MISSISSIPPI STATE -- Fields that appear lush and green from the highway may be deceiving: Plant roots could be struggling to grow and find resources because of underground soil compaction.

Compacted soil has usually been compressed when equipment travels over it, forming a dense layer somewhere below the surface. The depth of this layer and its thickness depend on a variety of factors, including soil texture, moisture, organic matter and past use.

Larry Oldham, soils specialist with the Mississippi State University Extension Service, said compacted soil restricts the effective rooting depth of plants.

“The roots of plants are designed to grow down through soil to anchor the plant and access nutrients and water stored in the soil,” Oldham said. “When the roots hit dense regions that they do not have the strength to push through, they grow laterally. This restricts the volume of soil that the plants can mine for soil moisture, nutrients and air.”

Plants growing in compacted soil are prone to moisture stress and nutrient deficiencies.

The problem is most pronounced in soils where crops are grown. Oldham said corn, rice and other grass crops with fibrous root systems can reach several feet into the soil when unrestricted. Soybeans and cotton form deep tap roots in healthy situations.

“When you compact the soil, it is much more difficult for the roots to expand their growth in the soil,” Oldham said. “You get to the point in the summer that the root systems go down 8 to 10 inches and take a hard right or left turn when they hit the compacted soil layer.”

Soil compaction can persist and become more severe when heavy equipment continues to move across the soil surface and when intensive, shallow tillage practices such as disking are used frequently.

Billy Kingery, an agronomist working in soil science with the Mississippi Agricultural and Forestry Experiment Station, said soil compaction is a significant problem in both the Delta and hill areas and has many facets.

“Below the compacted layer, soil is often dry even after a heavy rain,” Kingery said. “All the pore spaces between soil particles close and water just can’t get through.”

Organic matter and organisms in the soil bind these particles into aggregates to create pores that allow the infiltration of water and air.

“Excessive disking creates conditions that allow organic matter to break down rapidly, degrading soil health,” he said.

Kingery called the economic impact of soil compaction “stunning.”

Ernie Flint, an Extension regional agronomist, said the problem is so widespread that many producers consider it inevitable.

“Farmers often assume that compaction exists even though tests have not been done,” Flint said. “Tillage operations are often done when not needed because of this assumption. Very often, the best answer is less traffic and less tillage, not more.”