Processes such as nutrient uptake, transport of water and contaminants, root growth and air exchange are some of the soil functions that depend on the soil's pore system. A better understanding of the complex interactions between the pore system and the soil functions is crucial in the efforts to optimise plant production and minimise the effect on the surrounding environment. A combination of digital imaging, physical models, and laboratory measurements can help us take a step further. 

“There is no universal method that can be used to describe the pores in the soil. Therefore, we have investigated the possibility of using artificial and 3D printed models in combination with CT X-ray photography for studies of the connection between soil structure and transport of gas. Our assumption is that artificial pore systems and 3D-printed soil samples can be useful physical models that can help us better understand the soil functions. And we have investigated this by using gas transport as case,” says senior researcher Mathieu Lamandé from the Department of Agroecology at Aarhus University.

The soil pore system is just like blood vessels

The soil pore system is often compared to the human blood vessels. Large arteries provide transport over larger stretches, while so-called marginal pores branch off from the arteries. Some pores may be completely cut off from the arteries and the marginal pores. The distribution and coherence between the three groups of pores is of great importance for the functions of the soil. Soil compaction can for example reduce the transport of air by the marginal pores to the entire pore volume. When the soil is compacked, there can also often be a tendency for transport only in the artery pores.

“On cultivated soils, the pore system is very different in the upper, cultivated layer and the subsoil. And the difference between the two layers plays a significant role in relation to the soil functions and our understanding of how they work. And the regular cultivation of the top soil layer results in a very heterogeneous pore system with many marginal pores and artery pores that go in all directions,” explains Mathieu Lamandé.

The lower part of the soil, on the other hand, is more affected by plant roots and earthworms. It creates a more durable pore structure consisting of vertical macropores (arteries). And these are exactly the differences the researchers have wanted to study on the basis of natural samples as well as artificially created model samples. 

3D printed reconstruction

“A complete reconstruction of the soil's structure or architecture, we can call it, is not yet possible. But it is possible to construct a soil model as well as use 3D printing to make a partial reconstruction. By combining CT X-rays with measurements of air transport in the pores - mass flow and diffusion, we can actually get a good picture of the complicated geometry with a resolution of a few micrometers,” says Mathieu Lamandé.

To date, very few studies have investigated and demonstrated the potential of using 3D printing to better understand physical, mechanical, and biological processes in soil.

A method with great potential

The researchers CT-scanned eight natural 100 cm3 soil samples from both the plow layer and the subsoil, as well as eight artificially produced 100 cm3 samples in plastic and aerated concrete. On the basis of the CT scans, 3D prints were also made of both the natural and the artificially produced samples. 

“We then tested the air transport in the various soil samples, both the natural, artificial and 3D-printed ones. The measurements confirmed our hypotheses about the occurrence of artery pores and marginal pores and that the vertical artery pores mean a lot for the transport in the subsoil,” says Mathieu Lamandé. 

The study confirms that CT scans and 3D printing combined are suitable as a conceptual model to describe the impact of soil structure on gas transport. But even though the method has the potential to reconstruct the soil macropore network, there are some points to consider.

“CT X-ray photography and 3D printing also have imperfections. For example, one can experience blockages in pore spaces in the form of material residues. And then it is important that you take into account the CT images and the resolution of the 3D printer in relation to the type of pore structure you are trying to reconstruct. So, the method is not perfect, there are elements that need to be taken into account. But if you do, our study shows that there is great potential in using 3D-printed soil samples to reconstruct the soil macropore network for a better understanding of soil functions such as gas transport,” says Mathieu Lamandé.