Crop Comments: Let’s stop squeezing the life out of soils

According to John Idowu, Ph.D., plant science Extension agronomist at New Mexico State University, soil compaction can greatly challenge cropland performance and reduce yields, depending on the type of crop grown. However, there are many strategies that producers can use to minimize soil compaction issues on the farm.

Compaction often occurs with heavy traffic or when equipment is operated on wet soils. With this goal in mind, growers should stay off wet fields until they are dry enough for operating equipment. Idowu recommends using a penetrometer to provide information about the depth and extent of soil compaction – information critical for management decisions.

From Delaware coast visits years ago, I remember signs posted near where vehicles were allowed beach access. Signs read “Maximum tire pressure 10 PSI.” On the beach, sand compacted from harder tires will be less able to percolate seawater downward, increasing likelihood of shoreline erosion.

In a video filmed on North Carolina’s Outer Banks, a four wheel-drive truck gets stuck, its 60 PSI tires digging deeper and deeper, unable to back away from the mess. They then reduce tire pressure to 15 PSI. The truck easily backs out of its ruts and repositions on a new surface, effortlessly negotiating the sandy slope. The tire track widths left by the two different pressures are noted: the 15 PSI track is approximately 40% wider than the 60 PSI track. This greater contact area between tire and soil surface markedly lowers soil compaction.

A well-written bulletin published by the University of Minnesota (UMN) is titled “Soil Compaction.” Compaction concerns have been growing in Minnesota as both annual precipitation and farm equipment size have dramatically increased. Wet soils are particularly susceptible to compaction. Heavy equipment and tillage implements amplify damage to soil structure, decreasing pore space, further limiting soil and water volume. Well-structured soils hold and conduct the water, nutrients and air necessary for healthy plant root activity.

Soil compaction occurs when soil particles press together, reducing the pore space between them. Heavily compacted soils contain few large pores, thus less total pore volume – and consequently, a greater density. Compacted soils have reduced rates of water infiltration and drainage, because large pores move water more effectively downward through the soil than smaller pores do. The exchange of gases slows down in compacted soils, increasing the likelihood of aeration-related problems. Compacted soils mean roots must exert greater force to penetrate the hardened layer.

UMN Extension explained away one common myth: That in temperate climates, freeze/thaw cycles alleviate most soil compaction created by machinery. Although soils in the northern U.S. are subject to annual freeze/thaw cycles (with freeze depths of three feet or more), only the top two to five inches experience more than one freeze/thaw cycle per year. The belief that freeze/thaw cycles loosen compacted soils likely developed years ago when compaction was relatively shallow, machinery weighed less and more grass and deep-rooted legumes grew in rotations. Now heavy axle loads and wet soil conditions increase compaction’s downward reach in soil profiles.

USDA-NRCS research shows respiration generally increases as soil moisture increases; however, oxygen was limited when the soil pores filled with water, interfering with the ability of soil organisms to respire. Ideal soil moisture content is reached when approximately 60% of pore space fills with water. When water fills more than 80% of pore space, soil respiration reduces to a minimum level and most aerobic microorganisms begin to use nitrate (NO3) instead of oxygen, resulting in nitrogen loss (as N2 and nitrogen oxide gas), emission of potent greenhouse gases, reduced yields and an increased need for nitrogen fertilizer.

A load of 10 tons/axle or more on wet soils extends compaction to depths of at least two feet. Because this is well below normal tillage depth, the compaction is more likely to persist, compared to shallow compaction that can largely be removed by tillage. Raindrops landing hard on bare soil cause compaction naturally, as evidenced by a soil crust – usually less than a half-inch thick at the soil surface – which may prevent seedling emergence. Fortunately, rotary hoeing usually alleviates this problem.

There are tests to measure soil oxygen, most of them doing so indirectly by assaying soil carbon dioxide (CO2). Most soil microbes behave more like animals than plants, emitting CO2, not oxygen. Much of the CO2 reacts with soil moisture to form carbonic acid, which is quantified to become a measure of soil biology. Higher levels of soil carbonic acid mean more oxygen was respired, hence more active soil life. More CO2 remaining in the soil means that plants have more building blocks with which to perform photosynthesis, and that less carbon escapes into the atmosphere as greenhouse gases CO2 and methane.

Mark Jeschke, Ph.D., agronomist at Pioneer Seeds, helps put a number face on this problem: “Farm machinery is much heavier today, creating larger axle loads.” To prove his point, Jeschke tracked down the weight of a tractor that might have been used for tillage 40 years ago vs. recent times, using Case IH models. “The average weight of the International Harvester 1086, made from 1976 to 1981 and often operated with single tires, was 12,715 pounds. The Case IH Magnum 380, made from 2014 to 2017, weighed in at 32,200 pounds. That’s 2.7 times as much weight per pass compared to the typical IH 1086.”

Research dating back to UMN in 1986 indicates that if axle weights are five tons or less, soil compaction usually stays near the surface. Axle loads greater than 10 tons penetrate the subsoil, creating harder-to-remove, deep compaction.

“One take-home message is staying off wet soils, if possible,” Jeschke said. “Do what you can to limit chances for soil compaction if you must be in the field. That may include running with tracks, wider tires or at lower tire pressure.”

by Paris Reidhead