Before long, most corn growers will be combining and/or row-chopping their crop. This will be a good time to conduct a small experiment while the person doing the chopping (or combining) waits for an empty wagon to be returned. Pull up two or three randomly selected still-standing plants. This simple test helps the grower determine whether yields have been maximized prior to counting number of loads taken from a particular field.
Several times I have done this and I have pulled up plants with almost no effort, because the majority of roots were in the top three inches. This meant that the roots’ ability to forage for nutrients and water has been severely limited. For these plants, there’s no functioning eight inches of topsoil – a fact which certainly limits yields.
Many agronomists believe that autumn soil sampling is the best time to maximize fertilizer inputs smartly and effectively. There’s something besides whatever the lab tells us that’s present or lacking in those two or three ounces of extracted earth. The actual probing often reveals soil limitations as the instrument hits a compacted layer or can’t go in the ground, period.
Soil compaction impacts root depth and available water. It can severely limit the available nutrients. They are there, but unreachable. As farms and tractor sizes get bigger – and growers are obsessed with a “we have to get this done now” attitude – compacted soils will come up and bite them in the backside. We often blame bad weather, such as too much or too little rain. It’s a good idea to use a penetrometer (or a poor man’s penetrometer, otherwise known as a shovel) to feel the compaction and to look at roots’ growth and patterns.
Two years ago, I watched a short video that was filmed on the Atlantic’s Piedmont coast. In that mini movie, a four-wheel-drive pick-up truck gets stuck, with its 60 PSI tires burrowing deeper into the surface of a gently sloping sand bank, unable to even back away from the mess. The narrator then reduces the truck’s tire pressure to 15 PSI. At that point the truck is easily backed out of its ruts, is repositioned on a new surface and effortlessly negotiates the sandy slope.
He 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 meeting 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.
In the Upper Midwest, soil compaction receives attention from University of Minnesota (UMN) agronomists. In their Extension bulletin titled “Soil Compaction,” these workers do the subject justice. Soil compaction concerns have been growing in Minnesota, as both precipitation and farm equipment size have increased in recent years. These researchers stress that wet soils are particularly susceptible to compaction. Moreover, that 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.
UMN 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 – and consequently greater density. A compacted soil has a reduced rate of both water infiltration and drainage. This happens because large pores are more effective than smaller pores at moving water downward through the soil.
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, a compacted soil also means that roots must exert greater force to penetrate the harder layer.
UMN workers explained away one prevalent 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 frost 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 depth in the soil profile.
For example, a load of 10 tons/axle or more on wet soils can extend compaction to depths of at least two feet. Because this is below the depth of normal tillage, 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 half-inch thick at the surface – that may prevent seedling emergence. Fortunately, rotary hoeing can often alleviate this problem.
The UMN scientists also pointed out that the increasingly common minimized crop rotations spawn two unfortunate side effects. First, 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.
To assess the damage caused by soil compaction, there are tests to measure soil oxygen. Most of them do so indirectly by measuring carbon dioxide (CO2). 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 – and less carbon escapes into the atmosphere as a greenhouse gas.