China’s food security is rooted in its soil. Sadly, more than 40 percent of China’s soil is degraded from overuse, erosion, and pollution. The government’s 2014 soil survey revealed that 19 percent of China’s farmland was contaminated by metals such as lead, cadmium, and arsenic as well as organic and inorganic chemical pollutants. As part of its growing war on pollution, China’s central government enacted a new soil pollution law on January 1, 2019, to clean up contaminated sites. However, this new law targets just one of the many critical soil quality issues that reduce agricultural yield but does not address the problem of compacted soil.

Wide-spread construction severely compacts soil, which reduces crop yield by more than 60 percent. In China, more than 2.47 million hectares of farmland – the size of the state of Vermont – have been reclaimed under the Dynamic Balance of Total Farmland Area policy. Under this policy, when any arable land is converted to a non-farm use, like a factory or housing, new arable land of equal or greater area must be compensated for. Cities pay to tear down old farm villages as one way to find this new land.

Unfortunately, this reclaimed farmland is not fully productive, preventing it from fully contributing to China’s food security. Researchers at Zhejiang University found that only 37 percent of reclaimed land is used for agriculture. Much (44 percent) is just bare soil and 19 percent is now forest. Just as a forest ecosystem takes time to grow and develop after a forest is harvested, the same is true for soil.

Reclaiming Disturbed Soils

Soils develop over hundreds to thousands of years. However, since it is buried beneath our feet, its complex structure and ecosystem are difficult to see and appreciate. Productive soils that support plant growth contain pore spaces with interconnected pathways that bring air and water to roots, insects, and microbes. The soil that took nature thousands of years to develop from its parent material can be destroyed in a matter of hours with excavation equipment. To build homes and roadways, the top soil is stripped and subsoil compacted to create a firm foundation. Once disturbed and compacted, it takes decades to centuries for natural forces to recreate the different layers, aggregates and interconnected pores of soil. Piling up a bunch of dirt does not make it soil.

A farmer who faces a bare lot stripped of top soil with a compacted subsoil confronts a daunting task. On these lots, even when someone cultivates it and buys seeds, fertilizers and pesticides, the crop yield will be low. A three-part approach – compost, cover crops and time – is needed to successfully restore compacted construction sites, reports Rattan Lal, author of the book Urban Soils.

His research found compacted residential sites can be rehabilitated by applying yard and food waste compost, then planting a cover crop of sorghum-sudangrass and tillage radish for two years. The compost provides nutrients and food for plants and microbes, and the cover crop’s penetrating roots create pathways. After the two-year treatment, the crop yield was 8 to 10 times higher than untreated lots. Unfortunately, treated lots still yielded 60 percent less than an average farm field. Farmers will need to keep applying compost and planting cover crops for years to fully restore the soil’s productivity.

China’s Soil Management Legacy

With 4,000+ years of agricultural experience, China has long worked to maximize production. To maintain high yields, the Chinese practically invented sustainable farming. Starting in the Qing Dynasty (1644 to 1912), Chinese farmers carefully collected animal and human waste from cities to improve the soil and fertilize their crops. This practice did not diminish until the 1970s.

Unfortunately, as China has modernized its agriculture and moved to commercial fertilizers, farmers drastically reduced the old practices of collecting, composting, and applying waste material to crop fields.

Conservation Agriculture

Conservation agriculture is a new approach that blends the ancient practices like compost application with new technology. Conservation agriculture practices are sustainable and profitable with minimal soil disturbance, permanent soil cover and crop rotation. Crop residues are left on the field as mulch, and cover crops are planted after harvest.

Chinese farmers and local governments have adopted conservation agriculture practices on only 8 million hectares as of 2016, about the area of South Carolina. Adoption has been stymied for a few reasons. First many farmers falsely believe that plowing is necessary for a successful crop. Second, crop residue and straw that should cover the soil as a natural mulch are traditionally collected and burned as fuel for cooking and heating. Finally, special planting equipment is needed to cut through the natural mulch to place the seeds in the soil. Overall, it takes years of applying conservation agriculture techniques for the compacted soil to recover and to restore normal crop yields.

Since 2002, the Ministry of Agriculture has supported demonstrations for progressive farmers to see the benefits, learn how to use the new equipment, and increase adoption of conservation agriculture. Today at almost 600 sites in rural China, agricultural extension meetings educate and encourage influential farmers to return to their village and teach others about conservation agriculture.

Connecting Farm to City

Finding enough compost to apply to the millions of hectares of reclaimed soils is a challenge in China. One source could renew the vital farm to city connection. Today 70 percent of China’s municipal solid waste is food waste, however cities have mainly landfilled or incinerated food wastes instead of separating it out. As part of the war on pollution, 46 Chinese cities are piloting municipal solid waste sorting, which is an important first step in generating compost for rural areas.

Once damaged by construction, agricultural land takes time to recover. By understanding soil and agricultural processes, Chinese farmers can speed up reclamation by applying compost and growing cover crops. Through conservation agricultural techniques, China could continue to grow crops for another 40 centuries.