Scientists use CT scanning to study effects of soil compaction

MADISON, Wis. — The large, air-filled spaces, or “macropores,” in untilled soil often resemble the branching vessels of the human circulatory system.

Method

Taking advantage of this similarity, a team of Nordic researchers led by Per Schjonning (www.poseidon-nordic.dk) 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.

In farm settings, soil can become compressed and unnaturally dense when heavy farm machinery is driven over it. But what the system of pores looks like in compacted soil hasn’t been well studied.

Results

When the Nordic scientists examined cores of compacted, heavy clay subsoil from a research site in Finland, they found the macropores were greatly affected compared with a non-compacted, control soil.

In particular, the compacted soil contained mostly long, vertical “arterial” pores, or pipes, with significantly fewer “marginal” pores branching from them.

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 water flows mostly downward, with relatively little reaching the surrounding soil matrix. Both of these changes can reduce crop productivity.

Concern

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.

What this all says is that while subsoil compaction is easy to ignore because it’s hard to see, it definitely deserves more study, say the researchers.