How soil compaction impacts fertilizer decisions

Image credit: Paul McDivitt/University of Minnesota Extension

By: Extension specialist Dan Kaiser, Extension educator Brad Carlson, and UMN Soil scientist Jeff Vetsch

Another wet fall means that harvesting crops from saturated fields could lead to compaction issues.

Many farmers believe the old adage that nothing you do in the fall can hurt the soil because the freeze-and-thaw cycle of winter will even everything out before the next growing season. While this may have been largely true 50 years ago, when equipment was small and light, modern equipment can cause long-lasting problems through traffic or tillage when the soil is too wet.

Should you be concerned about soil compaction when making fertilizer decisions?

It depends. The uptake of plant nutrients relies on a healthy, actively-growing plant root system. Compaction creates barriers that limit root growth, impacting the uptake of nutrients and limiting yield. There are two big-picture issues. First, are you fertilizing a crop with limited potential because of its growing environment? Second, are you applying fertilizer to a field that has an increased risk of loss?

Most nutrients with limited mobility in the soil move via diffusion through soil water. Compaction makes the soil less porous, which could impact the movement of some nutrients. Less porous soil also interferes with drainage, causing extended periods of saturation, leading to nitrogen (N) loss via denitrification. Decreased root growth and possible increased denitrification are two reasons we have seen more yellowing at the tops of plants late in the growing season the last two years.

Image credit: Jodi DeJong-Hughes/University of Minnesota Extension

Can anything be done to increase yield with fertilizer application in compacted soil?

Data from Wisconsin has shown that adding potassium (K) to the soil increases alfalfa yield. Other data have shown a similar link between increased yield and higher levels of magnesium (Mg) in compacted situations. High levels of immobile nutrients like K and Mg likely help maintain uptake if root growth in the soil is limited, but the University of Minnesota does not have specific research-based recommendations for this.

Could banding or applying higher rates of nutrients help increase yield in compacted soil?

Probably not. Banding can increase availability of nutrients in a small soil volume, but it is not the sole answer to increasing yield. Similarly, high rates of nutrients may increase yield by making more available to the limited plant roots, but it’s important to remember that you are fertilizing a crop that is unlikely to reach optimum yield, putting the economics of this in doubt.

One method of avoiding fertilizer loss due to compaction issues is delaying fertilizer application until the field is in better condition. A field with compaction problems is likely to stay wet longer, so applying anhydrous ammonia in early November could lead to significant N loss via denitrification if spring is prolonged or excessively wet. In this case, broadcasting urea in early April decreases the likelihood of N loss. Additionally, a saturated field has a higher likelihood of runoff, meaning there could be a problem with dissolved phosphorus in the runoff water, giving an advantage to later applied fertilizer.

University of Minnesota research has shown that severe compaction can impact yield for many years. Ultimately, the best plan is to try to avoid severe compaction at all costs. This may test your patience at the moment, but your crops will thank you for years to come.

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Support for this project was provided in part by the Agricultural Fertilizer Research & Education Council (AFREC).

This article was first published in October 2019.