But how much will you reduce the infiltration rate if you run a tillage tool through well-structured soil just one time? And how much will a cover crop improve infiltration if soil already has a healthy crumb-like structure?
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 the six tillage and cover crop treatments on water infiltration below:
- Four years of no-till
- Four years of no-till, including a cereal rye cover crop the past two years
- Three years of no-till followed by a soil finisher run about 4" deep after the third year
- Three years of no-till followed by moldboard plowing in fall 2014 and a soil finisher in spring 2015
- Three years of no-till followed by a chisel plow in fall 2014 and a soil finisher in spring 2015
- Three years of no-till followed by a chisel plow in fall 2014 and vertical tillage in spring 2015
“No-tilling over existing soil density layers can hinder success,” Ferrie says.
“The exception is if your conservation plan for highly erodible land forbids tillage. If so, cover crops may help remove the dense layers,” he adds.
“If you’re forced to do some tillage for a year or two to repair ruts or other problems, keep the tillage in a vertical format so you can return to no-till after the problem is fixed, Ferrie says.
Building on the Systems Approach, the Soil Health series will detail the chemical, physical and biological components of soil and how to give your crop a fighting chance.
“I think the infiltration rate of continuous no-till continued to improve over time because of night crawler activity and biochannels, to the point a cover crop had less of an impact,” he says.
Before termination, soil in the cover crop strips was noticeably drier in the top few inches than the no-till. But after the cover crop was killed, soil in the top 3" was wetter than in no-till because the cover crop transpired water while it was alive, Ferrie says. How Did Tillage Affect Water Infiltration? In early spring 2015, before any tillage was done, Ferrie took his first measurements to establish a baseline for water infiltration:
- The top 3" of soil was driest where they ran the moldboard plow the previous fall. 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 the moldboard plowed ground was considerably wetter. That showed the plow sole created at 8" or 9" was holding back water, preventing it from infiltrating deeper. At the 6" depth, that soil remained the wettest through April.
- After running the soil finisher, the 3" moisture sensor spiked upward with every rain. 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 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.
- 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,” Ferrie says. 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 ran off or evaporated.
The three ears on the left were grown after three years of no-till. The other ears were grown in soil that had been no-tilled for two years, followed by one soil finisher pass the third spring. The difference in ear fill likely resulted from more water infiltrating the no- till soil.
At that time, Ferrie began to 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. 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.”
Ferrie was also impressed by how much four years of no-till, through increased biological activity and root biochannels, improved the downward movement of moisture—and shocked by how much one pass of a soil finisher reduced that improvement.
“If we have to leave no-till and return to a spring horizontal tillage system, we should do fall tillage to avoid creating a dramatic change in bulk density,” he says.
Interestingly, while chisel plowing followed by vertical tillage in the spring caused water to move downward faster and more uniformly, it did not stand out significantly compared with four years of no-till.
During a dry June, soil from 4" on down retained more moisture under four years of no-till (above right) than it did where a soil finisher was used after three years of no-till (above left). The soil where the tillage tool had been run lost its healthy crumb structure as it began to collapse into a massive, or blocky, state—a typical response of this soil type to dry conditions. It’s interesting to note these soil pits were only 20' apart.
The 4" of water that failed to infiltrate because tillage created a soil density change could be important in a dry growing season.
“An inch of water in an acre of soil is 27,000 gal.,” Ferrie explains. “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.”
Water-holding capacity and the amount of rainfall determine crop yield. But if water can’t percolate into the crop’s root zone, rainfall won’t be very effective.
Ferrie’s assistant Thomas Zerebny ran some calculations based on average rainfall during the growing season for the past 30 years at the study location—24.28". He determined 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 of 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 a decrease of 6.8" of water for the crop, which equals 183,600 gal. (at 27,000 gal. per inch), or enough water to produce 61 bu. of corn (at 3,000 gal. per bushel).
“If it rains enough, the soil’s infiltration rate doesn’t matter,” Ferrie notes. “But this water is insurance in case it doesn’t.”
More residue on the soil surface in continuous no-till conditions (below left), as opposed to three years of no-till followed by a pass with a soil finisher (below right), prevented the surface from sealing and allowed more water to infiltrate.