Managing the Impact of Construction Compaction on Crop Land

Dick Wolkowski, Extension Soil Scientist

In the last several months I have received numerous queries about soil compaction caused by the construction of pipelines, high voltage transmission lines, highways, and wind turbines. Producers are concerned that compaction from heavy equipment will negatively affect soil quality factors such as bulk density, porosity, aggregation, and drainage; causing yield reduction that may persist for several years. While the effect will vary by soil type, soil moisture content, and vehicle load; soil compaction has been shown to have negative impact on yield in almost all situations. Where the soil has been aggressively disturbed, as it will be when a pipeline is buried, the effects can be long-lasting. Examples of compaction effects on crop yield collected from various research studies are shown in Table 1.

			Compaction load
County	Crop	Units	Minimal	Moderate	Heavy
Manitowoc	Corn	bu/a	120	103	69
Grant	Corn	bu/a	106	101	91
Winnebago	Corn	bu/a	156	152	142
Columbia	Corn	bu/a	156	--	112
Columbia	Alfalfa	ton/a	3.7	--	3.3
Waushara	Potato	cwt/a	458	--	440

Compaction has the effect of re-arranging the soil aggregates to form a denser, less porous soil. Severe compaction will destroy these aggregates further reducing porosity. The denser soil has fewer large pores to conduct water, so increased runoff and ponding in depressions are common symptoms of compaction. The smaller pores hold water tighter resulting in a Awetter@ soil that dries more slowly. Soil gas exchange is slowed and oxygen can become limiting to roots. The soil strength increases, requiring more power for tillage and impeding root growth. These problems are made worse when the soil is near field capacity as the water acts as a lubricant between soil particles. The problem is also worsened as the clay content of the soil increases.

It has been estimated that over 70 percent of the compaction effect occurs in the first pass across the field. The heavier the equipment, the more severe the effect. Factors such as increased tire size, proper inflation pressure, and the addition of tracks, duals, or tandem axles can offset some of the effect, but in many cases with very heavy equipment there will be a limited reduction in compaction. 

A treatment that is often suggested to alleviate compaction is deep tillage, more commonly known as subsoiling. If done use a subsoiler with L-shaped legs (e.g. para-plow) that lifts the soil with minimal surface disturbance. Subsoiling will loosen the soil, but it will not re-create structure or the biopores from roots and earthworm activity. These redevelop over time once the site is returned to crop production. Natural forces, such as wetting/drying, freezing/thawing, and biological activity will take many years to restore a severely compacted soil. A study conducted in western Minnesota demonstrated that the compaction caused by covered wagons in the 1880’s could still be detected over a century later. 

The best advice for managing compaction is to avoid compaction. When possible stay off wet soils, operate with lighter loads, confine traffic to existing lanes, and use tracked vehicles when possible. These recommendations are easily disregarded in the construction process. Producers should attempt to estimate a fair price to compensate for the yield loss from compaction based on the area affected, commodity prices, and other inconveniences. A yield reduction estimate of 12 to 25 % is not unreasonable. Some situation may lend themselves to comparing yield in affected and un-affected areas. Be sure to average multiple paired yield measurements to account for natural field variability.

It is recommended that producers have a clear agreement with contractors before construction begins. The agreement should outline construction particulars, and describe compensation for crop loss and responsibility for mitigating impacts. Producers should keep records of discussions with representatives of the construction companies. According to State Statutes the WDATCP is required to prepare an Agricultural Impact Statement when the potential for the exercise of eminent domain powers would result in the acquisition of more than five acres.  These documents often contain the practices that the company will use to manage post-construction compaction.