Substrate survey team digs deep across 83 key black soil counties to unearth secrets of the earth and learn how to improve farming

It takes nature about 200 to 400 years to build about 1 centimeter of black soil, a key element in China's food security. In many parts of Northeast China, that soil is thinning. But the key to saving it, a geologist argues, lies not only in the soil itself, but also in the ground beneath it.

A ground substrate survey team from the Langfang Natural Resources Comprehensive Survey Center of the China Geological Survey has completed a survey across 83 key black soil counties on more than 410,000 square kilometers of land. Their job was not to look at the soil, but to drill through it, down to dozens of meters, to see what lies underneath.

"On the surface, the black soil looks uniform. Drill into it, and the profile changes completely," said Chen Peng, a member of the center's ground substrate survey team.

Some fields sit on a thick layer of clay. Water and nutrients stay put. Others sit on sand and gravel. Water and fertilizer drain straight through. In some places, bedrock is just below the black soil. And once the top layer thins, recovery is painfully slow, he added.

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"Underneath, one field can be completely different from another. We cannot protect them in the same way," the 44-year-old said.

He added that the survey is distinct from routine agricultural soil testing. Agricultural departments focus on the topsoil — how to plow it, how to manage it day to day. The substrate survey goes deeper, through the plow layer and into the material that controls whether water is retained, whether nutrients stay and whether the land can keep producing.

"One looks at how to farm the land. The other looks at whether the land can continue to support farming," Chen said.

He explained that in addition to conventional physical and chemical measurements, ground substrate surveys pay more attention to soil formation age and mineral composition.

"Since the current soil's formation typically takes several thousand to about 10,000 years, we focus on that longer time scale to understand how soils have been formed and evolved. In our surveys, we classified regions into similar environmental categories. Under these comparable conditions, if the soil in one region has been completely eroded away, we explore whether soil from another region could be relocated to make up for the loss."

Over the roughly 40 years from the 1980s to the 2020s, the black soil layer in Northeast China experienced an overall decline in thickness, showing a clear thinning trend, with reductions ranging from 2 to 19 centimeters, according to 2025 research findings by the Northeast Institute of Geography and Agroecology of the Chinese Academy of Sciences.

Chen's team does not carry out protection work itself. It submits recommendations to local authorities. In areas where black soil faces easy erosion, it suggests topsoil stripping — removing and storing the precious layer before it is lost. In other places, it advises where protection should be prioritized, and where farming methods need to change.

Over two years of fieldwork in Northeast China's black soil region since 2022, the team amassed nearly 185,000 samples. For the first time, the team quantitatively mapped the boundaries of black soil resources, pinpointing a total area of 387,500 square kilometers.

The survey, involving 145 core technical members from the center, was completed in two years in collaboration with institutions like the Chinese Academy of Sciences, China Agricultural University and local natural resources authorities.

The team developed a comprehensive model for evaluating black soil stability. The model categorizes the land into four types: stable, relatively stable, vulnerable and degraded. They also built the nation's first database of ground substrate samples for black soil.

In early spring in Northeast China, the ground was still frozen solid. A pickaxe could leave only a white mark. The backpack drills shook hard, and the members' hands often cracked. They just wrapped tape around them and kept going. When the ground thawed and the rains came, they carried heavy gear through the mud. If someone slipped, they wiped off the mud and moved on to the next spot.

In 2023, on the Songnen Plain, they drilled over a hundred boreholes. But what they actually measured differed greatly from the model's predictions about the black soil layer.

The model was supposed to predict black soil conditions across the larger area around the borehole, but the reality on the ground was too complex to fit the model's simulation results.

One night in Qiqihar, Heilongjiang province, someone suggested shrinking the survey area to avoid the difficulty. Shrinking the scope would have meant sidestepping the trickiest unknown, but that would leave a gap in the report.

"We refused to give up. We then realized our old way was to just drill to a set depth of 5 meters no matter what the ground was like. That turned out to be inefficient because it ignored the regional background. So later, we made the depth depend on the actual geological conditions," he said.

For example, in weathered rock areas, soil is often only 1 to 2 meters deep. In river floodplains, where sediments run deep, investigation depth went down 20 to 50 meters.

"Our new survey system was developed through trial and error — and it is now logically self-consistent."

The survey is part of a shift in how China reads its land.

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In 2020, the Ministry of Natural Resources proposed the survey. The Langfang center ran a pilot in Baoding, Hebei province, then expanded to areas including the northeast's black soil region, the northwest's loess region and the coastal zone in Zhejiang province's Ningbo city.

In 2024, the team published China's first working methodology for ground substrate surveys, and developed the technical requirements for surveying the ground substrate in the black soil region of Northeast China, closing a critical technical gap that had existed in the country.

In the same year, they also tackled the problem of surface salt loss after rain and deep-layer salt resurgence after clear days, offering long-term solutions for saline-alkaline land improvement in places like Cangzhou, Hebei province and Lianyungang, Jiangsu province.

In the future, the team will extend efforts to key farming regions, such as the North China Plain, the middle and lower Yangtze River Plain, the hills along the southeastern coast, and Hainan Island.

"As long as there is land beneath our feet that needs guarding, we geologists will be the first to rush to the front line — to uncover the secrets of the earth and to take stock of all it holds," Chen said.

Contact the writers at lihongyang@chinadaily.com.cn