The hidden part of the soil

Interest in the soil often stops at the top 30cm – where ploughing, sowing, and some­times harvesting take place. But below this lies an area that is equally impor­tant for plants: The subsoil. It contains large reserves of water, nutri­ents, and organic matter.

In dry years, it can be crucial for plant survival, provided the roots can reach it. However, compacted layers – caused by the use of heavy machinery, among other things – frequently prevent this from happening. Conse­quently, the root system remains shallow, and the plant only utilises some of the avail­able water.

Topsoil and subsoil – two worlds beneath the field

Topsoil (0–30cm) is quite homo­ge­neous, nutrient-rich, and gener­ally allows for good root pene­tra­tion. Here, the farmer focuses on soil struc­ture as well as air and water balance, ensuring optimal condi­tions for plant growth.

Below this lies the subsoil, which can extend several metres down to the parent rock. It stores about half of the water avail­able to plants, contains 20–30% of the nitrogen and phos­phorus reserves, and an average of around 40% of the organic carbon. The problem is that around 70% of German arable soils have root-inhibiting layers, meaning that plants often cannot utilise this dormant under­ground reserve.

Farmers can use certain measures to influ­ence the struc­ture of the subsoil with the aim of increasing yields. These measures are known as subsoil melio­ra­tion. “If the subsoil is pene­trable by roots, the plant can endure a dry phase much better,” empha­sises Prof Dr Axel Don from the Thünen Insti­tute of Climate-Smart Agri­cul­ture. Measure­ments show that improved sites expe­ri­ence signif­i­cantly reduced yield losses during drought years.

“In normal times, it might be enough just to have the topsoil. But in extremely dry condi­tions, the plant can no longer manage with the water in the topsoil. Then it needs the subsoil,” explains the soil scien­tist. A loose subsoil also offers advan­tages during heavy rain. A loos­ened struc­ture improves the aera­tion of the subsoil and its water absorp­tion capacity, providing good protec­tion against water erosion.

Paths into the depths – how plants can reach the subsoil again

There are various approaches to breaking up compaction in the subsoil. These include mechan­ical deep tillage methods as well as biolog­ical measures.

The Soil³ project – compost as a struc­ture stabiliser

As part of the ‘Soil as a sustain­able resource for the bioe­conomy’ (BonaRes) programme, funded by the German Federal Ministry of Educa­tion and Research (BMBF), the Soil³ project (2015–2025) aimed to make deeper soil layers usable as a resource. The researchers devel­oped a tech­nique for subsoil melio­ra­tion that is gentler than deep ploughing and has a longer-lasting effect than deep tillage.

The result isthe so-called Soil³ tech­nique: First, the topsoil is pushed aside with a ploughshare on 30cm wide strips. A deep spader then loosens the subsoil and mixes in compost to a depth of 50–60cm. This both loosens and stabilises the soil, leading to measur­able long-term effects, as shown by field trial results.

• + 20–25% higher grain yields compared to the control
• + 50% higher maize yield in the first, very dry year
• Effect unchanged even after eight years
• Amor­ti­sa­tion after three to five years (at €700 – 800/ha / £608 – 695, plus costs for compost)

The measure is partic­u­larly advan­ta­geous where soil was previ­ously compacted. Sandy soils bene­fited more than loess soils. No increase in nitrate leaching from the subsoil was observed – on the contrary, plants were able to make better use of water and nutri­ents.

Soil³ exper­i­ment in maize at the Thyrow site with several treat­ment vari­ants (August 2020). From left to right: Conven­tional tillage, para­plough, deep tillage with straw incor­po­ra­tion for stabil­i­sa­tion, deep tillage, deep tillage with biocom­post incor­po­ra­tion for stabil­i­sa­tion (photo: Michael Sommer/ZALF)

Between the furrows that were worked with the Soil³ tech­nique, the winter rye dried out in July 2021 (photo: Dr Kathlin Schweitzer/HU Berlin)

The furrows filled with compost are about 50cm deep. (Photo: Dr Kathlin Schweitzer/HU Berlin)

Root growth is often restricted to the topsoil because plants cannot pene­trate the compacted, deeper soil layers (photo: Dr Kathlin Schweitzer/HU Berlin)

The method is not yet ready for prac­tical use. Suffi­cient quan­ti­ties of compost are not yet avail­able for large-scale appli­ca­tions, and the specialised tech­nology is also not yet acces­sible. Addi­tion­ally, there are uncer­tain­ties in terms of legal cate­gori­sa­tion. According to the German Federal Soil Protec­tion Act (BBod­SchV), the intro­duc­tion of organic mate­rials into the subsoil is currently prohib­ited. Whether compost falls under this or is covered by the Fertiliser Ordi­nance in this process remains uncer­tain.

Alter­na­tive: Partial deep tillage

An option without compost is partial deep tillage, where the soil is ploughed in strips to a depth of 60cm. This allows topsoil mate­rial to enter the subsoil. The same applies here: Caution is advised with clay soils, as the soil struc­ture can be easily damaged. A suit­able plough is expected to be intro­duced to the market in the coming years.

Lilian Guzmán from the Gross Machnow agri­cul­tural co-oper­a­tive is testing the method on the sandy soils of Bran­den­burg – with an average of 28 ground control points. “We are hoping for an increase in yield,” explains the farmer. And in the first year, 2024, the green-cut rye on the deep-tilled strips yielded 9% more than the variant with the conven­tional plough and 44% more compared to the field culti­vator. Grain maize, on the other hand, did equally well in all vari­ants in the wet year of 2025. “The year wasn’t bad enough,” concludes Ms Guzmán.

In addi­tion to mechan­ical methods, the Thünen team is inves­ti­gating arti­fi­cially created earth­worm chan­nels. To achieve this, fine holes (8mm across, 80cm deep) are made in the ground. Just three months later, 90% of the pores were occu­pied by roots, which resulted in 2–15% higher yields. Earth­worms also quickly colonised the tunnels. “They are like motor­ways for the roots in the subsoil,” says Prof Don enthu­si­as­ti­cally. “That could be the solu­tion. The system returns to a natural state, so to speak.” Ques­tions about the optimum number of holes, depth and stability are still unan­swered.

Looking into the depths is worth­while

While the Soil³ tech­nique, deep ploughing, deep tillage, and crumb deep­ening are partic­u­larly bene­fi­cial for sandy soils in north-eastern Germany, creating biopores is more suit­able for clay soils. All these methods are costly and energy-inten­sive. General recom­men­da­tions are diffi­cult because of the signif­i­cant natural vari­ability of the subsoil, even within a single field.

Deter­mining the best measure and its expected impact is there­fore always a site-specific and farm-specific ques­tion. The bene­fits and risks of such an inter­ven­tion in the soil should be care­fully consid­ered. Tech­nical, economic, and – in some cases – legal hurdles still remain. Those who dig deeper – both liter­ally and figu­ra­tively – and focus on their subsoil, lay the foun­da­tion for stable yields under increas­ingly extreme weather condi­tions.