalks in the spring will help make them easier to move but will also turn them into better insulation.
12. Can't we fix compaction?
It's hard to accept, but it's likely that we will simply need to live with most of the compaction now in fields and to hope that we can do a good job of deep tillage this fall to relieve it. Unlikely as it seems, there can sometimes be a slight upside to compaction: it improves capillary connections and so can help water move through the soil from deeper in the profile to the roots. In most cases, though, the much bigger downside is that compaction restricts root growth and makes the crop more vulnerable to dry periods.
13. What about nitrogen?
In many cases it may be better to plant as soon as it's ready and to apply N after planting, especially if we're using anhydrous ammonia and it stays wet into late April. If it dries up enough to allow us to do a good job of secondary tillage, it might be possible to broadcast N and work it in. The large amounts of residue that we'll likely be dealing with will make it riskier to apply urea or UAN after planting, especially if there is a period of dry weather. Residue contains a considerable amount of urease, which can break down urea and release the N into the air. Urease inhibitor can be added to fertilizer if it's necessary to surface-apply urea or UAN.
14. Will soybean be a better option than corn following cornstalks now?
It is easier to establish an adequate stand of soybeans with little or no tillage of cornstalks than it is to establish a good corn stand. Soybeans also suffer less yield loss with planting delays than corn. One problem that will be common to both crops is ruts; it will be difficult to plant soybeans with little or no tillage in rutted fields. With less wheat in the field there will be more total corn and soybean acres, and a considerable number of corn acres will follow corn. Some may line up seed of both crops just in case, and switch to soybean if the spring starts to resemble the spring of 2009.
We'll remain optimistic that the weather will warm and turn dry and that we'll get into the fields by early April. Even if that happens, we will need to use skill and some luck in getting the corn crop off to a great start in 2010 because of what happened in 2009.--Emerson Nafziger