In 2018, scientists added up to 15 tons of cacao-shell biochar per hectare to degraded tropical soil; maize yields improved over several growing seasons

In one corner of Sumatra, Indonesia, soil scientists turned cacao-shell waste into biochar and applied it to severely degraded, acidic farmland. Then they waited.

A 2018 field study titled “Fading positive effect of biochar on crop yield and soil acidity during five growth seasons in an Indonesian Ultisol,” published in the journal Science of The Total Environment, found that maize grown in treated, heavily degraded Ultisol soil performed significantly better than maize grown in untreated plots over five growing seasons. The strongest results came from cacao-shell biochar, although the benefits changed over time.

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Cocoa and cacao farming generate substantial amounts of agricultural residue, including shells and husks, which can be left to decompose or burn. The Sumatra experiment explored whether some of that biomass could instead be converted into biochar: a charcoal-like substance created by burning organic matter in a low-oxygen environment, ensuring that it doesn't burn completely. The idea of returning charred organic material to soil is not new; Amazonian soils associated with ancient human activity, including the famous Terra Preta, have long been part of scientific discussions on this practice.

What actually happened in the dirt

A comparison of two types of biochars, made from rice husks and cacao shells, at application rates of 5 and 15 tons per hectare was conducted in highly acidic Ultisol soil. The cacao-shell biochar performed particularly well. It had a higher pH and greater acid-neutralizing capacity than the rice-husk biochar, giving it greater liming potential. The researchers linked the stronger yield response to changes in soil acidity and more favorable calcium-to-aluminium ratios, which can help reduce the effects of aluminium toxicity in acidic soils.

The maize yield from plots receiving 15 tons per hectare of cacao-shell biochar peaked in the second season, continued to show a positive effect in seasons three and four, and the effect faded by the fifth season.

There's always a catch

There was no one-and-done cure here. The same study found that the positive effects faded after several growing seasons. The researchers estimated that reapplying 15 tons per hectare of cacao-shell biochar approximately every third season would be necessary to maintain its positive effect on yield. That recommendation is consistent with the observed pattern: the strongest yield response occurred in the second season, remained positive through seasons three and four, and faded in season five.

Biochar is also not a homogeneous product, and its effects depend heavily on the material from which it is produced and the soil receiving it. In this experiment, cacao-shell biochar had considerably greater liming potential than rice-husk biochar. The findings therefore apply specifically to the highly acidic tropical Ultisol examined in Sumatra and cannot automatically be transferred to very different soils, such as those found across the U.S. Midwest Corn Belt.

More recent field research points in a similar direction while adding another wrinkle: biochar may work particularly well as part of a broader soil-management strategy. A study in Northern Ghana in the 2025 Scientific Reports paper titled “Integrating biochar, compost, and chemical fertilizer improves maize yield and soil health in the guinea savannah: evidence from two cropping seasons in Northern Ghana,” found that combining biochar, compost and chemical fertilizer increased maize grain yields by 105.7% in 2023 and 127.4% in 2024 compared with the untreated control. The integrated treatment also improved several soil properties, including organic carbon, nitrogen and phosphorus.

A separate 2025 field experiment in southern Togo, published in the Geoderma Regional paper titled “The influence of rice husk biochar, compost, and their mixture on maize yields and soil organic carbon content in southern Togo,” showed that applying 20 tons per hectare of rice-husk biochar significantly increased soil organic carbon. Its effect on maize yield was significant in the second growing season but became marginal in the third, while mineral fertilizer produced significant effects across all three seasons. The researchers concluded that biochar could contribute to improved soil carbon and maize production, but also recommended reapplication after one year under the conditions tested.

Why this means

Soil degradation does not exist in isolation, confined to far-flung smallholder farms abroad. About one-third of the world's land is degraded, according to widely cited global assessments, although estimates vary depending on how land degradation is defined and measured. The fact that biochar can reduce soil acidity and improve crop yields under certain conditions is worth attention from anyone seriously considering food security, rather than dismissing the idea as merely a feel-good sustainability headline.

It's not just that biochar could make agricultural waste more useful. It turns out that a residue many of us would otherwise overlook can have measurable value in the soil, under the right conditions, with the right feedstock, application rate and management.