Gasping for Air

Gasping for AirThere’s a cute commercial on TV in which an all-wheel-drive SUV pulls a landscape gravel rake across an oceanside sandy beach. The rake is shown gathering huge quantities of litter, mostly bottles and cans. The now-clean stretch of beach is shown with swarms of baby sea turtles “winging” their way to the surf, unimpeded by debris abandoned by humans. I found interesting that the tires on this vehicle appeared to be at full inflation. Not having been to the ocean in quite some time, I still remember signs posted near where vehicles were permitted to gain beach access. Those signs read “Maximum tire pressure 10 PSI.”

Most likely the main reason for reducing tire pressure on the beach is that sand compacted from harder tires will be less able to percolate draining seawater downwards, thus increasing the likelihood of shore line erosion. Another reason will be that such hardened sand makes it more difficult for female sea turtles to dig into the sand and lay their eggs. A third reason for reduced tire pressure on beach sand is that there is less likelihood of vehicles miring, as hard tires burrow into the sand.

One video clip I watched took place on North Carolina’s Outer Banks. In that video, a four-wheel-drive truck gets stuck, with 60 PSI tires digging deeper and deeper into the surface of a gently sloping sand bank, unable to even back away from the mess. The narrator then reduces the tire pressure to 15 PSI. At that point the truck is easily backed out of its ruts, gets repositioned on a new surface and effortless negotiates 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. The additional surface contact – caused by more rubber hitting the sand – wonderfully improved the truck’s mobility. Another point that the narrator makes is that this greater contact area between tire and soil surface markedly lowers soil compaction. This is something which the egg-laden mama sea turtles appreciate.

Agronomically, the subject of compaction now receives well-deserved attention. Case in point: a well-written Cooperative Extension bulletin published by the University of Minnesota (UM) simply titled “Soil Compaction.” Soil 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. Improving soil structure is the best defense against soil compaction. Well-structured soils hold and conduct the water, nutrients and air necessary for healthy plant root activity.

UM agronomists defined soil compaction as taking place when soil particles are pressed together, reducing pore space between them. Heavily compacted soils contain few large pores and less total pore volume – consequently, a greater density. A compacted soil has a reduced rate of both water infiltration and drainage. This happens because large pores more effectively move water downward through the soil than smaller pores. In addition, the exchange of gases slows down in compacted soils, increasing the likelihood of aeration-related problems. Finally, while soil compaction increases soil strength (the ability of soil to resist being moved by an applied force), a compacted soil also means roots must exert greater force to penetrate the compacted layer.

UM Extension explained away one common soil compaction myth, commonly believed in temperate climate states – that freeze-thaw cycles alleviate most soil compaction created by machinery. Although soils in the northern half of the continental 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 may have developed years ago when compaction was relatively shallow. At that time, machinery weighed less and more grass and deep-rooted legumes were grown in crop rotations. The combination of heavy axle loads and wet soil conditions increases compaction’s depth in the soil profile.

For example, a load of 10 tons per axle or more on wet soils can extend compaction to depths of two feet or more. Because this is well below the depth of normal tillage, the compaction is more likely to persist, compared to shallow compaction that can largely be removed by tillage. Raindrops landing hard on bare soil are a natural cause of compaction, evidenced with a soil crust – usually less than a half-inch thick at the soil surface – that may prevent seedling emergence. The good news is that rotary hoeing can often alleviate this problem. The UM writers also point out that the increasingly common minimized crop rotations spawn two unfortunate side effects. The first is limiting different rooting systems and their beneficial effects heightens subsoil compaction. Second, there is increased potential for compaction early in the cropping season, due to more tillage activity and field traffic.

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, in that they omit CO2, not oxygen. Much of the CO2 reacts with soil moisture to form carbonic acid, which can be quantified to become a measure of soil biology. Higher levels of soil carbonic acid mean that more oxygen got respired, hence more living is taking place in the soil. More CO2 remaining in the soil means that plants have more building blocks with which to perform photosynthesis. It also means that less carbon escaped into the atmosphere as greenhouse gases CO2 and methane. Many weeds also give growers a test on 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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