This was the cover story of the October 2016 issue of Ag Professional.
Good soil health encompasses a lot of factors—including biological, physical and chemical— and its definition often varies based on the individual defi ning it. One thing that is certain today is farmers and retailers alike are giving the topic more attention than ever before and working to improve it.
“Anytime you introduce tillage to a field, you’re going to impact water infiltration and soil quality,” says Mark Baer, sales manager for Sun Ag, based in Tremont, Ill. “A grower wanting to improve soil health will go to minimum- or no-till, use cover crops extensively and realize it’ll be a multi-year undertaking to accomplish what he wants to achieve.” The need for more information contributed to a recently completed Farm Journal study that addresses the interactions between tillage practices and water infiltration rates and how they impact soil health and structure.
Among the lessons learned, the research documented that one year of horizontal tillage can significantly diminish the structural benefits and the water infiltration rate produced by four years of no-till.
Six treatments. Farm Journal Field Agronomist Ken Ferrie set up the study by placing moisture sensors 3" and 6" deep in a silt loam soil in a central Illinois field, following a corn/soybean rotation. Then he documented the effect of six tillage and cover crop treatments on water infiltration. See the sidebar, right, for a list of the treatments. Ferrie took his first measurements early last spring, before any spring tillage was done. “We found that the top 3" of soil was driest where we moldboard plowed in the fall,” he says. “The second driest soil in the top 3" was in the chisel-plowed area.
“At the 6" depth, the only significant moisture difference among the treatments was that the moldboard plowed ground was much wetter. That showed that the plow sole (hardpan) we had created at about 8" or 9" was holding back water, preventing it from infiltrating deeper into the soil. At the 6" depth, that soil remained the wettest all through April.”
Unexpected results. Ferrie was surprised to discover that adding the two years of cover crops to the no-till strips did little to improve water infiltration compared to four years of no-till; both infiltration rates were good. Those findings differed from an earlier study, which compared one year of no-till with one year of no-till plus an annual ryegrass cover crop.
The earlier study showed that the cover significantly improved water infiltration. “I think the infiltration rate of continuous no-till continued to improve over time because of night crawler activity and bio-channels, to the point that a cover crop had less of an impact,” he says.
Tillage had a significant impact on how much water was able to percolate into the lower level of soil.
“If your goal is to increase water infiltration, you need to know that horizontal tillage, with any type of shovel, affects the flow of water negatively,” Baer says.
In his experience, the type of tillage tool used is important. “A ‘wavy’ coulter-style blade won’t affect soil structure as much as a concaved one,” he adds.
Where spring tillage was done, Ferrie says the sensors were removed first and then reinserted afterward.
“After we ran the soil finisher, the 3" moisture sensor spiked upward with every rain,” Ferrie says. “Then the soil dried out faster than with any other treatment. These rain events registered quicker at the 6" level in every treatment except the one with the soil finisher.
“In the top 3" of soil, the soil was wettest with one pass of the soil finisher; second wettest with the chisel plow and one soil-finisher pass; and third wettest with the moldboard plow and one soil-finisher pass,” Ferrie continues. “On all three tillage treatments, the infiltration rate was higher at the surface than with no-till. But with all these tillage treatments, water movement beyond the tillage layer was delayed.
“Where we had four years of no-till, with and without a cover crop, and where we ran the chisel plow followed with vertical tillage in the spring, the moisture moved uniformly from the surface down to the 6" level. There was no change in the soil’s bulk density to delay the water’s movement.
“One pass with a soil finisher caused the biggest delay. Some of the water never made it down; it ran off or evaporated.”
Moisture loss. A dry June showed the importance of early-season infiltration. During that month, the rate of evapotranspiration was almost ½" per day—equal to a drought that ravaged the area in 2012.
“The corn began showing signs of stress about July 4,” Ferrie says. At that time, Ferrie began to see differences in the lower portion of the soil. “As time went by, we could visually see the difference between four years of no-till and one pass with the soil finisher. Where we ran the soil finisher, from 4" on down, the soil began to collapse into the massive state, which is what this type of soil does when it dries out.
“We lost the healthy crumb structure we had developed with no-till. The soil looked like it had received 4" less water than the no-till soil next to it,” Ferrie notes. The soil finisher had changed the infiltration rate so water was not penetrating 6" to 8" deep. Instead of penetrating, 4" of rainfall had run off or evaporated.
The 4" of water that failed to infiltrate because tillage created a soil density change could be very important in a dry growing season. An inch of water in an acre of soil is 27,000 gal. So 4" of water is 108,000 gal. It takes 3,000 gal. of water to produce a bushel of corn. So 4" of water is enough to produce 36 bu. of corn (108,000 divided by 3,000) if water is the limiting factor.
Ferrie’s assistant, Thomas Zerebny, ran some calculations based on average rainfall during the growing season for the last 30 years at the study location—24.28". He determined that 75% of that rain fell at a rate of 3" or less per hour and 47% at 1.7" per hour or less.
“If the soil’s infiltration rate is 3" or more per hour, the soil can capture
75% of that 24.28" of rain,” Ferrie says. “But if the soil’s infiltration rate falls to 1.7" per hour because you have put in a tillage layer, only 47% of that 24.28" of rain will enter the soil.
“If we get 24.28" of rain during the growing season, soil that can infiltrate 3" per hour takes in 18.2" (because 75% of the rain falls at a rate of 3" per hour or less). But soils that infiltrate only 1.7" per hour absorb only 11.4", and the rest runs off.
That’s 6.8 fewer inches of water for the crop. And 6.8" of soil water equals 183,600 gal. (at 27,000 gal. per inch), or enough water to produce 61 bu. of corn (at 3,000 gal. per bu.).
Lessons learned. Ferrie was surprised by how many things the study taught him. “I hadn’t fully realized how much moisture a plow sole holds up, preventing the water from percolating deeper into the soil,” he says. “It’s deceiving to look at because the surface looks drier and the soil works drier. But moisture is sitting on that plow sole just beneath the surface. This is one reason we fight wheel track compaction in moldboard plowed ground.”
Ferrie was impressed by how much four years of no-till, through increased biological activity and root bio-channels, influenced and improved the downward movement of moisture—and shocked by how much one pass of a soil finisher reduced that improvement.
“The study suggested that if we have to leave no-till for some reason, and return to a spring horizontal tillage system, we should do fall tillage so we don’t create such a dramatic change in bulk density,” he says.
In summary, the study reinforced the value of continuous no-till to improve soil structure and infiltration. This success was possible because the field had been placed into a vertical format, with horizontal layers removed.
“No-tilling over existing soil-density layers makes it harder to be successful,” Ferrie notes. “The only exception is if your Highly Erodible Land conservation plan forbids tillage under any circumstances. In that situation, cover crops may help remove the dense layers.” If a time comes that a farmer needs to do tillage in a long-term no-till field, Ferrie advises caution.
“If forced to repair ruts or other problems, be very careful not to erase years of soil improvement,” he concludes. “If you must go back to tillage for a year or two, keep the tillage in a vertical format so you can go back to no-till after the problem is fixed.”