Heavy machines in the fields compact the soil. And the crops have a hard time growing in heavily compacted soil. When the soil is compacted, the number and size of the pores are reduced, and this hinders the transport of water and oxygen in the soil.

"Compaction of the soil below normal ploughing depth is therefore a serious problem because it inhibits agricultural production and affects the environment, but also because the natural recovery rate is slow," says researcher Mansonia Pulido-Moncada from the Department of Agroecology at Aarhus University, who along with researchers from Aarhus, Arkansas and Ghent University have investigated whether certain crops can help loosen and restore the subsoil structure after compaction.

Mitigation strategies

According to the researchers behind the study, one can improve a compacted soil in several ways.

“You can do different things to avoid compaction of the soil, but once the damage is done, you have to resort to other methods to improve the soil and rebuild the pore structure. This can be done, for example, by means of a mechanical deep soil loosening. Nature can also help with drought, rain, frost, and thaw. Or as a third you can use plants and their roots. And it is the latter that has been the focus of this study, ”says Professor Lars J. Munkholm from the Department of Agroecology. 

However, according to the researchers, it is not without challenges to use plants to improve the pore structure in a heavily compacted soil. The poor conditions in the soil help to limit the root development of the plants.

“However, how much a plant's root growth is affected by poor soil conditions differs from species to species. And we have chosen crops are able to grow in compacted soil,” says Lars J. Munkholm. 

CT scan of the soil

In this study, the researchers evaluated whether different plant species can affect and improve the pore network in heavily compacted subsoil under northern European agricultural conditions. 

In the two-year trial, the researchers used chicory, lucerne, fodder radish, tall fescue, and kernza as test crops, while spring barley was used as a reference. The purpose was, among other things, to assess how these five crops can help to loosen compacted soil, as well as improve pore functionality at three different depths with main focus on compacted subsoil layers (0.25-0.35 cm depth).

"We have assessed and quantified the pore properties at three different soil depths using a CT scanner, which is commonly used in medical science," says Mansonia Pulido-Moncada. Who explains that with the help of the scanner they could clearly see how the roots from the different crops affected the soil.

In the experiment, the researchers grew the five crops as well as the reference crop in undisturbed soil columns taken from a soil that was heavily packed due to driving with heavy machinery. Each soil column was scanned both before and after the two years of growing the potential ‘bio-subsoilers’had elapsed. And the CT pore properties were qualified using image analysis.

CT images show potential

"Our experiments showed the potential to use crops as a potential restoration strategy for soil structure, but it very much depends on which crop one chooses to use," explains Mansonia Pulido-Moncada. "It was clear already after the first year that lucerne and chicory had a significantly better ability to increase pore volume and porosity to a depth of 0.25-0.35 m."

However, after the first year, the compacted layer further down (0.35-0.50 m) showed only small changes in the pore system of all the crops examined, which according to the researchers indicates that the soil was still very compacted after one year of cultivation.

After the second year, the researchers found that lucerne and chicory had a greater positive impact on the pore system, while fodder radish, kernza and tall fescue did no differently than spring barley.

The roots of the plants can help to some extent

In the attempt to use crops as a treatment for the porosity of the soil, the plants showed different results. Where fodder radish, tall fescue and kernza showed no significant impact in any of the three scanned depths relative to the spring barley reference, the amount of macropores and complexity of the pore system was greater for chicory and lucerne compared to spring barley.

“Chicory and lucerne seem to contribute to the development of a large number of complex pores, with improved functionality, and this indicates that the two crops perform better than the other three when used as a crop treatment. They create a larger, more connected, and complex pore network. But there is a need to cultivate a mixture of species with potential as “sub-soilers” should be a further topic of investigation,” explains Lars Juhl Munkholm.

“Lucerne clearly has the greatest underground potential for loosening compacted soil below normal ploughing depth, our experiments show. And investigation is ongoing with two papers in review status at the moment,” says Mansonia Pulido-Moncada.