unstable soil versus stable soil Lynn Betts
(Left) Tilled soils without cover or aggregate-building glues from soil microbes are unstable. They “melt” during rains into gooey soil that has no pores to move air and water. (Right) Soils protected with no-till and cover crops build a stable structure with aggregates that resist breaking down with heavy rains.

Soil penalties for over-tillage

Fluffing the soil with tillage degrades soil structure and will not result in long-term benefits.

Think Different
After 35 years of research in management of chemical and physical properties of soils, Dr. John Grove has learned a good deal about the effects of tillage on soil structure and compaction. Over time, the director of the Research and Education Center for the University of Kentucky has become convinced there are fewer and fewer reasons to till. The long-term price to pay for working the soil with large equipment each year, he says, includes loss of soil aggregation, compaction below the soil surface, and a tendency for soil to crust at the surface.

 

“I think many farmers who are tilling will admit that tillage can harm soil structure,” says Dr. John Grove, a long-time soil properties researcher at the University of Kentucky. “And farmers who have soils more sensitive to soil structure damage admit this more quickly. They’re the ones who are more aggressive in trying to make up for the soil degradation—more often than not with mechanical fixes.”

While his 35 years of research have pointed to fewer and fewer reasons for tillage, Grove believes tillage continues because many farmers believe breaking up the soil creates a better seedbed. “What they need to realize is that they’ve created fluff above, and compaction below, the depth of tillage,” Grove says. “The fluffy part of that tilled soil settles. With no protection, the surface is vulnerable to crusting from hard rains, closing pore spaces that would allow air and water to enter the soil. On the other hand, crop residues from no-till shield the soil from pelting rains.”

Tactical tillage
“We need to be more tactical in deciding when and where we till,” Grove says. “If we have a weed problem that calls for tillage, then yes, we can use tillage to take care of that problem. Or maybe occasionally to loosen compaction in certain parts of a field. But we should only till when we have a good reason. Tilling because it’s what we’ve always done isn’t a good reason. Certain fields in certain situations call for tillage. Otherwise we ought to leave tillage implements parked, because there are penalties to using these tools.”

Carry a probe, penetrometer
Grove says the first of three serious problems caused by tillage is the loss of soil aggregation. Soil aggregates are groups of soil particles bound or “cemented” to each other in a way that creates pore space for air and water movement in the soil. Each time you till, you’ll cause a decline in soil aggregation, Grove says. “The larger soil particles break into smaller particles, and those small particles settle back to become more closely packed together.” That reduces pore space, and leads to two other problems, compaction below the soil surface and a tendency for soil crusting.

“Every farmer should carry a soil probe and a penetrometer to the field with them,” Grove says. “Walk over the field when the crop is growing, and soil conditions are somewhat moist, and check the soil profile at different field locations. If you get penetration resistance with the penetrometer at the depth you tilled, that tells you something—you may have created a compaction layer. You might not notice a difference in the looks of the crop or in yield every year, because in a year with good rainfall, the crop roots may be able to push through that compacted layer. But in a dry year they may not. That’s one reason I like cover crops—they create root channels for new crop roots to follow lower into the profile and help maintain soil aggregation.”

Grove knows one producer who carries a penetrometer to the field to find and map where compaction exists. Then he rips only those parts of the field. Grove also knows farmers who use cover crops in only the parts of fields that have compaction or erosion issues.

Recognize good soil structure
While he knows biological life in the soil creates good structure, Grove thinks of good soil structure in physical terms. A good test, he says, is to see how the soil breaks apart when it’s fairly dry. “If you take a spade full of soil, and it comes up in big clods, or breaks apart in big clods, you have poor soil structure. Another indication of poor structure is a plate-like appearance with horizontal layers that have been pressed together. With good soil structure, you can see the soil aggregates as soil crumbles apart,” he says.

Grove says excessively sandy, silty, or clay soils are most sensitive to degradation with tillage. “You might not think it, but some sandy soils are very easy to compact,” Grove says, “because they have particular distribution of sand particle sizes. Soils with a mixture of sand, silt and clay in them, like many in the Midwest, respond to regeneration more quickly.”

Lynn Betts

(Left) A freshly tilled and planted soil in central Iowa was loose and fluffed up after planting in early June. (Right) A day later, after just one three-quarter inch rain, the freshly tilled field was already settling, on its way to lower porosity and a tendency to crust.

Restore soil aggregation
Depending on the soil, if poor structure is at or near the surface, it can be improved within a year or two, Grove says. An aggressive structure rebuild program he would recommend includes minimizing tillage, making sure the soil is properly drained, and growing carbon with cover crops. “If you can’t grow cover crops for some reason, try to add manure, compost, or some other form of organic matter,” Grove says. “What you really need to build soil structure is a combination of less tillage and more organic matter.”

In an emergency situation where parts of the field indicate shallow compaction, Grove advises mapping out the field with a penetrometer to locate problem areas. “You could rip up the shallow pan in the fall when the soil is dry with a chisel plow in those areas,” he says. “Put the tips 2” to 3” below the depth of compaction, with a goal to shatter the soil to that depth. But then sow that area immediately to rye, wheat or some other cover crop whose roots can find their way down through that soil.”

Vertical tillage solution?
“We don’t know as much as we need to know about vertical tillage,” Grove says. “We don’t know how much structural damage vertical tillage can do. On one hand, vertical tillage straight on is the same as a drill at a 7-inch spacing. But if you run the blades at an angle at high speed, more like a disk, you should expect the same results as a disk. The more aggressively you displace the soil, the more damage you do. It may be shallow tillage, but if you expose the soil for drying, you’re losing carbon. It’s true you can cut through a shallow compaction layer, but just know that if you get 400 PSI pressure at the blade tip, you’ll get the same compaction at that point as with a disk tip that delivers 400 PSI. There’s much more to learn.”

Organic matter limits
Grove is a proponent of building healthier soils through no-till, cover crops and other methods, but cautions producers and others on trying to achieve overly high soil organic matter levels. “There’s a limit to the economic value of raising organic matter levels, and a limit to how much carbon can be stored in the soil,” Grove says. “It has a lot to do with rainfall and temperature; it can be a waste of resources to pursue extremely high OM levels. In Kentucky, where we have degraded soils, we’ve learned from soils that were mapped many years ago that we should feel good about getting back to 2.5 or 3% organic matter levels. Though a soil may be terribly degraded, organic matter concentrations just won’t be stable at higher levels, where organic matter is more easily lost.”

He also expects that poorly drained soils in the upper Midwest that were formed under more oxygen limited conditions will drift towards lower optimum organic matter levels. “There’s little research on this, and I wouldn’t know what those levels will eventually be,” Grove says, “but the oxygen that tile drainage has brought into those soils should result in a new equilibrium concentration of organic matter.”

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