Soil Compaction: Agronomic and Environmental Foe

In the summer of 2008, after wet weather in much of the central United States, soils began to dry, and farmers felt an urgent need to get in the field as quickly as possible to prepare soils and plant as the optimum planting window narrowed. As a result, some soils may have been tilled at moisture levels that were prime for increased compaction at the bottom of the implement’s depth of travel. Soil compaction may have also increased more than normal beneath tractor wheels and the tracks of heavy fertilizer, herbicide and seed tender machinery.

Soil compaction is like a silent thief whose robbery is not discovered until the symptoms of damage are severe. Compaction increases soil bulk density, decreases soil porosity (especially the large or macropores), lowers the total water-holding capacity, lowers the plant-available water capacity, and causes significant resistance to root penetration and elongation. It can severely limit soil infiltration of rainfall and irrigation water and contribute to increased runoff losses. A close examination root systems following a wet season can expose yield-robbing soil compaction problems. Look for “flat-bottomed” root patterns in monocots like corn and sorghum, or J- or L- shaped taproots in dicots like soybean and cotton. As summer heat becomes more intense, in the periods between rain showers certain plants may begin to wilt in some field areas quicker than in others, hinting at compaction problems in those spots. Crops like corn and sorghum may roll their leaves in response to drought stress, and in crops like soybean and cotton, the plants’ flowers, pods and young bolls may abort excessively.

When it’s too late prevent soil compaction, knowing what to look for can help farmers disrupt it. After a wet spring, keep an especially watchful eye on your crops, both above-ground and below-ground. You may recognize soil compaction problems that, one, limit crop yields; two, decrease N and other nutrient use efficiency; and three, increase the risk of N₂O emissions. Once you identify the problem areas, you can develop a deep chisel tillage or shallow subsoiling strategy to disrupt the compaction in the fall, when soils are dry and most responsive to this tillage practice. While university and USDA research has shown that usually no benefit comes from tilling any deeper than an inch or two beneath the depth of the surface soil compaction, opinions on maximum tilling mitigation depths vary. Most recommend no deeper than 6 to 9 inches, depending on the specific tillage implement or equipment traffic pattern. Another option is zone or strip tillage that may, in the long term, help prevent large areas of your fields from being damaged by soil compaction.

Besides limiting yields, soil compaction has also been identified as a key factor that aggravates or increases the soil emission of nitrous oxide (N₂O), a potent greenhouse gas. Because soil compaction results in lower soil oxygen status, reduced root growth rates and reduced nutrient absorption rates, any nitrate present in the surface soil under warm and wet to near-saturated conditions (and which is not rapidly absorbed by roots) can be quickly converted by certain soil microorganisms to N₂O. Even mild compaction can increase N₂O emissions by more than 20 percent.

Consult your extension agent, certified crop adviser, or other agronomic professionals about ways to remedy and to limit soil compaction. Be ready this fall to eliminate this factor from the list of things that could hurt crop production, your profits and the environment next season. Good soil management provides both agronomic and environmental benefits.

Source: Dr. Clifford S. Snyder, Nitrogen Program Director, P.O. Drawer 2440, Conway, AR 2033-2440, International Plant Nutrition Institute (IPNI)

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