Two years ago, a TV commercial aired in which an all-wheel-drive SUV pulled a landscape gravel rake across an oceanside sandy beach, gathering huge quantities of litter. The now-clean beach was shown with swarms of baby sea turtles “winging” their way surf-ward, no longer impeded by human debris. The vehicle’s tires appeared fully inflated.
From shore visits years ago, I remember signs posted near where vehicles were allowed beach access. Signs read “Maximum tire pressure 10 PSI.” The main reason for reducing tire pressure on the beach is that sand compacted from harder tires will be less able to percolate seawater downward, increasing the likelihood of shoreline erosion. Such hardened sand makes it more difficult for female sea turtles to dig and lay their eggs.
In one video clip filmed on North Carolina’s Outer Banks, a 4WD truck gets stuck, with 60 PSI tires digging deeper and deeper, unable to even back away from the mess. The narrator then reduces tire pressure to 15 PSI. Then the truck easily backs out of its ruts and repositions on a new surface, effortlessly negotiating the sandy slope. He then points out the tire track widths left by the two different pressures: 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 fact which egg-laden mama sea turtles appreciate.
A well-written Cooperative Extension bulletin published by the University of Minnesota (UM) is simply 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 more effectively move water downward through the soil than do smaller pores. 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.
UM Extension explained away one common soil compaction myth: 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 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 do 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 escaped into the atmosphere as greenhouse gases CO2 and methane.
Some weeds help growers score soil biology. For example, nutsedge and fall panicum are encouraged by the presence of anaerobic (low- or no-oxygen) soil environments. These weeds thank crop people for compacting oxygen away. Crops don’t share that gratitude.
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