The large, air-filled spaces, or “macropores,” in untilled soil often resemble the branching vessels of the human circulatory system. Taking advantage of this similarity, a team of Nordic researchers led by Per Schjønning combined computed tomography (CT) scanning with traditional measurements of air exchange to “diagnose” the long-term impacts of soil compaction on the hidden, but vital, soil pore network.
Compaction also reduced the size of the vertical arteries, and just as in the human body, this constriction of the soil’s “circulatory” system can have ill effects. Blocked and narrowed pores likely impede the diffusion of air through bulk soil, the scientists say. The dominance of vertical pipes in the compacted soil also suggests that water flows mostly downward, with relatively little reaching the surrounding soil matrix. Both of these changes can reduce crop productivity. But most troubling to the researchers was how lasting the impacts of compaction appear to be.
In the study, the group examined soil cores taken from a depth of 0.3 to 0.4 meters (0.9 to 1.2 feet) in plots where 30 years earlier a heavy tractor-trailer drove over the ground four times in an experimental treatment. (Only smaller farm equipment was used in subsequent years.) Despite all the elapsed time, macropores in the compacted subsoil were still highly altered compared with control soils, indicating a poor ability of this heavy clay soil to recover its original structure.
What’s more, the damage was done by wheel loads (3.2 Mg per tractor rear wheel and 4.8 Mg per trailer wheel) that are considerably lower than those used in agriculture today.