Crop Comments: Softening Hard Soils
Northeast corn silage growers are currently about three-quarters done with chopping. Corn grain growers are about one-quarter done with combining. This is a good time to conduct a small experiment while the person doing the chopping or combining waits for an empty forage wagon or gravity wagon to be returned. Pull up two or three randomly selected still-standing plants. This test helps growers determine whether yields have been maximized, prior to actually counting loads taken from a particular field.
Several times I have done this, and have pulled up plants effortlessly, because the majority of roots were in the top three inches. This meant that the roots’ ability to probe for nutrients and water had been severely limited. For these plants, there’s less than eight functional inches of topsoil – seriously limiting yields. Most 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. Actual probing can reveal 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, severely limiting available nutrients: present, but unreachable. Intensified by bigger and bigger tractors, compacted soils serve as the figurative swinging door hitting growers from behind. We often instinctively blame bad weather, like too much or too little rain. Let’s be scientific instead, using a penetrometer — or a poor man’s penetrometer, commonly called a shovel — to feel the compaction and look at roots’ growth and patterns.
In the Upper Midwest, soil compaction receives attention from scientists explaining that this problem increasingly frustrates Minnesota farmers, as both precipitation and farm equipment size have increased in recent years. These researchers stress that wet soils are particularly susceptible to compaction.
Heavier equipment and tillage implements amplify soil structure damage, decreasing pore space, 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.
University of Minnesota (UMN) agronomists (John Lamb et al.) stress that soil compaction occurs when particles are pressed together, reducing pore space between them. Heavily compacted soils reduce rates of water infiltration and drainage. This happens because large pores move water, downward through soil, more effectively than smaller pores do.
The exchange of gases slows down in compacted soils, undermining aeration.
While soil compaction increases soil strength, compacted soils mean that roots must exert greater force to penetrate the harder layer. UMN workers challenged one prevalent soil compaction myth —prevalent in temperate climate states, that freeze/thaw cycles alleviate soil compaction created by machinery.
Soils in the northern half of the U.S. are subject to annual freeze/thaw cycles — with frost depths at least three feet, but 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. Back then 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 soil extends compaction to depths exceeding two feet. Because this is below normal tillage depth, com- paction is more likely to persist, compared to shallow compaction that’s largely removed by tillage. Raindrops landing hard on bare soil intensify compaction, as evidenced with a soil crust – usually less than half-inch thick at the surface – that may prevent seedling emergence. Often rotary hoeing alleviates this problem. UMN scientists emphasized 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 CO2 reacts with soil moisture to form carbonic acid, which can be quantified to measure soil biology. Higher levels of soil carbonic acid mean that more oxygen got respired, hence more living is taking place in soil – and less carbon escapes into the atmosphere as greenhouse gas.
Iowa State University agronomists recommend a “ribbon test” to estimate soil texture and the amount of clay in a soil. First, place a small handful of soil in your palm. If it is dry. add water, drop-wise, until it resembles modeling clay. Roll the clay into a cigar shape with a 1/2-3/4-inch diameter. Place the cigar-shaped soil between your thumb and fore-finger, and start gently pressing the “cigar” into a flat ribbon shape. As the ribbon develops, let it extend over your forefinger until it breaks from its own weight. If this soil sample does not form a ribbon, its texture is sandy. If it does form a ribbon that is less than one inch and feels gritty, the texture is sandy loam. If it’s smooth, the soil texture is silty loam. And if neither gritty nor smooth, the soil texture is loam. If you are able to form a ribbon that is 1-2 inches long, and the soil feels gritty, its texture is sandy clay loam. If smooth, the soil texture is silty clay loam, and if soil does not have either a gritty or smooth feel to it, you have clay loam. If you are able to form a ribbon that is more than 2 inches long and the soil feels gritty, its texture is sandy clay; if smooth, the soil texture is silty clay. And if the soil lacks either a gritty or smooth feel to it, the texture is clay. Pretty handy in areas lacking detailed soil type maps.
